Linux Virtualization - Apr 2018 - [RFC v2] virtio: support packed ring

Hello everyone,

This RFC implements packed ring support for virtio driver.

The code was tested with DPDK vhost (testpmd/vhost-PMD) implemented
by Jens at http://dpdk.org/ml/archives/dev/2018-January/089417.html
Minor changes are needed for the vhost code, e.g. to kick the guest.

TODO:
- Refinements and bug fixes;
- Split into small patches;
- Test indirect descriptor support;
- Test/fix event suppression support;
- Test devices other than net;

RFC v1 -> RFC v2:
- Add indirect descriptor support - compile test only;
- Add event suppression supprt - compile test only;
- Move vring_packed_init() out of uapi (Jason, MST);
- Merge two loops into one in virtqueue_add_packed() (Jason);
- Split vring_unmap_one() for packed ring and split ring (Jason);
- Avoid using '%' operator (Jason);
- Rename free_head -> next_avail_idx (Jason);
- Add comments for virtio_wmb() in virtqueue_add_packed() (Jason);
- Some other refinements and bug fixes;

Thanks!

Signed-off-by: Tiwei Bie <tiwei.bie at intel.com>
---
 drivers/virtio/virtio_ring.c       | 1094 +++++++++++++++++++++++++++++-------
 include/linux/virtio_ring.h        |    8 +-
 include/uapi/linux/virtio_config.h |   12 +-
 include/uapi/linux/virtio_ring.h   |   61 ++
 4 files changed, 980 insertions(+), 195 deletions(-)

diff --git a/drivers/virtio/virtio_ring.c b/drivers/virtio/virtio_ring.c
index 71458f493cf8..0515dca34d77 100644
--- a/drivers/virtio/virtio_ring.c
+++ b/drivers/virtio/virtio_ring.c
@@ -58,14 +58,15 @@
 
 struct vring_desc_state {
 	void *data;			/* Data for callback. */
-	struct vring_desc *indir_desc;	/* Indirect descriptor, if any. */
+	void *indir_desc;		/* Indirect descriptor, if any. */
+	int num;			/* Descriptor list length. */
 };
 
 struct vring_virtqueue {
 	struct virtqueue vq;
 
-	/* Actual memory layout for this queue */
-	struct vring vring;
+	/* Is this a packed ring? */
+	bool packed;
 
 	/* Can we use weak barriers? */
 	bool weak_barriers;
@@ -79,19 +80,45 @@ struct vring_virtqueue {
 	/* Host publishes avail event idx */
 	bool event;
 
-	/* Head of free buffer list. */
-	unsigned int free_head;
 	/* Number we've added since last sync. */
 	unsigned int num_added;
 
 	/* Last used index we've seen. */
 	u16 last_used_idx;
 
-	/* Last written value to avail->flags */
-	u16 avail_flags_shadow;
+	union {
+		/* Available for split ring */
+		struct {
+			/* Actual memory layout for this queue. */
+			struct vring vring;
 
-	/* Last written value to avail->idx in guest byte order */
-	u16 avail_idx_shadow;
+			/* Head of free buffer list. */
+			unsigned int free_head;
+
+			/* Last written value to avail->flags */
+			u16 avail_flags_shadow;
+
+			/* Last written value to avail->idx in
+			 * guest byte order. */
+			u16 avail_idx_shadow;
+		};
+
+		/* Available for packed ring */
+		struct {
+			/* Actual memory layout for this queue. */
+			struct vring_packed vring_packed;
+
+			/* Driver ring wrap counter. */
+			u8 wrap_counter;
+
+			/* Index of the next avail descriptor. */
+			unsigned int next_avail_idx;
+
+			/* Last written value to driver->flags in
+			 * guest byte order. */
+			u16 event_flags_shadow;
+		};
+	};
 
 	/* How to notify other side. FIXME: commonalize hcalls! */
 	bool (*notify)(struct virtqueue *vq);
@@ -201,8 +228,33 @@ static dma_addr_t vring_map_single(const struct
vring_virtqueue *vq,
 			      cpu_addr, size, direction);
 }
 
-static void vring_unmap_one(const struct vring_virtqueue *vq,
-			    struct vring_desc *desc)
+static void vring_unmap_one_split(const struct vring_virtqueue *vq,
+				  struct vring_desc *desc)
+{
+	u16 flags;
+
+	if (!vring_use_dma_api(vq->vq.vdev))
+		return;
+
+	flags = virtio16_to_cpu(vq->vq.vdev, desc->flags);
+
+	if (flags & VRING_DESC_F_INDIRECT) {
+		dma_unmap_single(vring_dma_dev(vq),
+				 virtio64_to_cpu(vq->vq.vdev, desc->addr),
+				 virtio32_to_cpu(vq->vq.vdev, desc->len),
+				 (flags & VRING_DESC_F_WRITE) ?
+				 DMA_FROM_DEVICE : DMA_TO_DEVICE);
+	} else {
+		dma_unmap_page(vring_dma_dev(vq),
+			       virtio64_to_cpu(vq->vq.vdev, desc->addr),
+			       virtio32_to_cpu(vq->vq.vdev, desc->len),
+			       (flags & VRING_DESC_F_WRITE) ?
+			       DMA_FROM_DEVICE : DMA_TO_DEVICE);
+	}
+}
+
+static void vring_unmap_one_packed(const struct vring_virtqueue *vq,
+				   struct vring_packed_desc *desc)
 {
 	u16 flags;
 
@@ -235,8 +287,9 @@ static int vring_mapping_error(const struct vring_virtqueue
*vq,
 	return dma_mapping_error(vring_dma_dev(vq), addr);
 }
 
-static struct vring_desc *alloc_indirect(struct virtqueue *_vq,
-					 unsigned int total_sg, gfp_t gfp)
+static struct vring_desc *alloc_indirect_split(struct virtqueue *_vq,
+					       unsigned int total_sg,
+					       gfp_t gfp)
 {
 	struct vring_desc *desc;
 	unsigned int i;
@@ -257,14 +310,32 @@ static struct vring_desc *alloc_indirect(struct virtqueue
*_vq,
 	return desc;
 }
 
-static inline int virtqueue_add(struct virtqueue *_vq,
-				struct scatterlist *sgs[],
-				unsigned int total_sg,
-				unsigned int out_sgs,
-				unsigned int in_sgs,
-				void *data,
-				void *ctx,
-				gfp_t gfp)
+static struct vring_packed_desc *alloc_indirect_packed(struct virtqueue *_vq,
+						       unsigned int total_sg,
+						       gfp_t gfp)
+{
+	struct vring_packed_desc *desc;
+
+	/*
+	 * We require lowmem mappings for the descriptors because
+	 * otherwise virt_to_phys will give us bogus addresses in the
+	 * virtqueue.
+	 */
+	gfp &= ~__GFP_HIGHMEM;
+
+	desc = kmalloc(total_sg * sizeof(struct vring_packed_desc), gfp);
+
+	return desc;
+}
+
+static inline int virtqueue_add_split(struct virtqueue *_vq,
+				      struct scatterlist *sgs[],
+				      unsigned int total_sg,
+				      unsigned int out_sgs,
+				      unsigned int in_sgs,
+				      void *data,
+				      void *ctx,
+				      gfp_t gfp)
 {
 	struct vring_virtqueue *vq = to_vvq(_vq);
 	struct scatterlist *sg;
@@ -303,7 +374,7 @@ static inline int virtqueue_add(struct virtqueue *_vq,
 	/* If the host supports indirect descriptor tables, and we have multiple
 	 * buffers, then go indirect. FIXME: tune this threshold */
 	if (vq->indirect && total_sg > 1 && vq->vq.num_free)
-		desc = alloc_indirect(_vq, total_sg, gfp);
+		desc = alloc_indirect_split(_vq, total_sg, gfp);
 	else {
 		desc = NULL;
 		WARN_ON_ONCE(total_sg > vq->vring.num && !vq->indirect);
@@ -424,7 +495,7 @@ static inline int virtqueue_add(struct virtqueue *_vq,
 	for (n = 0; n < total_sg; n++) {
 		if (i == err_idx)
 			break;
-		vring_unmap_one(vq, &desc[i]);
+		vring_unmap_one_split(vq, &desc[i]);
 		i = virtio16_to_cpu(_vq->vdev, vq->vring.desc[i].next);
 	}
 
@@ -435,6 +506,210 @@ static inline int virtqueue_add(struct virtqueue *_vq,
 	return -EIO;
 }
 
+static inline int virtqueue_add_packed(struct virtqueue *_vq,
+				       struct scatterlist *sgs[],
+				       unsigned int total_sg,
+				       unsigned int out_sgs,
+				       unsigned int in_sgs,
+				       void *data,
+				       void *ctx,
+				       gfp_t gfp)
+{
+	struct vring_virtqueue *vq = to_vvq(_vq);
+	struct vring_packed_desc *desc;
+	struct scatterlist *sg;
+	unsigned int i, n, descs_used, uninitialized_var(prev), err_idx;
+	__virtio16 uninitialized_var(head_flags), flags;
+	int head, wrap_counter;
+	bool indirect;
+
+	START_USE(vq);
+
+	BUG_ON(data == NULL);
+	BUG_ON(ctx && vq->indirect);
+
+	if (unlikely(vq->broken)) {
+		END_USE(vq);
+		return -EIO;
+	}
+
+#ifdef DEBUG
+	{
+		ktime_t now = ktime_get();
+
+		/* No kick or get, with .1 second between?  Warn. */
+		if (vq->last_add_time_valid)
+			WARN_ON(ktime_to_ms(ktime_sub(now, vq->last_add_time))
+					    > 100);
+		vq->last_add_time = now;
+		vq->last_add_time_valid = true;
+	}
+#endif
+
+	BUG_ON(total_sg == 0);
+
+	head = vq->next_avail_idx;
+	wrap_counter = vq->wrap_counter;
+
+	/* If the host supports indirect descriptor tables, and we have multiple
+	 * buffers, then go indirect. FIXME: tune this threshold */
+	if (vq->indirect && total_sg > 1 && vq->vq.num_free)
+		desc = alloc_indirect_packed(_vq, total_sg, gfp);
+	else {
+		desc = NULL;
+		WARN_ON_ONCE(total_sg > vq->vring_packed.num &&
!vq->indirect);
+	}
+
+	if (desc) {
+		/* Use a single buffer which doesn't continue */
+		indirect = true;
+		/* Set up rest to use this indirect table. */
+		i = 0;
+		descs_used = 1;
+	} else {
+		indirect = false;
+		desc = vq->vring_packed.desc;
+		i = head;
+		descs_used = total_sg;
+	}
+
+	if (vq->vq.num_free < descs_used) {
+		pr_debug("Can't add buf len %i - avail = %i\n",
+			 descs_used, vq->vq.num_free);
+		/* FIXME: for historical reasons, we force a notify here if
+		 * there are outgoing parts to the buffer.  Presumably the
+		 * host should service the ring ASAP. */
+		if (out_sgs)
+			vq->notify(&vq->vq);
+		if (indirect)
+			kfree(desc);
+		END_USE(vq);
+		return -ENOSPC;
+	}
+
+	for (n = 0; n < out_sgs + in_sgs; n++) {
+		for (sg = sgs[n]; sg; sg = sg_next(sg)) {
+			dma_addr_t addr = vring_map_one_sg(vq, sg, n < out_sgs ?
+						DMA_TO_DEVICE : DMA_FROM_DEVICE);
+			if (vring_mapping_error(vq, addr))
+				goto unmap_release;
+
+			flags = cpu_to_virtio16(_vq->vdev, VRING_DESC_F_NEXT |
+					(n < out_sgs ? 0 : VRING_DESC_F_WRITE) |
+					VRING_DESC_F_AVAIL(vq->wrap_counter) |
+					VRING_DESC_F_USED(!vq->wrap_counter));
+			if (!indirect && i == head)
+				head_flags = flags;
+			else
+				desc[i].flags = flags;
+
+			desc[i].addr = cpu_to_virtio64(_vq->vdev, addr);
+			desc[i].len = cpu_to_virtio32(_vq->vdev, sg->length);
+			desc[i].id = cpu_to_virtio32(_vq->vdev, head);
+			prev = i;
+			i++;
+			if (!indirect && i >= vq->vring_packed.num) {
+				i = 0;
+				vq->wrap_counter ^= 1;
+			}
+		}
+	}
+	/* Last one doesn't continue. */
+	if (total_sg == 1)
+		head_flags &= cpu_to_virtio16(_vq->vdev, ~VRING_DESC_F_NEXT);
+	else
+		desc[prev].flags &= cpu_to_virtio16(_vq->vdev, ~VRING_DESC_F_NEXT);
+
+	if (indirect) {
+		/* Now that the indirect table is filled in, map it. */
+		dma_addr_t addr = vring_map_single(
+			vq, desc, total_sg * sizeof(struct vring_packed_desc),
+			DMA_TO_DEVICE);
+		if (vring_mapping_error(vq, addr))
+			goto unmap_release;
+
+		head_flags = cpu_to_virtio16(_vq->vdev, VRING_DESC_F_INDIRECT |
+					     VRING_DESC_F_AVAIL(wrap_counter) |
+					     VRING_DESC_F_USED(!wrap_counter));
+		vq->vring_packed.desc[head].addr = cpu_to_virtio64(_vq->vdev, addr);
+		vq->vring_packed.desc[head].len = cpu_to_virtio32(_vq->vdev,
+				total_sg * sizeof(struct vring_packed_desc));
+		vq->vring_packed.desc[head].id = cpu_to_virtio32(_vq->vdev, head);
+	}
+
+	/* We're using some buffers from the free list. */
+	vq->vq.num_free -= descs_used;
+
+	/* Update free pointer */
+	if (indirect) {
+		n = head + 1;
+		if (n >= vq->vring_packed.num) {
+			n = 0;
+			vq->wrap_counter ^= 1;
+		}
+		vq->next_avail_idx = n;
+	} else
+		vq->next_avail_idx = i;
+
+	/* Store token and indirect buffer state. */
+	vq->desc_state[head].num = descs_used;
+	vq->desc_state[head].data = data;
+	if (indirect)
+		vq->desc_state[head].indir_desc = desc;
+	else
+		vq->desc_state[head].indir_desc = ctx;
+
+	/* A driver MUST NOT make the first descriptor in the list
+	 * available before all subsequent descriptors comprising
+	 * the list are made available. */
+	virtio_wmb(vq->weak_barriers);
+	vq->vring_packed.desc[head].flags = head_flags;
+	vq->num_added++;
+
+	pr_debug("Added buffer head %i to %p\n", head, vq);
+	END_USE(vq);
+
+	return 0;
+
+unmap_release:
+	err_idx = i;
+	i = head;
+
+	for (n = 0; n < total_sg; n++) {
+		if (i == err_idx)
+			break;
+		vring_unmap_one_packed(vq, &desc[i]);
+		i++;
+		if (!indirect && i >= vq->vring_packed.num)
+			i = 0;
+	}
+
+	vq->wrap_counter = wrap_counter;
+
+	if (indirect)
+		kfree(desc);
+
+	END_USE(vq);
+	return -EIO;
+}
+
+static inline int virtqueue_add(struct virtqueue *_vq,
+				struct scatterlist *sgs[],
+				unsigned int total_sg,
+				unsigned int out_sgs,
+				unsigned int in_sgs,
+				void *data,
+				void *ctx,
+				gfp_t gfp)
+{
+	struct vring_virtqueue *vq = to_vvq(_vq);
+
+	return vq->packed ? virtqueue_add_packed(_vq, sgs, total_sg, out_sgs,
+						 in_sgs, data, ctx, gfp) :
+			    virtqueue_add_split(_vq, sgs, total_sg, out_sgs,
+						in_sgs, data, ctx, gfp);
+}
+
 /**
  * virtqueue_add_sgs - expose buffers to other end
  * @vq: the struct virtqueue we're talking about.
@@ -537,18 +812,7 @@ int virtqueue_add_inbuf_ctx(struct virtqueue *vq,
 }
 EXPORT_SYMBOL_GPL(virtqueue_add_inbuf_ctx);
 
-/**
- * virtqueue_kick_prepare - first half of split virtqueue_kick call.
- * @vq: the struct virtqueue
- *
- * Instead of virtqueue_kick(), you can do:
- *	if (virtqueue_kick_prepare(vq))
- *		virtqueue_notify(vq);
- *
- * This is sometimes useful because the virtqueue_kick_prepare() needs
- * to be serialized, but the actual virtqueue_notify() call does not.
- */
-bool virtqueue_kick_prepare(struct virtqueue *_vq)
+static bool virtqueue_kick_prepare_split(struct virtqueue *_vq)
 {
 	struct vring_virtqueue *vq = to_vvq(_vq);
 	u16 new, old;
@@ -580,6 +844,62 @@ bool virtqueue_kick_prepare(struct virtqueue *_vq)
 	END_USE(vq);
 	return needs_kick;
 }
+
+static bool virtqueue_kick_prepare_packed(struct virtqueue *_vq)
+{
+	struct vring_virtqueue *vq = to_vvq(_vq);
+	u16 new, old, off_wrap;
+	bool needs_kick;
+
+	START_USE(vq);
+	/* We need to expose the new flags value before checking notification
+	 * suppressions. */
+	virtio_mb(vq->weak_barriers);
+
+	old = vq->next_avail_idx - vq->num_added;
+	new = vq->next_avail_idx;
+	vq->num_added = 0;
+
+#ifdef DEBUG
+	if (vq->last_add_time_valid) {
+		WARN_ON(ktime_to_ms(ktime_sub(ktime_get(),
+					      vq->last_add_time)) > 100);
+	}
+	vq->last_add_time_valid = false;
+#endif
+
+	off_wrap = virtio16_to_cpu(_vq->vdev,
vq->vring_packed.device->off_wrap);
+
+	if (vq->event) {
+		// FIXME: fix this!
+		needs_kick = ((off_wrap >> 15) == vq->wrap_counter) &&
+			     vring_need_event(off_wrap & ~(1<<15), new, old);
+	} else {
+		needs_kick = (vq->vring_packed.device->flags !+			     
cpu_to_virtio16(_vq->vdev, VRING_EVENT_F_DISABLE));
+	}
+	END_USE(vq);
+	return needs_kick;
+}
+
+/**
+ * virtqueue_kick_prepare - first half of split virtqueue_kick call.
+ * @vq: the struct virtqueue
+ *
+ * Instead of virtqueue_kick(), you can do:
+ *	if (virtqueue_kick_prepare(vq))
+ *		virtqueue_notify(vq);
+ *
+ * This is sometimes useful because the virtqueue_kick_prepare() needs
+ * to be serialized, but the actual virtqueue_notify() call does not.
+ */
+bool virtqueue_kick_prepare(struct virtqueue *_vq)
+{
+	struct vring_virtqueue *vq = to_vvq(_vq);
+
+	return vq->packed ? virtqueue_kick_prepare_packed(_vq) :
+			    virtqueue_kick_prepare_split(_vq);
+}
 EXPORT_SYMBOL_GPL(virtqueue_kick_prepare);
 
 /**
@@ -626,8 +946,8 @@ bool virtqueue_kick(struct virtqueue *vq)
 }
 EXPORT_SYMBOL_GPL(virtqueue_kick);
 
-static void detach_buf(struct vring_virtqueue *vq, unsigned int head,
-		       void **ctx)
+static void detach_buf_split(struct vring_virtqueue *vq, unsigned int head,
+			     void **ctx)
 {
 	unsigned int i, j;
 	__virtio16 nextflag = cpu_to_virtio16(vq->vq.vdev, VRING_DESC_F_NEXT);
@@ -639,12 +959,12 @@ static void detach_buf(struct vring_virtqueue *vq,
unsigned int head,
 	i = head;
 
 	while (vq->vring.desc[i].flags & nextflag) {
-		vring_unmap_one(vq, &vq->vring.desc[i]);
+		vring_unmap_one_split(vq, &vq->vring.desc[i]);
 		i = virtio16_to_cpu(vq->vq.vdev, vq->vring.desc[i].next);
 		vq->vq.num_free++;
 	}
 
-	vring_unmap_one(vq, &vq->vring.desc[i]);
+	vring_unmap_one_split(vq, &vq->vring.desc[i]);
 	vq->vring.desc[i].next = cpu_to_virtio16(vq->vq.vdev, vq->free_head);
 	vq->free_head = head;
 
@@ -666,7 +986,7 @@ static void detach_buf(struct vring_virtqueue *vq, unsigned
int head,
 		BUG_ON(len == 0 || len % sizeof(struct vring_desc));
 
 		for (j = 0; j < len / sizeof(struct vring_desc); j++)
-			vring_unmap_one(vq, &indir_desc[j]);
+			vring_unmap_one_split(vq, &indir_desc[j]);
 
 		kfree(indir_desc);
 		vq->desc_state[head].indir_desc = NULL;
@@ -675,11 +995,207 @@ static void detach_buf(struct vring_virtqueue *vq,
unsigned int head,
 	}
 }
 
-static inline bool more_used(const struct vring_virtqueue *vq)
+static int detach_buf_packed(struct vring_virtqueue *vq, unsigned int head,
+			      void **ctx)
+{
+	struct vring_packed_desc *desc;
+	unsigned int i, j;
+
+	/* Clear data ptr. */
+	vq->desc_state[head].data = NULL;
+
+	i = head;
+
+	for (j = 0; j < vq->desc_state[head].num; j++) {
+		desc = &vq->vring_packed.desc[i];
+		vring_unmap_one_packed(vq, desc);
+		desc->flags = 0x0;
+		i++;
+		if (i >= vq->vring_packed.num)
+			i = 0;
+	}
+
+	vq->vq.num_free += vq->desc_state[head].num;
+
+	if (vq->indirect) {
+		u32 len;
+
+		desc = vq->desc_state[head].indir_desc;
+		/* Free the indirect table, if any, now that it's unmapped. */
+		if (!desc)
+			goto out;
+
+		len = virtio32_to_cpu(vq->vq.vdev,
+				      vq->vring_packed.desc[head].len);
+
+		BUG_ON(!(vq->vring_packed.desc[head].flags &
+			 cpu_to_virtio16(vq->vq.vdev, VRING_DESC_F_INDIRECT)));
+		BUG_ON(len == 0 || len % sizeof(struct vring_packed_desc));
+
+		for (j = 0; j < len / sizeof(struct vring_packed_desc); j++)
+			vring_unmap_one_packed(vq, &desc[j]);
+
+		kfree(desc);
+		vq->desc_state[head].indir_desc = NULL;
+	} else if (ctx) {
+		*ctx = vq->desc_state[head].indir_desc;
+	}
+
+out:
+	return vq->desc_state[head].num;
+}
+
+static inline bool more_used_split(const struct vring_virtqueue *vq)
 {
 	return vq->last_used_idx != virtio16_to_cpu(vq->vq.vdev,
vq->vring.used->idx);
 }
 
+static inline bool more_used_packed(const struct vring_virtqueue *vq)
+{
+	u16 last_used, flags;
+	bool avail, used;
+
+	if (vq->vq.num_free == vq->vring_packed.num)
+		return false;
+
+	last_used = vq->last_used_idx;
+	flags = virtio16_to_cpu(vq->vq.vdev,
+				vq->vring_packed.desc[last_used].flags);
+	avail = flags & VRING_DESC_F_AVAIL(1);
+	used = flags & VRING_DESC_F_USED(1);
+
+	return avail == used;
+}
+
+static inline bool more_used(const struct vring_virtqueue *vq)
+{
+	return vq->packed ? more_used_packed(vq) : more_used_split(vq);
+}
+
+void *virtqueue_get_buf_ctx_split(struct virtqueue *_vq, unsigned int *len,
+				  void **ctx)
+{
+	struct vring_virtqueue *vq = to_vvq(_vq);
+	void *ret;
+	unsigned int i;
+	u16 last_used;
+
+	START_USE(vq);
+
+	if (unlikely(vq->broken)) {
+		END_USE(vq);
+		return NULL;
+	}
+
+	if (!more_used(vq)) {
+		pr_debug("No more buffers in queue\n");
+		END_USE(vq);
+		return NULL;
+	}
+
+	/* Only get used array entries after they have been exposed by host. */
+	virtio_rmb(vq->weak_barriers);
+
+	last_used = (vq->last_used_idx & (vq->vring.num - 1));
+	i = virtio32_to_cpu(_vq->vdev, vq->vring.used->ring[last_used].id);
+	*len = virtio32_to_cpu(_vq->vdev,
vq->vring.used->ring[last_used].len);
+
+	if (unlikely(i >= vq->vring.num)) {
+		BAD_RING(vq, "id %u out of range\n", i);
+		return NULL;
+	}
+	if (unlikely(!vq->desc_state[i].data)) {
+		BAD_RING(vq, "id %u is not a head!\n", i);
+		return NULL;
+	}
+
+	/* detach_buf_split clears data, so grab it now. */
+	ret = vq->desc_state[i].data;
+	detach_buf_split(vq, i, ctx);
+	vq->last_used_idx++;
+	/* If we expect an interrupt for the next entry, tell host
+	 * by writing event index and flush out the write before
+	 * the read in the next get_buf call. */
+	if (!(vq->avail_flags_shadow & VRING_AVAIL_F_NO_INTERRUPT))
+		virtio_store_mb(vq->weak_barriers,
+				&vring_used_event(&vq->vring),
+				cpu_to_virtio16(_vq->vdev, vq->last_used_idx));
+
+#ifdef DEBUG
+	vq->last_add_time_valid = false;
+#endif
+
+	END_USE(vq);
+	return ret;
+}
+
+void *virtqueue_get_buf_ctx_packed(struct virtqueue *_vq, unsigned int *len,
+				   void **ctx)
+{
+	struct vring_virtqueue *vq = to_vvq(_vq);
+	uint16_t wrap_counter;
+	void *ret;
+	unsigned int i;
+	u16 last_used;
+
+	START_USE(vq);
+
+	if (unlikely(vq->broken)) {
+		END_USE(vq);
+		return NULL;
+	}
+
+	if (!more_used(vq)) {
+		pr_debug("No more buffers in queue\n");
+		END_USE(vq);
+		return NULL;
+	}
+
+	/* Only get used elements after they have been exposed by host. */
+	virtio_rmb(vq->weak_barriers);
+
+	last_used = vq->last_used_idx;
+	i = virtio32_to_cpu(_vq->vdev, vq->vring_packed.desc[last_used].id);
+	*len = virtio32_to_cpu(_vq->vdev, vq->vring_packed.desc[last_used].len);
+
+	if (unlikely(i >= vq->vring_packed.num)) {
+		BAD_RING(vq, "id %u out of range\n", i);
+		return NULL;
+	}
+	if (unlikely(!vq->desc_state[i].data)) {
+		BAD_RING(vq, "id %u is not a head!\n", i);
+		return NULL;
+	}
+
+	/* detach_buf_packed clears data, so grab it now. */
+	ret = vq->desc_state[i].data;
+	detach_buf_packed(vq, i, ctx);
+
+	vq->last_used_idx += vq->desc_state[i].num;
+	if (vq->last_used_idx >= vq->vring_packed.num)
+		vq->last_used_idx -= vq->vring_packed.num;
+
+	wrap_counter = vq->wrap_counter;
+	if (vq->last_used_idx > vq->next_avail_idx)
+		wrap_counter ^= 1;
+
+	/* If we expect an interrupt for the next entry, tell host
+	 * by writing event index and flush out the write before
+	 * the read in the next get_buf call. */
+	if (vq->event_flags_shadow == VRING_EVENT_F_DESC)
+		virtio_store_mb(vq->weak_barriers,
+				&vq->vring_packed.driver->off_wrap,
+				cpu_to_virtio16(_vq->vdev, vq->last_used_idx |
+						wrap_counter << 15));
+
+#ifdef DEBUG
+	vq->last_add_time_valid = false;
+#endif
+
+	END_USE(vq);
+	return ret;
+}
+
 /**
  * virtqueue_get_buf - get the next used buffer
  * @vq: the struct virtqueue we're talking about.
@@ -700,57 +1216,9 @@ void *virtqueue_get_buf_ctx(struct virtqueue *_vq,
unsigned int *len,
 			    void **ctx)
 {
 	struct vring_virtqueue *vq = to_vvq(_vq);
-	void *ret;
-	unsigned int i;
-	u16 last_used;
 
-	START_USE(vq);
-
-	if (unlikely(vq->broken)) {
-		END_USE(vq);
-		return NULL;
-	}
-
-	if (!more_used(vq)) {
-		pr_debug("No more buffers in queue\n");
-		END_USE(vq);
-		return NULL;
-	}
-
-	/* Only get used array entries after they have been exposed by host. */
-	virtio_rmb(vq->weak_barriers);
-
-	last_used = (vq->last_used_idx & (vq->vring.num - 1));
-	i = virtio32_to_cpu(_vq->vdev, vq->vring.used->ring[last_used].id);
-	*len = virtio32_to_cpu(_vq->vdev,
vq->vring.used->ring[last_used].len);
-
-	if (unlikely(i >= vq->vring.num)) {
-		BAD_RING(vq, "id %u out of range\n", i);
-		return NULL;
-	}
-	if (unlikely(!vq->desc_state[i].data)) {
-		BAD_RING(vq, "id %u is not a head!\n", i);
-		return NULL;
-	}
-
-	/* detach_buf clears data, so grab it now. */
-	ret = vq->desc_state[i].data;
-	detach_buf(vq, i, ctx);
-	vq->last_used_idx++;
-	/* If we expect an interrupt for the next entry, tell host
-	 * by writing event index and flush out the write before
-	 * the read in the next get_buf call. */
-	if (!(vq->avail_flags_shadow & VRING_AVAIL_F_NO_INTERRUPT))
-		virtio_store_mb(vq->weak_barriers,
-				&vring_used_event(&vq->vring),
-				cpu_to_virtio16(_vq->vdev, vq->last_used_idx));
-
-#ifdef DEBUG
-	vq->last_add_time_valid = false;
-#endif
-
-	END_USE(vq);
-	return ret;
+	return vq->packed ? virtqueue_get_buf_ctx_packed(_vq, len, ctx) :
+			    virtqueue_get_buf_ctx_split(_vq, len, ctx);
 }
 EXPORT_SYMBOL_GPL(virtqueue_get_buf_ctx);
 
@@ -759,6 +1227,29 @@ void *virtqueue_get_buf(struct virtqueue *_vq, unsigned
int *len)
 	return virtqueue_get_buf_ctx(_vq, len, NULL);
 }
 EXPORT_SYMBOL_GPL(virtqueue_get_buf);
+
+static void virtqueue_disable_cb_split(struct virtqueue *_vq)
+{
+	struct vring_virtqueue *vq = to_vvq(_vq);
+
+	if (!(vq->avail_flags_shadow & VRING_AVAIL_F_NO_INTERRUPT)) {
+		vq->avail_flags_shadow |= VRING_AVAIL_F_NO_INTERRUPT;
+		if (!vq->event)
+			vq->vring.avail->flags = cpu_to_virtio16(_vq->vdev,
vq->avail_flags_shadow);
+	}
+}
+
+static void virtqueue_disable_cb_packed(struct virtqueue *_vq)
+{
+	struct vring_virtqueue *vq = to_vvq(_vq);
+
+	if (vq->event_flags_shadow != VRING_EVENT_F_DISABLE) {
+		vq->event_flags_shadow = VRING_EVENT_F_DISABLE;
+		vq->vring_packed.driver->flags = cpu_to_virtio16(_vq->vdev,
+							vq->event_flags_shadow);
+	}
+}
+
 /**
  * virtqueue_disable_cb - disable callbacks
  * @vq: the struct virtqueue we're talking about.
@@ -772,15 +1263,66 @@ void virtqueue_disable_cb(struct virtqueue *_vq)
 {
 	struct vring_virtqueue *vq = to_vvq(_vq);
 
-	if (!(vq->avail_flags_shadow & VRING_AVAIL_F_NO_INTERRUPT)) {
-		vq->avail_flags_shadow |= VRING_AVAIL_F_NO_INTERRUPT;
-		if (!vq->event)
-			vq->vring.avail->flags = cpu_to_virtio16(_vq->vdev,
vq->avail_flags_shadow);
-	}
-
+	if (vq->packed)
+		virtqueue_disable_cb_packed(_vq);
+	else
+		virtqueue_disable_cb_split(_vq);
 }
 EXPORT_SYMBOL_GPL(virtqueue_disable_cb);
 
+static unsigned virtqueue_enable_cb_prepare_split(struct virtqueue *_vq)
+{
+	struct vring_virtqueue *vq = to_vvq(_vq);
+	u16 last_used_idx;
+
+	START_USE(vq);
+
+	/* We optimistically turn back on interrupts, then check if there was
+	 * more to do. */
+	/* Depending on the VIRTIO_RING_F_EVENT_IDX feature, we need to
+	 * either clear the flags bit or point the event index at the next
+	 * entry. Always do both to keep code simple. */
+	if (vq->avail_flags_shadow & VRING_AVAIL_F_NO_INTERRUPT) {
+		vq->avail_flags_shadow &= ~VRING_AVAIL_F_NO_INTERRUPT;
+		if (!vq->event)
+			vq->vring.avail->flags = cpu_to_virtio16(_vq->vdev,
vq->avail_flags_shadow);
+	}
+	vring_used_event(&vq->vring) = cpu_to_virtio16(_vq->vdev,
last_used_idx = vq->last_used_idx);
+	END_USE(vq);
+	return last_used_idx;
+}
+
+static unsigned virtqueue_enable_cb_prepare_packed(struct virtqueue *_vq)
+{
+	struct vring_virtqueue *vq = to_vvq(_vq);
+	u16 last_used_idx, wrap_counter, off_wrap;
+
+	START_USE(vq);
+
+	last_used_idx = vq->last_used_idx;
+	wrap_counter = vq->wrap_counter;
+
+	if (last_used_idx > vq->next_avail_idx)
+		wrap_counter ^= 1;
+
+	off_wrap = last_used_idx | (wrap_counter << 15);
+
+	/* We optimistically turn back on interrupts, then check if there was
+	 * more to do. */
+	/* Depending on the VIRTIO_RING_F_EVENT_IDX feature, we need to
+	 * either clear the flags bit or point the event index at the next
+	 * entry. Always do both to keep code simple. */
+	if (vq->event_flags_shadow == VRING_EVENT_F_DISABLE) {
+		vq->event_flags_shadow = vq->event ? VRING_EVENT_F_DESC:
+						     VRING_EVENT_F_ENABLE;
+		vq->vring_packed.driver->flags = cpu_to_virtio16(_vq->vdev,
+							vq->event_flags_shadow);
+	}
+	vq->vring_packed.driver->off_wrap = cpu_to_virtio16(_vq->vdev,
off_wrap);
+	END_USE(vq);
+	return last_used_idx;
+}
+
 /**
  * virtqueue_enable_cb_prepare - restart callbacks after disable_cb
  * @vq: the struct virtqueue we're talking about.
@@ -796,26 +1338,34 @@ EXPORT_SYMBOL_GPL(virtqueue_disable_cb);
 unsigned virtqueue_enable_cb_prepare(struct virtqueue *_vq)
 {
 	struct vring_virtqueue *vq = to_vvq(_vq);
-	u16 last_used_idx;
 
-	START_USE(vq);
-
-	/* We optimistically turn back on interrupts, then check if there was
-	 * more to do. */
-	/* Depending on the VIRTIO_RING_F_EVENT_IDX feature, we need to
-	 * either clear the flags bit or point the event index at the next
-	 * entry. Always do both to keep code simple. */
-	if (vq->avail_flags_shadow & VRING_AVAIL_F_NO_INTERRUPT) {
-		vq->avail_flags_shadow &= ~VRING_AVAIL_F_NO_INTERRUPT;
-		if (!vq->event)
-			vq->vring.avail->flags = cpu_to_virtio16(_vq->vdev,
vq->avail_flags_shadow);
-	}
-	vring_used_event(&vq->vring) = cpu_to_virtio16(_vq->vdev,
last_used_idx = vq->last_used_idx);
-	END_USE(vq);
-	return last_used_idx;
+	return vq->packed ? virtqueue_enable_cb_prepare_packed(_vq) :
+			    virtqueue_enable_cb_prepare_split(_vq);
 }
 EXPORT_SYMBOL_GPL(virtqueue_enable_cb_prepare);
 
+static bool virtqueue_poll_split(struct virtqueue *_vq, unsigned last_used_idx)
+{
+	struct vring_virtqueue *vq = to_vvq(_vq);
+
+	virtio_mb(vq->weak_barriers);
+	return (u16)last_used_idx != virtio16_to_cpu(_vq->vdev,
vq->vring.used->idx);
+}
+
+static bool virtqueue_poll_packed(struct virtqueue *_vq, unsigned
last_used_idx)
+{
+	struct vring_virtqueue *vq = to_vvq(_vq);
+	bool avail, used;
+	u16 flags;
+
+	virtio_mb(vq->weak_barriers);
+	flags = virtio16_to_cpu(vq->vq.vdev,
+			vq->vring_packed.desc[last_used_idx].flags);
+	avail = flags & VRING_DESC_F_AVAIL(1);
+	used = flags & VRING_DESC_F_USED(1);
+	return avail == used;
+}
+
 /**
  * virtqueue_poll - query pending used buffers
  * @vq: the struct virtqueue we're talking about.
@@ -829,8 +1379,8 @@ bool virtqueue_poll(struct virtqueue *_vq, unsigned
last_used_idx)
 {
 	struct vring_virtqueue *vq = to_vvq(_vq);
 
-	virtio_mb(vq->weak_barriers);
-	return (u16)last_used_idx != virtio16_to_cpu(_vq->vdev,
vq->vring.used->idx);
+	return vq->packed ? virtqueue_poll_packed(_vq, last_used_idx) :
+			    virtqueue_poll_split(_vq, last_used_idx);
 }
 EXPORT_SYMBOL_GPL(virtqueue_poll);
 
@@ -852,6 +1402,83 @@ bool virtqueue_enable_cb(struct virtqueue *_vq)
 }
 EXPORT_SYMBOL_GPL(virtqueue_enable_cb);
 
+static bool virtqueue_enable_cb_delayed_split(struct virtqueue *_vq)
+{
+	struct vring_virtqueue *vq = to_vvq(_vq);
+	u16 bufs;
+
+	START_USE(vq);
+
+	/* We optimistically turn back on interrupts, then check if there was
+	 * more to do. */
+	/* Depending on the VIRTIO_RING_F_USED_EVENT_IDX feature, we need to
+	 * either clear the flags bit or point the event index at the next
+	 * entry. Always update the event index to keep code simple. */
+	if (vq->avail_flags_shadow & VRING_AVAIL_F_NO_INTERRUPT) {
+		vq->avail_flags_shadow &= ~VRING_AVAIL_F_NO_INTERRUPT;
+		if (!vq->event)
+			vq->vring.avail->flags = cpu_to_virtio16(_vq->vdev,
vq->avail_flags_shadow);
+	}
+	/* TODO: tune this threshold */
+	bufs = (u16)(vq->avail_idx_shadow - vq->last_used_idx) * 3 / 4;
+
+	virtio_store_mb(vq->weak_barriers,
+			&vring_used_event(&vq->vring),
+			cpu_to_virtio16(_vq->vdev, vq->last_used_idx + bufs));
+
+	if (unlikely((u16)(virtio16_to_cpu(_vq->vdev, vq->vring.used->idx) -
vq->last_used_idx) > bufs)) {
+		END_USE(vq);
+		return false;
+	}
+
+	END_USE(vq);
+	return true;
+}
+
+static bool virtqueue_enable_cb_delayed_packed(struct virtqueue *_vq)
+{
+	struct vring_virtqueue *vq = to_vvq(_vq);
+	u16 bufs, off_wrap, used_idx, wrap_counter;
+
+	START_USE(vq);
+
+	/* We optimistically turn back on interrupts, then check if there was
+	 * more to do. */
+	/* Depending on the VIRTIO_RING_F_USED_EVENT_IDX feature, we need to
+	 * either clear the flags bit or point the event index at the next
+	 * entry. Always update the event index to keep code simple. */
+	if (vq->event_flags_shadow == VRING_EVENT_F_DISABLE) {
+		vq->event_flags_shadow = vq->event ? VRING_EVENT_F_DESC:
+						     VRING_EVENT_F_ENABLE;
+		vq->vring_packed.driver->flags = cpu_to_virtio16(_vq->vdev,
+							vq->event_flags_shadow);
+	}
+
+	/* TODO: tune this threshold */
+	bufs = (u16)(vq->next_avail_idx - vq->last_used_idx) * 3 / 4;
+
+	used_idx = vq->last_used_idx + bufs;
+	if (used_idx >= vq->vring_packed.num)
+		used_idx -= vq->vring_packed.num;
+
+	wrap_counter = vq->wrap_counter;
+	if (used_idx > vq->next_avail_idx)
+		wrap_counter ^= 1;
+
+	off_wrap = used_idx | (wrap_counter << 15);
+
+	virtio_store_mb(vq->weak_barriers,
&vq->vring_packed.driver->off_wrap,
+			cpu_to_virtio16(_vq->vdev, off_wrap));
+
+	if (more_used_packed(vq)) {
+		END_USE(vq);
+		return false;
+	}
+
+	END_USE(vq);
+	return true;
+}
+
 /**
  * virtqueue_enable_cb_delayed - restart callbacks after disable_cb.
  * @vq: the struct virtqueue we're talking about.
@@ -868,37 +1495,69 @@ EXPORT_SYMBOL_GPL(virtqueue_enable_cb);
 bool virtqueue_enable_cb_delayed(struct virtqueue *_vq)
 {
 	struct vring_virtqueue *vq = to_vvq(_vq);
-	u16 bufs;
 
-	START_USE(vq);
-
-	/* We optimistically turn back on interrupts, then check if there was
-	 * more to do. */
-	/* Depending on the VIRTIO_RING_F_USED_EVENT_IDX feature, we need to
-	 * either clear the flags bit or point the event index at the next
-	 * entry. Always update the event index to keep code simple. */
-	if (vq->avail_flags_shadow & VRING_AVAIL_F_NO_INTERRUPT) {
-		vq->avail_flags_shadow &= ~VRING_AVAIL_F_NO_INTERRUPT;
-		if (!vq->event)
-			vq->vring.avail->flags = cpu_to_virtio16(_vq->vdev,
vq->avail_flags_shadow);
-	}
-	/* TODO: tune this threshold */
-	bufs = (u16)(vq->avail_idx_shadow - vq->last_used_idx) * 3 / 4;
-
-	virtio_store_mb(vq->weak_barriers,
-			&vring_used_event(&vq->vring),
-			cpu_to_virtio16(_vq->vdev, vq->last_used_idx + bufs));
-
-	if (unlikely((u16)(virtio16_to_cpu(_vq->vdev, vq->vring.used->idx) -
vq->last_used_idx) > bufs)) {
-		END_USE(vq);
-		return false;
-	}
-
-	END_USE(vq);
-	return true;
+	return vq->packed ? virtqueue_enable_cb_delayed_packed(_vq) :
+			    virtqueue_enable_cb_delayed_split(_vq);
 }
 EXPORT_SYMBOL_GPL(virtqueue_enable_cb_delayed);
 
+static void *virtqueue_detach_unused_buf_split(struct virtqueue *_vq)
+{
+	struct vring_virtqueue *vq = to_vvq(_vq);
+	unsigned int i;
+	void *buf;
+
+	START_USE(vq);
+
+	for (i = 0; i < vq->vring.num; i++) {
+		if (!vq->desc_state[i].data)
+			continue;
+		/* detach_buf clears data, so grab it now. */
+		buf = vq->desc_state[i].data;
+		detach_buf_split(vq, i, NULL);
+		vq->avail_idx_shadow--;
+		vq->vring.avail->idx = cpu_to_virtio16(_vq->vdev,
vq->avail_idx_shadow);
+		END_USE(vq);
+		return buf;
+	}
+	/* That should have freed everything. */
+	BUG_ON(vq->vq.num_free != vq->vring.num);
+
+	END_USE(vq);
+	return NULL;
+}
+
+static void *virtqueue_detach_unused_buf_packed(struct virtqueue *_vq)
+{
+	struct vring_virtqueue *vq = to_vvq(_vq);
+	unsigned int i, num;
+	void *buf;
+
+	START_USE(vq);
+
+	for (i = 0; i < vq->vring_packed.num; i++) {
+		if (!vq->desc_state[i].data)
+			continue;
+		/* detach_buf clears data, so grab it now. */
+		buf = vq->desc_state[i].data;
+		num = detach_buf_packed(vq, i, NULL);
+		if (vq->next_avail_idx < num) {
+			vq->next_avail_idx = vq->vring_packed.num -
+					(num - vq->next_avail_idx);
+			vq->wrap_counter ^= 1;
+		} else {
+			vq->next_avail_idx -= num;
+		}
+		END_USE(vq);
+		return buf;
+	}
+	/* That should have freed everything. */
+	BUG_ON(vq->vq.num_free != vq->vring_packed.num);
+
+	END_USE(vq);
+	return NULL;
+}
+
 /**
  * virtqueue_detach_unused_buf - detach first unused buffer
  * @vq: the struct virtqueue we're talking about.
@@ -910,27 +1569,9 @@ EXPORT_SYMBOL_GPL(virtqueue_enable_cb_delayed);
 void *virtqueue_detach_unused_buf(struct virtqueue *_vq)
 {
 	struct vring_virtqueue *vq = to_vvq(_vq);
-	unsigned int i;
-	void *buf;
 
-	START_USE(vq);
-
-	for (i = 0; i < vq->vring.num; i++) {
-		if (!vq->desc_state[i].data)
-			continue;
-		/* detach_buf clears data, so grab it now. */
-		buf = vq->desc_state[i].data;
-		detach_buf(vq, i, NULL);
-		vq->avail_idx_shadow--;
-		vq->vring.avail->idx = cpu_to_virtio16(_vq->vdev,
vq->avail_idx_shadow);
-		END_USE(vq);
-		return buf;
-	}
-	/* That should have freed everything. */
-	BUG_ON(vq->vq.num_free != vq->vring.num);
-
-	END_USE(vq);
-	return NULL;
+	return vq->packed ? virtqueue_detach_unused_buf_packed(_vq) :
+			    virtqueue_detach_unused_buf_split(_vq);
 }
 EXPORT_SYMBOL_GPL(virtqueue_detach_unused_buf);
 
@@ -955,7 +1596,8 @@ irqreturn_t vring_interrupt(int irq, void *_vq)
 EXPORT_SYMBOL_GPL(vring_interrupt);
 
 struct virtqueue *__vring_new_virtqueue(unsigned int index,
-					struct vring vring,
+					union vring_union vring,
+					bool packed,
 					struct virtio_device *vdev,
 					bool weak_barriers,
 					bool context,
@@ -963,19 +1605,20 @@ struct virtqueue *__vring_new_virtqueue(unsigned int
index,
 					void (*callback)(struct virtqueue *),
 					const char *name)
 {
-	unsigned int i;
+	unsigned int num, i;
 	struct vring_virtqueue *vq;
 
-	vq = kmalloc(sizeof(*vq) + vring.num * sizeof(struct vring_desc_state),
+	num = packed ? vring.vring_packed.num : vring.vring_split.num;
+
+	vq = kmalloc(sizeof(*vq) + num * sizeof(struct vring_desc_state),
 		     GFP_KERNEL);
 	if (!vq)
 		return NULL;
 
-	vq->vring = vring;
 	vq->vq.callback = callback;
 	vq->vq.vdev = vdev;
 	vq->vq.name = name;
-	vq->vq.num_free = vring.num;
+	vq->vq.num_free = num;
 	vq->vq.index = index;
 	vq->we_own_ring = false;
 	vq->queue_dma_addr = 0;
@@ -984,9 +1627,8 @@ struct virtqueue *__vring_new_virtqueue(unsigned int index,
 	vq->weak_barriers = weak_barriers;
 	vq->broken = false;
 	vq->last_used_idx = 0;
-	vq->avail_flags_shadow = 0;
-	vq->avail_idx_shadow = 0;
 	vq->num_added = 0;
+	vq->packed = packed;
 	list_add_tail(&vq->vq.list, &vdev->vqs);
 #ifdef DEBUG
 	vq->in_use = false;
@@ -997,18 +1639,37 @@ struct virtqueue *__vring_new_virtqueue(unsigned int
index,
 		!context;
 	vq->event = virtio_has_feature(vdev, VIRTIO_RING_F_EVENT_IDX);
 
+	if (vq->packed) {
+		vq->vring_packed = vring.vring_packed;
+		vq->next_avail_idx = 0;
+		vq->wrap_counter = 1;
+		vq->event_flags_shadow = 0;
+	} else {
+		vq->vring = vring.vring_split;
+		vq->avail_flags_shadow = 0;
+		vq->avail_idx_shadow = 0;
+
+		/* Put everything in free lists. */
+		vq->free_head = 0;
+		for (i = 0; i < num-1; i++)
+			vq->vring.desc[i].next = cpu_to_virtio16(vdev, i + 1);
+	}
+
 	/* No callback?  Tell other side not to bother us. */
 	if (!callback) {
-		vq->avail_flags_shadow |= VRING_AVAIL_F_NO_INTERRUPT;
-		if (!vq->event)
-			vq->vring.avail->flags = cpu_to_virtio16(vdev,
vq->avail_flags_shadow);
+		if (packed) {
+			vq->event_flags_shadow = VRING_EVENT_F_DISABLE;
+			vq->vring_packed.driver->flags = cpu_to_virtio16(vdev,
+						vq->event_flags_shadow);
+		} else {
+			vq->avail_flags_shadow |= VRING_AVAIL_F_NO_INTERRUPT;
+			if (!vq->event)
+				vq->vring.avail->flags = cpu_to_virtio16(vdev,
+						vq->avail_flags_shadow);
+		}
 	}
 
-	/* Put everything in free lists. */
-	vq->free_head = 0;
-	for (i = 0; i < vring.num-1; i++)
-		vq->vring.desc[i].next = cpu_to_virtio16(vdev, i + 1);
-	memset(vq->desc_state, 0, vring.num * sizeof(struct vring_desc_state));
+	memset(vq->desc_state, 0, num * sizeof(struct vring_desc_state));
 
 	return &vq->vq;
 }
@@ -1056,6 +1717,22 @@ static void vring_free_queue(struct virtio_device *vdev,
size_t size,
 	}
 }
 
+static inline int
+__vring_size(unsigned int num, unsigned long align, bool packed)
+{
+	return packed ? vring_packed_size(num, align) : vring_size(num, align);
+}
+
+static inline void vring_packed_init(struct vring_packed *vr, unsigned int num,
+				     void *p, unsigned long align)
+{
+	vr->num = num;
+	vr->desc = p;
+	vr->driver = (void *)(((uintptr_t)p + sizeof(struct vring_packed_desc)
+		* num + align - 1) & ~(align - 1));
+	vr->device = vr->driver + 1;
+}
+
 struct virtqueue *vring_create_virtqueue(
 	unsigned int index,
 	unsigned int num,
@@ -1072,7 +1749,8 @@ struct virtqueue *vring_create_virtqueue(
 	void *queue = NULL;
 	dma_addr_t dma_addr;
 	size_t queue_size_in_bytes;
-	struct vring vring;
+	union vring_union vring;
+	bool packed;
 
 	/* We assume num is a power of 2. */
 	if (num & (num - 1)) {
@@ -1080,9 +1758,13 @@ struct virtqueue *vring_create_virtqueue(
 		return NULL;
 	}
 
+	packed = virtio_has_feature(vdev, VIRTIO_F_RING_PACKED);
+
 	/* TODO: allocate each queue chunk individually */
-	for (; num && vring_size(num, vring_align) > PAGE_SIZE; num /= 2) {
-		queue = vring_alloc_queue(vdev, vring_size(num, vring_align),
+	for (; num && __vring_size(num, vring_align, packed) > PAGE_SIZE;
+			num /= 2) {
+		queue = vring_alloc_queue(vdev, __vring_size(num, vring_align,
+							     packed),
 					  &dma_addr,
 					  GFP_KERNEL|__GFP_NOWARN|__GFP_ZERO);
 		if (queue)
@@ -1094,17 +1776,21 @@ struct virtqueue *vring_create_virtqueue(
 
 	if (!queue) {
 		/* Try to get a single page. You are my only hope! */
-		queue = vring_alloc_queue(vdev, vring_size(num, vring_align),
+		queue = vring_alloc_queue(vdev, __vring_size(num, vring_align,
+							     packed),
 					  &dma_addr, GFP_KERNEL|__GFP_ZERO);
 	}
 	if (!queue)
 		return NULL;
 
-	queue_size_in_bytes = vring_size(num, vring_align);
-	vring_init(&vring, num, queue, vring_align);
+	queue_size_in_bytes = __vring_size(num, vring_align, packed);
+	if (packed)
+		vring_packed_init(&vring.vring_packed, num, queue, vring_align);
+	else
+		vring_init(&vring.vring_split, num, queue, vring_align);
 
-	vq = __vring_new_virtqueue(index, vring, vdev, weak_barriers, context,
-				   notify, callback, name);
+	vq = __vring_new_virtqueue(index, vring, packed, vdev, weak_barriers,
+				   context, notify, callback, name);
 	if (!vq) {
 		vring_free_queue(vdev, queue_size_in_bytes, queue,
 				 dma_addr);
@@ -1130,10 +1816,17 @@ struct virtqueue *vring_new_virtqueue(unsigned int
index,
 				      void (*callback)(struct virtqueue *vq),
 				      const char *name)
 {
-	struct vring vring;
-	vring_init(&vring, num, pages, vring_align);
-	return __vring_new_virtqueue(index, vring, vdev, weak_barriers, context,
-				     notify, callback, name);
+	union vring_union vring;
+	bool packed;
+
+	packed = virtio_has_feature(vdev, VIRTIO_F_RING_PACKED);
+	if (packed)
+		vring_packed_init(&vring.vring_packed, num, pages, vring_align);
+	else
+		vring_init(&vring.vring_split, num, pages, vring_align);
+
+	return __vring_new_virtqueue(index, vring, packed, vdev, weak_barriers,
+				     context, notify, callback, name);
 }
 EXPORT_SYMBOL_GPL(vring_new_virtqueue);
 
@@ -1143,7 +1836,9 @@ void vring_del_virtqueue(struct virtqueue *_vq)
 
 	if (vq->we_own_ring) {
 		vring_free_queue(vq->vq.vdev, vq->queue_size_in_bytes,
-				 vq->vring.desc, vq->queue_dma_addr);
+				 vq->packed ? (void *)vq->vring_packed.desc :
+					      (void *)vq->vring.desc,
+				 vq->queue_dma_addr);
 	}
 	list_del(&_vq->list);
 	kfree(vq);
@@ -1157,14 +1852,18 @@ void vring_transport_features(struct virtio_device
*vdev)
 
 	for (i = VIRTIO_TRANSPORT_F_START; i < VIRTIO_TRANSPORT_F_END; i++) {
 		switch (i) {
-		case VIRTIO_RING_F_INDIRECT_DESC:
+#if 0
+		case VIRTIO_RING_F_INDIRECT_DESC: // FIXME not tested yet.
 			break;
-		case VIRTIO_RING_F_EVENT_IDX:
+		case VIRTIO_RING_F_EVENT_IDX: // FIXME probably not work.
 			break;
+#endif
 		case VIRTIO_F_VERSION_1:
 			break;
 		case VIRTIO_F_IOMMU_PLATFORM:
 			break;
+		case VIRTIO_F_RING_PACKED:
+			break;
 		default:
 			/* We don't understand this bit. */
 			__virtio_clear_bit(vdev, i);
@@ -1185,7 +1884,7 @@ unsigned int virtqueue_get_vring_size(struct virtqueue
*_vq)
 
 	struct vring_virtqueue *vq = to_vvq(_vq);
 
-	return vq->vring.num;
+	return vq->packed ? vq->vring_packed.num : vq->vring.num;
 }
 EXPORT_SYMBOL_GPL(virtqueue_get_vring_size);
 
@@ -1228,6 +1927,10 @@ dma_addr_t virtqueue_get_avail_addr(struct virtqueue
*_vq)
 
 	BUG_ON(!vq->we_own_ring);
 
+	if (vq->packed)
+		return vq->queue_dma_addr + ((char *)vq->vring_packed.driver -
+				(char *)vq->vring_packed.desc);
+
 	return vq->queue_dma_addr +
 		((char *)vq->vring.avail - (char *)vq->vring.desc);
 }
@@ -1239,11 +1942,16 @@ dma_addr_t virtqueue_get_used_addr(struct virtqueue
*_vq)
 
 	BUG_ON(!vq->we_own_ring);
 
+	if (vq->packed)
+		return vq->queue_dma_addr + ((char *)vq->vring_packed.device -
+				(char *)vq->vring_packed.desc);
+
 	return vq->queue_dma_addr +
 		((char *)vq->vring.used - (char *)vq->vring.desc);
 }
 EXPORT_SYMBOL_GPL(virtqueue_get_used_addr);
 
+/* Only available for split ring */
 const struct vring *virtqueue_get_vring(struct virtqueue *vq)
 {
 	return &to_vvq(vq)->vring;
diff --git a/include/linux/virtio_ring.h b/include/linux/virtio_ring.h
index bbf32524ab27..a0075894ad16 100644
--- a/include/linux/virtio_ring.h
+++ b/include/linux/virtio_ring.h
@@ -60,6 +60,11 @@ static inline void virtio_store_mb(bool weak_barriers,
 struct virtio_device;
 struct virtqueue;
 
+union vring_union {
+	struct vring vring_split;
+	struct vring_packed vring_packed;
+};
+
 /*
  * Creates a virtqueue and allocates the descriptor ring.  If
  * may_reduce_num is set, then this may allocate a smaller ring than
@@ -79,7 +84,8 @@ struct virtqueue *vring_create_virtqueue(unsigned int index,
 
 /* Creates a virtqueue with a custom layout. */
 struct virtqueue *__vring_new_virtqueue(unsigned int index,
-					struct vring vring,
+					union vring_union vring,
+					bool packed,
 					struct virtio_device *vdev,
 					bool weak_barriers,
 					bool ctx,
diff --git a/include/uapi/linux/virtio_config.h
b/include/uapi/linux/virtio_config.h
index 308e2096291f..a6e392325e3a 100644
--- a/include/uapi/linux/virtio_config.h
+++ b/include/uapi/linux/virtio_config.h
@@ -49,7 +49,7 @@
  * transport being used (eg. virtio_ring), the rest are per-device feature
  * bits. */
 #define VIRTIO_TRANSPORT_F_START	28
-#define VIRTIO_TRANSPORT_F_END		34
+#define VIRTIO_TRANSPORT_F_END		36
 
 #ifndef VIRTIO_CONFIG_NO_LEGACY
 /* Do we get callbacks when the ring is completely used, even if we've
@@ -71,4 +71,14 @@
  * this is for compatibility with legacy systems.
  */
 #define VIRTIO_F_IOMMU_PLATFORM		33
+
+/* This feature indicates support for the packed virtqueue layout. */
+#define VIRTIO_F_RING_PACKED		34
+
+/*
+ * This feature indicates that all buffers are used by the device
+ * in the same order in which they have been made available.
+ */
+#define VIRTIO_F_IN_ORDER		35
+
 #endif /* _UAPI_LINUX_VIRTIO_CONFIG_H */
diff --git a/include/uapi/linux/virtio_ring.h b/include/uapi/linux/virtio_ring.h
index 6d5d5faa989b..735d4207c988 100644
--- a/include/uapi/linux/virtio_ring.h
+++ b/include/uapi/linux/virtio_ring.h
@@ -44,6 +44,9 @@
 /* This means the buffer contains a list of buffer descriptors. */
 #define VRING_DESC_F_INDIRECT	4
 
+#define VRING_DESC_F_AVAIL(b)	((b) << 7)
+#define VRING_DESC_F_USED(b)	((b) << 15)
+
 /* The Host uses this in used->flags to advise the Guest: don't kick me
when
  * you add a buffer.  It's unreliable, so it's simply an optimization. 
Guest
  * will still kick if it's out of buffers. */
@@ -53,6 +56,10 @@
  * optimization.  */
 #define VRING_AVAIL_F_NO_INTERRUPT	1
 
+#define VRING_EVENT_F_ENABLE	0x0
+#define VRING_EVENT_F_DISABLE	0x1
+#define VRING_EVENT_F_DESC	0x2
+
 /* We support indirect buffer descriptors */
 #define VIRTIO_RING_F_INDIRECT_DESC	28
 
@@ -171,4 +178,58 @@ static inline int vring_need_event(__u16 event_idx, __u16
new_idx, __u16 old)
 	return (__u16)(new_idx - event_idx - 1) < (__u16)(new_idx - old);
 }
 
+struct vring_packed_desc_event {
+	/* __virtio16 off  : 15; // Descriptor Event Offset
+	 * __virtio16 wrap : 1;  // Descriptor Event Wrap Counter */
+	__virtio16 off_wrap;
+	/* __virtio16 flags : 2; // Descriptor Event Flags */
+	__virtio16 flags;
+};
+
+struct vring_packed_desc {
+	/* Buffer Address. */
+	__virtio64 addr;
+	/* Buffer Length. */
+	__virtio32 len;
+	/* Buffer ID. */
+	__virtio16 id;
+	/* The flags depending on descriptor type. */
+	__virtio16 flags;
+};
+
+struct vring_packed {
+	unsigned int num;
+
+	struct vring_packed_desc *desc;
+
+	struct vring_packed_desc_event *driver;
+
+	struct vring_packed_desc_event *device;
+};
+
+/* The standard layout for the packed ring is a continuous chunk of memory
+ * which looks like this.
+ *
+ * struct vring_packed
+ * {
+ *	// The actual descriptors (16 bytes each)
+ *	struct vring_packed_desc desc[num];
+ *
+ *	// Padding to the next align boundary.
+ *	char pad[];
+ *
+ *	// Driver Event Suppression
+ *	struct vring_packed_desc_event driver;
+ *
+ *	// Device Event Suppression
+ *	struct vring_packed_desc_event device;
+ * };
+ */
+
+static inline unsigned vring_packed_size(unsigned int num, unsigned long align)
+{
+	return ((sizeof(struct vring_packed_desc) * num + align - 1)
+		& ~(align - 1)) + sizeof(struct vring_packed_desc_event) * 2;
+}
+
 #endif /* _UAPI_LINUX_VIRTIO_RING_H */
-- 
2.11.0

On 2018?04?01? 22:12, Tiwei Bie wrote:
> Hello everyone,
>
> This RFC implements packed ring support for virtio driver.
>
> The code was tested with DPDK vhost (testpmd/vhost-PMD) implemented
> by Jens at http://dpdk.org/ml/archives/dev/2018-January/089417.html
> Minor changes are needed for the vhost code, e.g. to kick the guest.
>
> TODO:
> - Refinements and bug fixes;
> - Split into small patches;
> - Test indirect descriptor support;
> - Test/fix event suppression support;
> - Test devices other than net;
>
> RFC v1 -> RFC v2:
> - Add indirect descriptor support - compile test only;
> - Add event suppression supprt - compile test only;
> - Move vring_packed_init() out of uapi (Jason, MST);
> - Merge two loops into one in virtqueue_add_packed() (Jason);
> - Split vring_unmap_one() for packed ring and split ring (Jason);
> - Avoid using '%' operator (Jason);
> - Rename free_head -> next_avail_idx (Jason);
> - Add comments for virtio_wmb() in virtqueue_add_packed() (Jason);
> - Some other refinements and bug fixes;
>
> Thanks!
Will try to review this later.

But it would be better if you can split it (more than 1000 lines is too 
big to be reviewed easily). E.g you can at least split it into three 
patches, new structures, datapath, and event suppression.

Thanks

>
> Signed-off-by: Tiwei Bie <tiwei.bie at intel.com>
> ---
>   drivers/virtio/virtio_ring.c       | 1094
+++++++++++++++++++++++++++++-------
>   include/linux/virtio_ring.h        |    8 +-
>   include/uapi/linux/virtio_config.h |   12 +-
>   include/uapi/linux/virtio_ring.h   |   61 ++
>   4 files changed, 980 insertions(+), 195 deletions(-)
>
> diff --git a/drivers/virtio/virtio_ring.c b/drivers/virtio/virtio_ring.c
> index 71458f493cf8..0515dca34d77 100644
> --- a/drivers/virtio/virtio_ring.c
> +++ b/drivers/virtio/virtio_ring.c
> @@ -58,14 +58,15 @@
>   
>   struct vring_desc_state {
>   	void *data;			/* Data for callback. */
> -	struct vring_desc *indir_desc;	/* Indirect descriptor, if any. */
> +	void *indir_desc;		/* Indirect descriptor, if any. */
> +	int num;			/* Descriptor list length. */
>   };
>   
>   struct vring_virtqueue {
>   	struct virtqueue vq;
>   
> -	/* Actual memory layout for this queue */
> -	struct vring vring;
> +	/* Is this a packed ring? */
> +	bool packed;
>   
>   	/* Can we use weak barriers? */
>   	bool weak_barriers;
> @@ -79,19 +80,45 @@ struct vring_virtqueue {
>   	/* Host publishes avail event idx */
>   	bool event;
>   
> -	/* Head of free buffer list. */
> -	unsigned int free_head;
>   	/* Number we've added since last sync. */
>   	unsigned int num_added;
>   
>   	/* Last used index we've seen. */
>   	u16 last_used_idx;
>   
> -	/* Last written value to avail->flags */
> -	u16 avail_flags_shadow;
> +	union {
> +		/* Available for split ring */
> +		struct {
> +			/* Actual memory layout for this queue. */
> +			struct vring vring;
>   
> -	/* Last written value to avail->idx in guest byte order */
> -	u16 avail_idx_shadow;
> +			/* Head of free buffer list. */
> +			unsigned int free_head;
> +
> +			/* Last written value to avail->flags */
> +			u16 avail_flags_shadow;
> +
> +			/* Last written value to avail->idx in
> +			 * guest byte order. */
> +			u16 avail_idx_shadow;
> +		};
> +
> +		/* Available for packed ring */
> +		struct {
> +			/* Actual memory layout for this queue. */
> +			struct vring_packed vring_packed;
> +
> +			/* Driver ring wrap counter. */
> +			u8 wrap_counter;
> +
> +			/* Index of the next avail descriptor. */
> +			unsigned int next_avail_idx;
> +
> +			/* Last written value to driver->flags in
> +			 * guest byte order. */
> +			u16 event_flags_shadow;
> +		};
> +	};
>   
>   	/* How to notify other side. FIXME: commonalize hcalls! */
>   	bool (*notify)(struct virtqueue *vq);
> @@ -201,8 +228,33 @@ static dma_addr_t vring_map_single(const struct
vring_virtqueue *vq,
>   			      cpu_addr, size, direction);
>   }
>   
> -static void vring_unmap_one(const struct vring_virtqueue *vq,
> -			    struct vring_desc *desc)
> +static void vring_unmap_one_split(const struct vring_virtqueue *vq,
> +				  struct vring_desc *desc)
> +{
> +	u16 flags;
> +
> +	if (!vring_use_dma_api(vq->vq.vdev))
> +		return;
> +
> +	flags = virtio16_to_cpu(vq->vq.vdev, desc->flags);
> +
> +	if (flags & VRING_DESC_F_INDIRECT) {
> +		dma_unmap_single(vring_dma_dev(vq),
> +				 virtio64_to_cpu(vq->vq.vdev, desc->addr),
> +				 virtio32_to_cpu(vq->vq.vdev, desc->len),
> +				 (flags & VRING_DESC_F_WRITE) ?
> +				 DMA_FROM_DEVICE : DMA_TO_DEVICE);
> +	} else {
> +		dma_unmap_page(vring_dma_dev(vq),
> +			       virtio64_to_cpu(vq->vq.vdev, desc->addr),
> +			       virtio32_to_cpu(vq->vq.vdev, desc->len),
> +			       (flags & VRING_DESC_F_WRITE) ?
> +			       DMA_FROM_DEVICE : DMA_TO_DEVICE);
> +	}
> +}
> +
> +static void vring_unmap_one_packed(const struct vring_virtqueue *vq,
> +				   struct vring_packed_desc *desc)
>   {
>   	u16 flags;
>   
> @@ -235,8 +287,9 @@ static int vring_mapping_error(const struct
vring_virtqueue *vq,
>   	return dma_mapping_error(vring_dma_dev(vq), addr);
>   }
>   
> -static struct vring_desc *alloc_indirect(struct virtqueue *_vq,
> -					 unsigned int total_sg, gfp_t gfp)
> +static struct vring_desc *alloc_indirect_split(struct virtqueue *_vq,
> +					       unsigned int total_sg,
> +					       gfp_t gfp)
>   {
>   	struct vring_desc *desc;
>   	unsigned int i;
> @@ -257,14 +310,32 @@ static struct vring_desc *alloc_indirect(struct
virtqueue *_vq,
>   	return desc;
>   }
>   
> -static inline int virtqueue_add(struct virtqueue *_vq,
> -				struct scatterlist *sgs[],
> -				unsigned int total_sg,
> -				unsigned int out_sgs,
> -				unsigned int in_sgs,
> -				void *data,
> -				void *ctx,
> -				gfp_t gfp)
> +static struct vring_packed_desc *alloc_indirect_packed(struct virtqueue
*_vq,
> +						       unsigned int total_sg,
> +						       gfp_t gfp)
> +{
> +	struct vring_packed_desc *desc;
> +
> +	/*
> +	 * We require lowmem mappings for the descriptors because
> +	 * otherwise virt_to_phys will give us bogus addresses in the
> +	 * virtqueue.
> +	 */
> +	gfp &= ~__GFP_HIGHMEM;
> +
> +	desc = kmalloc(total_sg * sizeof(struct vring_packed_desc), gfp);
> +
> +	return desc;
> +}
> +
> +static inline int virtqueue_add_split(struct virtqueue *_vq,
> +				      struct scatterlist *sgs[],
> +				      unsigned int total_sg,
> +				      unsigned int out_sgs,
> +				      unsigned int in_sgs,
> +				      void *data,
> +				      void *ctx,
> +				      gfp_t gfp)
>   {
>   	struct vring_virtqueue *vq = to_vvq(_vq);
>   	struct scatterlist *sg;
> @@ -303,7 +374,7 @@ static inline int virtqueue_add(struct virtqueue *_vq,
>   	/* If the host supports indirect descriptor tables, and we have multiple
>   	 * buffers, then go indirect. FIXME: tune this threshold */
>   	if (vq->indirect && total_sg > 1 &&
vq->vq.num_free)
> -		desc = alloc_indirect(_vq, total_sg, gfp);
> +		desc = alloc_indirect_split(_vq, total_sg, gfp);
>   	else {
>   		desc = NULL;
>   		WARN_ON_ONCE(total_sg > vq->vring.num &&
!vq->indirect);
> @@ -424,7 +495,7 @@ static inline int virtqueue_add(struct virtqueue *_vq,
>   	for (n = 0; n < total_sg; n++) {
>   		if (i == err_idx)
>   			break;
> -		vring_unmap_one(vq, &desc[i]);
> +		vring_unmap_one_split(vq, &desc[i]);
>   		i = virtio16_to_cpu(_vq->vdev, vq->vring.desc[i].next);
>   	}
>   
> @@ -435,6 +506,210 @@ static inline int virtqueue_add(struct virtqueue
*_vq,
>   	return -EIO;
>   }
>   
> +static inline int virtqueue_add_packed(struct virtqueue *_vq,
> +				       struct scatterlist *sgs[],
> +				       unsigned int total_sg,
> +				       unsigned int out_sgs,
> +				       unsigned int in_sgs,
> +				       void *data,
> +				       void *ctx,
> +				       gfp_t gfp)
> +{
> +	struct vring_virtqueue *vq = to_vvq(_vq);
> +	struct vring_packed_desc *desc;
> +	struct scatterlist *sg;
> +	unsigned int i, n, descs_used, uninitialized_var(prev), err_idx;
> +	__virtio16 uninitialized_var(head_flags), flags;
> +	int head, wrap_counter;
> +	bool indirect;
> +
> +	START_USE(vq);
> +
> +	BUG_ON(data == NULL);
> +	BUG_ON(ctx && vq->indirect);
> +
> +	if (unlikely(vq->broken)) {
> +		END_USE(vq);
> +		return -EIO;
> +	}
> +
> +#ifdef DEBUG
> +	{
> +		ktime_t now = ktime_get();
> +
> +		/* No kick or get, with .1 second between?  Warn. */
> +		if (vq->last_add_time_valid)
> +			WARN_ON(ktime_to_ms(ktime_sub(now, vq->last_add_time))
> +					    > 100);
> +		vq->last_add_time = now;
> +		vq->last_add_time_valid = true;
> +	}
> +#endif
> +
> +	BUG_ON(total_sg == 0);
> +
> +	head = vq->next_avail_idx;
> +	wrap_counter = vq->wrap_counter;
> +
> +	/* If the host supports indirect descriptor tables, and we have multiple
> +	 * buffers, then go indirect. FIXME: tune this threshold */
> +	if (vq->indirect && total_sg > 1 &&
vq->vq.num_free)
> +		desc = alloc_indirect_packed(_vq, total_sg, gfp);
> +	else {
> +		desc = NULL;
> +		WARN_ON_ONCE(total_sg > vq->vring_packed.num &&
!vq->indirect);
> +	}
> +
> +	if (desc) {
> +		/* Use a single buffer which doesn't continue */
> +		indirect = true;
> +		/* Set up rest to use this indirect table. */
> +		i = 0;
> +		descs_used = 1;
> +	} else {
> +		indirect = false;
> +		desc = vq->vring_packed.desc;
> +		i = head;
> +		descs_used = total_sg;
> +	}
> +
> +	if (vq->vq.num_free < descs_used) {
> +		pr_debug("Can't add buf len %i - avail = %i\n",
> +			 descs_used, vq->vq.num_free);
> +		/* FIXME: for historical reasons, we force a notify here if
> +		 * there are outgoing parts to the buffer.  Presumably the
> +		 * host should service the ring ASAP. */
> +		if (out_sgs)
> +			vq->notify(&vq->vq);
> +		if (indirect)
> +			kfree(desc);
> +		END_USE(vq);
> +		return -ENOSPC;
> +	}
> +
> +	for (n = 0; n < out_sgs + in_sgs; n++) {
> +		for (sg = sgs[n]; sg; sg = sg_next(sg)) {
> +			dma_addr_t addr = vring_map_one_sg(vq, sg, n < out_sgs ?
> +						DMA_TO_DEVICE : DMA_FROM_DEVICE);
> +			if (vring_mapping_error(vq, addr))
> +				goto unmap_release;
> +
> +			flags = cpu_to_virtio16(_vq->vdev, VRING_DESC_F_NEXT |
> +					(n < out_sgs ? 0 : VRING_DESC_F_WRITE) |
> +					VRING_DESC_F_AVAIL(vq->wrap_counter) |
> +					VRING_DESC_F_USED(!vq->wrap_counter));
> +			if (!indirect && i == head)
> +				head_flags = flags;
> +			else
> +				desc[i].flags = flags;
> +
> +			desc[i].addr = cpu_to_virtio64(_vq->vdev, addr);
> +			desc[i].len = cpu_to_virtio32(_vq->vdev, sg->length);
> +			desc[i].id = cpu_to_virtio32(_vq->vdev, head);
> +			prev = i;
> +			i++;
> +			if (!indirect && i >= vq->vring_packed.num) {
> +				i = 0;
> +				vq->wrap_counter ^= 1;
> +			}
> +		}
> +	}
> +	/* Last one doesn't continue. */
> +	if (total_sg == 1)
> +		head_flags &= cpu_to_virtio16(_vq->vdev, ~VRING_DESC_F_NEXT);
> +	else
> +		desc[prev].flags &= cpu_to_virtio16(_vq->vdev,
~VRING_DESC_F_NEXT);
> +
> +	if (indirect) {
> +		/* Now that the indirect table is filled in, map it. */
> +		dma_addr_t addr = vring_map_single(
> +			vq, desc, total_sg * sizeof(struct vring_packed_desc),
> +			DMA_TO_DEVICE);
> +		if (vring_mapping_error(vq, addr))
> +			goto unmap_release;
> +
> +		head_flags = cpu_to_virtio16(_vq->vdev, VRING_DESC_F_INDIRECT |
> +					     VRING_DESC_F_AVAIL(wrap_counter) |
> +					     VRING_DESC_F_USED(!wrap_counter));
> +		vq->vring_packed.desc[head].addr = cpu_to_virtio64(_vq->vdev,
addr);
> +		vq->vring_packed.desc[head].len = cpu_to_virtio32(_vq->vdev,
> +				total_sg * sizeof(struct vring_packed_desc));
> +		vq->vring_packed.desc[head].id = cpu_to_virtio32(_vq->vdev, head);
> +	}
> +
> +	/* We're using some buffers from the free list. */
> +	vq->vq.num_free -= descs_used;
> +
> +	/* Update free pointer */
> +	if (indirect) {
> +		n = head + 1;
> +		if (n >= vq->vring_packed.num) {
> +			n = 0;
> +			vq->wrap_counter ^= 1;
> +		}
> +		vq->next_avail_idx = n;
> +	} else
> +		vq->next_avail_idx = i;
> +
> +	/* Store token and indirect buffer state. */
> +	vq->desc_state[head].num = descs_used;
> +	vq->desc_state[head].data = data;
> +	if (indirect)
> +		vq->desc_state[head].indir_desc = desc;
> +	else
> +		vq->desc_state[head].indir_desc = ctx;
> +
> +	/* A driver MUST NOT make the first descriptor in the list
> +	 * available before all subsequent descriptors comprising
> +	 * the list are made available. */
> +	virtio_wmb(vq->weak_barriers);
> +	vq->vring_packed.desc[head].flags = head_flags;
> +	vq->num_added++;
> +
> +	pr_debug("Added buffer head %i to %p\n", head, vq);
> +	END_USE(vq);
> +
> +	return 0;
> +
> +unmap_release:
> +	err_idx = i;
> +	i = head;
> +
> +	for (n = 0; n < total_sg; n++) {
> +		if (i == err_idx)
> +			break;
> +		vring_unmap_one_packed(vq, &desc[i]);
> +		i++;
> +		if (!indirect && i >= vq->vring_packed.num)
> +			i = 0;
> +	}
> +
> +	vq->wrap_counter = wrap_counter;
> +
> +	if (indirect)
> +		kfree(desc);
> +
> +	END_USE(vq);
> +	return -EIO;
> +}
> +
> +static inline int virtqueue_add(struct virtqueue *_vq,
> +				struct scatterlist *sgs[],
> +				unsigned int total_sg,
> +				unsigned int out_sgs,
> +				unsigned int in_sgs,
> +				void *data,
> +				void *ctx,
> +				gfp_t gfp)
> +{
> +	struct vring_virtqueue *vq = to_vvq(_vq);
> +
> +	return vq->packed ? virtqueue_add_packed(_vq, sgs, total_sg, out_sgs,
> +						 in_sgs, data, ctx, gfp) :
> +			    virtqueue_add_split(_vq, sgs, total_sg, out_sgs,
> +						in_sgs, data, ctx, gfp);
> +}
> +
>   /**
>    * virtqueue_add_sgs - expose buffers to other end
>    * @vq: the struct virtqueue we're talking about.
> @@ -537,18 +812,7 @@ int virtqueue_add_inbuf_ctx(struct virtqueue *vq,
>   }
>   EXPORT_SYMBOL_GPL(virtqueue_add_inbuf_ctx);
>   
> -/**
> - * virtqueue_kick_prepare - first half of split virtqueue_kick call.
> - * @vq: the struct virtqueue
> - *
> - * Instead of virtqueue_kick(), you can do:
> - *	if (virtqueue_kick_prepare(vq))
> - *		virtqueue_notify(vq);
> - *
> - * This is sometimes useful because the virtqueue_kick_prepare() needs
> - * to be serialized, but the actual virtqueue_notify() call does not.
> - */
> -bool virtqueue_kick_prepare(struct virtqueue *_vq)
> +static bool virtqueue_kick_prepare_split(struct virtqueue *_vq)
>   {
>   	struct vring_virtqueue *vq = to_vvq(_vq);
>   	u16 new, old;
> @@ -580,6 +844,62 @@ bool virtqueue_kick_prepare(struct virtqueue *_vq)
>   	END_USE(vq);
>   	return needs_kick;
>   }
> +
> +static bool virtqueue_kick_prepare_packed(struct virtqueue *_vq)
> +{
> +	struct vring_virtqueue *vq = to_vvq(_vq);
> +	u16 new, old, off_wrap;
> +	bool needs_kick;
> +
> +	START_USE(vq);
> +	/* We need to expose the new flags value before checking notification
> +	 * suppressions. */
> +	virtio_mb(vq->weak_barriers);
> +
> +	old = vq->next_avail_idx - vq->num_added;
> +	new = vq->next_avail_idx;
> +	vq->num_added = 0;
> +
> +#ifdef DEBUG
> +	if (vq->last_add_time_valid) {
> +		WARN_ON(ktime_to_ms(ktime_sub(ktime_get(),
> +					      vq->last_add_time)) > 100);
> +	}
> +	vq->last_add_time_valid = false;
> +#endif
> +
> +	off_wrap = virtio16_to_cpu(_vq->vdev,
vq->vring_packed.device->off_wrap);
> +
> +	if (vq->event) {
> +		// FIXME: fix this!
> +		needs_kick = ((off_wrap >> 15) == vq->wrap_counter) &&
> +			     vring_need_event(off_wrap & ~(1<<15), new, old);
> +	} else {
> +		needs_kick = (vq->vring_packed.device->flags !> +			     
cpu_to_virtio16(_vq->vdev, VRING_EVENT_F_DISABLE));
> +	}
> +	END_USE(vq);
> +	return needs_kick;
> +}
> +
> +/**
> + * virtqueue_kick_prepare - first half of split virtqueue_kick call.
> + * @vq: the struct virtqueue
> + *
> + * Instead of virtqueue_kick(), you can do:
> + *	if (virtqueue_kick_prepare(vq))
> + *		virtqueue_notify(vq);
> + *
> + * This is sometimes useful because the virtqueue_kick_prepare() needs
> + * to be serialized, but the actual virtqueue_notify() call does not.
> + */
> +bool virtqueue_kick_prepare(struct virtqueue *_vq)
> +{
> +	struct vring_virtqueue *vq = to_vvq(_vq);
> +
> +	return vq->packed ? virtqueue_kick_prepare_packed(_vq) :
> +			    virtqueue_kick_prepare_split(_vq);
> +}
>   EXPORT_SYMBOL_GPL(virtqueue_kick_prepare);
>   
>   /**
> @@ -626,8 +946,8 @@ bool virtqueue_kick(struct virtqueue *vq)
>   }
>   EXPORT_SYMBOL_GPL(virtqueue_kick);
>   
> -static void detach_buf(struct vring_virtqueue *vq, unsigned int head,
> -		       void **ctx)
> +static void detach_buf_split(struct vring_virtqueue *vq, unsigned int
head,
> +			     void **ctx)
>   {
>   	unsigned int i, j;
>   	__virtio16 nextflag = cpu_to_virtio16(vq->vq.vdev,
VRING_DESC_F_NEXT);
> @@ -639,12 +959,12 @@ static void detach_buf(struct vring_virtqueue *vq,
unsigned int head,
>   	i = head;
>   
>   	while (vq->vring.desc[i].flags & nextflag) {
> -		vring_unmap_one(vq, &vq->vring.desc[i]);
> +		vring_unmap_one_split(vq, &vq->vring.desc[i]);
>   		i = virtio16_to_cpu(vq->vq.vdev, vq->vring.desc[i].next);
>   		vq->vq.num_free++;
>   	}
>   
> -	vring_unmap_one(vq, &vq->vring.desc[i]);
> +	vring_unmap_one_split(vq, &vq->vring.desc[i]);
>   	vq->vring.desc[i].next = cpu_to_virtio16(vq->vq.vdev,
vq->free_head);
>   	vq->free_head = head;
>   
> @@ -666,7 +986,7 @@ static void detach_buf(struct vring_virtqueue *vq,
unsigned int head,
>   		BUG_ON(len == 0 || len % sizeof(struct vring_desc));
>   
>   		for (j = 0; j < len / sizeof(struct vring_desc); j++)
> -			vring_unmap_one(vq, &indir_desc[j]);
> +			vring_unmap_one_split(vq, &indir_desc[j]);
>   
>   		kfree(indir_desc);
>   		vq->desc_state[head].indir_desc = NULL;
> @@ -675,11 +995,207 @@ static void detach_buf(struct vring_virtqueue *vq,
unsigned int head,
>   	}
>   }
>   
> -static inline bool more_used(const struct vring_virtqueue *vq)
> +static int detach_buf_packed(struct vring_virtqueue *vq, unsigned int
head,
> +			      void **ctx)
> +{
> +	struct vring_packed_desc *desc;
> +	unsigned int i, j;
> +
> +	/* Clear data ptr. */
> +	vq->desc_state[head].data = NULL;
> +
> +	i = head;
> +
> +	for (j = 0; j < vq->desc_state[head].num; j++) {
> +		desc = &vq->vring_packed.desc[i];
> +		vring_unmap_one_packed(vq, desc);
> +		desc->flags = 0x0;
> +		i++;
> +		if (i >= vq->vring_packed.num)
> +			i = 0;
> +	}
> +
> +	vq->vq.num_free += vq->desc_state[head].num;
> +
> +	if (vq->indirect) {
> +		u32 len;
> +
> +		desc = vq->desc_state[head].indir_desc;
> +		/* Free the indirect table, if any, now that it's unmapped. */
> +		if (!desc)
> +			goto out;
> +
> +		len = virtio32_to_cpu(vq->vq.vdev,
> +				      vq->vring_packed.desc[head].len);
> +
> +		BUG_ON(!(vq->vring_packed.desc[head].flags &
> +			 cpu_to_virtio16(vq->vq.vdev, VRING_DESC_F_INDIRECT)));
> +		BUG_ON(len == 0 || len % sizeof(struct vring_packed_desc));
> +
> +		for (j = 0; j < len / sizeof(struct vring_packed_desc); j++)
> +			vring_unmap_one_packed(vq, &desc[j]);
> +
> +		kfree(desc);
> +		vq->desc_state[head].indir_desc = NULL;
> +	} else if (ctx) {
> +		*ctx = vq->desc_state[head].indir_desc;
> +	}
> +
> +out:
> +	return vq->desc_state[head].num;
> +}
> +
> +static inline bool more_used_split(const struct vring_virtqueue *vq)
>   {
>   	return vq->last_used_idx != virtio16_to_cpu(vq->vq.vdev,
vq->vring.used->idx);
>   }
>   
> +static inline bool more_used_packed(const struct vring_virtqueue *vq)
> +{
> +	u16 last_used, flags;
> +	bool avail, used;
> +
> +	if (vq->vq.num_free == vq->vring_packed.num)
> +		return false;
> +
> +	last_used = vq->last_used_idx;
> +	flags = virtio16_to_cpu(vq->vq.vdev,
> +				vq->vring_packed.desc[last_used].flags);
> +	avail = flags & VRING_DESC_F_AVAIL(1);
> +	used = flags & VRING_DESC_F_USED(1);
> +
> +	return avail == used;
> +}
> +
> +static inline bool more_used(const struct vring_virtqueue *vq)
> +{
> +	return vq->packed ? more_used_packed(vq) : more_used_split(vq);
> +}
> +
> +void *virtqueue_get_buf_ctx_split(struct virtqueue *_vq, unsigned int
*len,
> +				  void **ctx)
> +{
> +	struct vring_virtqueue *vq = to_vvq(_vq);
> +	void *ret;
> +	unsigned int i;
> +	u16 last_used;
> +
> +	START_USE(vq);
> +
> +	if (unlikely(vq->broken)) {
> +		END_USE(vq);
> +		return NULL;
> +	}
> +
> +	if (!more_used(vq)) {
> +		pr_debug("No more buffers in queue\n");
> +		END_USE(vq);
> +		return NULL;
> +	}
> +
> +	/* Only get used array entries after they have been exposed by host. */
> +	virtio_rmb(vq->weak_barriers);
> +
> +	last_used = (vq->last_used_idx & (vq->vring.num - 1));
> +	i = virtio32_to_cpu(_vq->vdev,
vq->vring.used->ring[last_used].id);
> +	*len = virtio32_to_cpu(_vq->vdev,
vq->vring.used->ring[last_used].len);
> +
> +	if (unlikely(i >= vq->vring.num)) {
> +		BAD_RING(vq, "id %u out of range\n", i);
> +		return NULL;
> +	}
> +	if (unlikely(!vq->desc_state[i].data)) {
> +		BAD_RING(vq, "id %u is not a head!\n", i);
> +		return NULL;
> +	}
> +
> +	/* detach_buf_split clears data, so grab it now. */
> +	ret = vq->desc_state[i].data;
> +	detach_buf_split(vq, i, ctx);
> +	vq->last_used_idx++;
> +	/* If we expect an interrupt for the next entry, tell host
> +	 * by writing event index and flush out the write before
> +	 * the read in the next get_buf call. */
> +	if (!(vq->avail_flags_shadow & VRING_AVAIL_F_NO_INTERRUPT))
> +		virtio_store_mb(vq->weak_barriers,
> +				&vring_used_event(&vq->vring),
> +				cpu_to_virtio16(_vq->vdev, vq->last_used_idx));
> +
> +#ifdef DEBUG
> +	vq->last_add_time_valid = false;
> +#endif
> +
> +	END_USE(vq);
> +	return ret;
> +}
> +
> +void *virtqueue_get_buf_ctx_packed(struct virtqueue *_vq, unsigned int
*len,
> +				   void **ctx)
> +{
> +	struct vring_virtqueue *vq = to_vvq(_vq);
> +	uint16_t wrap_counter;
> +	void *ret;
> +	unsigned int i;
> +	u16 last_used;
> +
> +	START_USE(vq);
> +
> +	if (unlikely(vq->broken)) {
> +		END_USE(vq);
> +		return NULL;
> +	}
> +
> +	if (!more_used(vq)) {
> +		pr_debug("No more buffers in queue\n");
> +		END_USE(vq);
> +		return NULL;
> +	}
> +
> +	/* Only get used elements after they have been exposed by host. */
> +	virtio_rmb(vq->weak_barriers);
> +
> +	last_used = vq->last_used_idx;
> +	i = virtio32_to_cpu(_vq->vdev,
vq->vring_packed.desc[last_used].id);
> +	*len = virtio32_to_cpu(_vq->vdev,
vq->vring_packed.desc[last_used].len);
> +
> +	if (unlikely(i >= vq->vring_packed.num)) {
> +		BAD_RING(vq, "id %u out of range\n", i);
> +		return NULL;
> +	}
> +	if (unlikely(!vq->desc_state[i].data)) {
> +		BAD_RING(vq, "id %u is not a head!\n", i);
> +		return NULL;
> +	}
> +
> +	/* detach_buf_packed clears data, so grab it now. */
> +	ret = vq->desc_state[i].data;
> +	detach_buf_packed(vq, i, ctx);
> +
> +	vq->last_used_idx += vq->desc_state[i].num;
> +	if (vq->last_used_idx >= vq->vring_packed.num)
> +		vq->last_used_idx -= vq->vring_packed.num;
> +
> +	wrap_counter = vq->wrap_counter;
> +	if (vq->last_used_idx > vq->next_avail_idx)
> +		wrap_counter ^= 1;
> +
> +	/* If we expect an interrupt for the next entry, tell host
> +	 * by writing event index and flush out the write before
> +	 * the read in the next get_buf call. */
> +	if (vq->event_flags_shadow == VRING_EVENT_F_DESC)
> +		virtio_store_mb(vq->weak_barriers,
> +				&vq->vring_packed.driver->off_wrap,
> +				cpu_to_virtio16(_vq->vdev, vq->last_used_idx |
> +						wrap_counter << 15));
> +
> +#ifdef DEBUG
> +	vq->last_add_time_valid = false;
> +#endif
> +
> +	END_USE(vq);
> +	return ret;
> +}
> +
>   /**
>    * virtqueue_get_buf - get the next used buffer
>    * @vq: the struct virtqueue we're talking about.
> @@ -700,57 +1216,9 @@ void *virtqueue_get_buf_ctx(struct virtqueue *_vq,
unsigned int *len,
>   			    void **ctx)
>   {
>   	struct vring_virtqueue *vq = to_vvq(_vq);
> -	void *ret;
> -	unsigned int i;
> -	u16 last_used;
>   
> -	START_USE(vq);
> -
> -	if (unlikely(vq->broken)) {
> -		END_USE(vq);
> -		return NULL;
> -	}
> -
> -	if (!more_used(vq)) {
> -		pr_debug("No more buffers in queue\n");
> -		END_USE(vq);
> -		return NULL;
> -	}
> -
> -	/* Only get used array entries after they have been exposed by host. */
> -	virtio_rmb(vq->weak_barriers);
> -
> -	last_used = (vq->last_used_idx & (vq->vring.num - 1));
> -	i = virtio32_to_cpu(_vq->vdev,
vq->vring.used->ring[last_used].id);
> -	*len = virtio32_to_cpu(_vq->vdev,
vq->vring.used->ring[last_used].len);
> -
> -	if (unlikely(i >= vq->vring.num)) {
> -		BAD_RING(vq, "id %u out of range\n", i);
> -		return NULL;
> -	}
> -	if (unlikely(!vq->desc_state[i].data)) {
> -		BAD_RING(vq, "id %u is not a head!\n", i);
> -		return NULL;
> -	}
> -
> -	/* detach_buf clears data, so grab it now. */
> -	ret = vq->desc_state[i].data;
> -	detach_buf(vq, i, ctx);
> -	vq->last_used_idx++;
> -	/* If we expect an interrupt for the next entry, tell host
> -	 * by writing event index and flush out the write before
> -	 * the read in the next get_buf call. */
> -	if (!(vq->avail_flags_shadow & VRING_AVAIL_F_NO_INTERRUPT))
> -		virtio_store_mb(vq->weak_barriers,
> -				&vring_used_event(&vq->vring),
> -				cpu_to_virtio16(_vq->vdev, vq->last_used_idx));
> -
> -#ifdef DEBUG
> -	vq->last_add_time_valid = false;
> -#endif
> -
> -	END_USE(vq);
> -	return ret;
> +	return vq->packed ? virtqueue_get_buf_ctx_packed(_vq, len, ctx) :
> +			    virtqueue_get_buf_ctx_split(_vq, len, ctx);
>   }
>   EXPORT_SYMBOL_GPL(virtqueue_get_buf_ctx);
>   
> @@ -759,6 +1227,29 @@ void *virtqueue_get_buf(struct virtqueue *_vq,
unsigned int *len)
>   	return virtqueue_get_buf_ctx(_vq, len, NULL);
>   }
>   EXPORT_SYMBOL_GPL(virtqueue_get_buf);
> +
> +static void virtqueue_disable_cb_split(struct virtqueue *_vq)
> +{
> +	struct vring_virtqueue *vq = to_vvq(_vq);
> +
> +	if (!(vq->avail_flags_shadow & VRING_AVAIL_F_NO_INTERRUPT)) {
> +		vq->avail_flags_shadow |= VRING_AVAIL_F_NO_INTERRUPT;
> +		if (!vq->event)
> +			vq->vring.avail->flags = cpu_to_virtio16(_vq->vdev,
vq->avail_flags_shadow);
> +	}
> +}
> +
> +static void virtqueue_disable_cb_packed(struct virtqueue *_vq)
> +{
> +	struct vring_virtqueue *vq = to_vvq(_vq);
> +
> +	if (vq->event_flags_shadow != VRING_EVENT_F_DISABLE) {
> +		vq->event_flags_shadow = VRING_EVENT_F_DISABLE;
> +		vq->vring_packed.driver->flags = cpu_to_virtio16(_vq->vdev,
> +							vq->event_flags_shadow);
> +	}
> +}
> +
>   /**
>    * virtqueue_disable_cb - disable callbacks
>    * @vq: the struct virtqueue we're talking about.
> @@ -772,15 +1263,66 @@ void virtqueue_disable_cb(struct virtqueue *_vq)
>   {
>   	struct vring_virtqueue *vq = to_vvq(_vq);
>   
> -	if (!(vq->avail_flags_shadow & VRING_AVAIL_F_NO_INTERRUPT)) {
> -		vq->avail_flags_shadow |= VRING_AVAIL_F_NO_INTERRUPT;
> -		if (!vq->event)
> -			vq->vring.avail->flags = cpu_to_virtio16(_vq->vdev,
vq->avail_flags_shadow);
> -	}
> -
> +	if (vq->packed)
> +		virtqueue_disable_cb_packed(_vq);
> +	else
> +		virtqueue_disable_cb_split(_vq);
>   }
>   EXPORT_SYMBOL_GPL(virtqueue_disable_cb);
>   
> +static unsigned virtqueue_enable_cb_prepare_split(struct virtqueue *_vq)
> +{
> +	struct vring_virtqueue *vq = to_vvq(_vq);
> +	u16 last_used_idx;
> +
> +	START_USE(vq);
> +
> +	/* We optimistically turn back on interrupts, then check if there was
> +	 * more to do. */
> +	/* Depending on the VIRTIO_RING_F_EVENT_IDX feature, we need to
> +	 * either clear the flags bit or point the event index at the next
> +	 * entry. Always do both to keep code simple. */
> +	if (vq->avail_flags_shadow & VRING_AVAIL_F_NO_INTERRUPT) {
> +		vq->avail_flags_shadow &= ~VRING_AVAIL_F_NO_INTERRUPT;
> +		if (!vq->event)
> +			vq->vring.avail->flags = cpu_to_virtio16(_vq->vdev,
vq->avail_flags_shadow);
> +	}
> +	vring_used_event(&vq->vring) = cpu_to_virtio16(_vq->vdev,
last_used_idx = vq->last_used_idx);
> +	END_USE(vq);
> +	return last_used_idx;
> +}
> +
> +static unsigned virtqueue_enable_cb_prepare_packed(struct virtqueue *_vq)
> +{
> +	struct vring_virtqueue *vq = to_vvq(_vq);
> +	u16 last_used_idx, wrap_counter, off_wrap;
> +
> +	START_USE(vq);
> +
> +	last_used_idx = vq->last_used_idx;
> +	wrap_counter = vq->wrap_counter;
> +
> +	if (last_used_idx > vq->next_avail_idx)
> +		wrap_counter ^= 1;
> +
> +	off_wrap = last_used_idx | (wrap_counter << 15);
> +
> +	/* We optimistically turn back on interrupts, then check if there was
> +	 * more to do. */
> +	/* Depending on the VIRTIO_RING_F_EVENT_IDX feature, we need to
> +	 * either clear the flags bit or point the event index at the next
> +	 * entry. Always do both to keep code simple. */
> +	if (vq->event_flags_shadow == VRING_EVENT_F_DISABLE) {
> +		vq->event_flags_shadow = vq->event ? VRING_EVENT_F_DESC:
> +						     VRING_EVENT_F_ENABLE;
> +		vq->vring_packed.driver->flags = cpu_to_virtio16(_vq->vdev,
> +							vq->event_flags_shadow);
> +	}
> +	vq->vring_packed.driver->off_wrap = cpu_to_virtio16(_vq->vdev,
off_wrap);
> +	END_USE(vq);
> +	return last_used_idx;
> +}
> +
>   /**
>    * virtqueue_enable_cb_prepare - restart callbacks after disable_cb
>    * @vq: the struct virtqueue we're talking about.
> @@ -796,26 +1338,34 @@ EXPORT_SYMBOL_GPL(virtqueue_disable_cb);
>   unsigned virtqueue_enable_cb_prepare(struct virtqueue *_vq)
>   {
>   	struct vring_virtqueue *vq = to_vvq(_vq);
> -	u16 last_used_idx;
>   
> -	START_USE(vq);
> -
> -	/* We optimistically turn back on interrupts, then check if there was
> -	 * more to do. */
> -	/* Depending on the VIRTIO_RING_F_EVENT_IDX feature, we need to
> -	 * either clear the flags bit or point the event index at the next
> -	 * entry. Always do both to keep code simple. */
> -	if (vq->avail_flags_shadow & VRING_AVAIL_F_NO_INTERRUPT) {
> -		vq->avail_flags_shadow &= ~VRING_AVAIL_F_NO_INTERRUPT;
> -		if (!vq->event)
> -			vq->vring.avail->flags = cpu_to_virtio16(_vq->vdev,
vq->avail_flags_shadow);
> -	}
> -	vring_used_event(&vq->vring) = cpu_to_virtio16(_vq->vdev,
last_used_idx = vq->last_used_idx);
> -	END_USE(vq);
> -	return last_used_idx;
> +	return vq->packed ? virtqueue_enable_cb_prepare_packed(_vq) :
> +			    virtqueue_enable_cb_prepare_split(_vq);
>   }
>   EXPORT_SYMBOL_GPL(virtqueue_enable_cb_prepare);
>   
> +static bool virtqueue_poll_split(struct virtqueue *_vq, unsigned
last_used_idx)
> +{
> +	struct vring_virtqueue *vq = to_vvq(_vq);
> +
> +	virtio_mb(vq->weak_barriers);
> +	return (u16)last_used_idx != virtio16_to_cpu(_vq->vdev,
vq->vring.used->idx);
> +}
> +
> +static bool virtqueue_poll_packed(struct virtqueue *_vq, unsigned
last_used_idx)
> +{
> +	struct vring_virtqueue *vq = to_vvq(_vq);
> +	bool avail, used;
> +	u16 flags;
> +
> +	virtio_mb(vq->weak_barriers);
> +	flags = virtio16_to_cpu(vq->vq.vdev,
> +			vq->vring_packed.desc[last_used_idx].flags);
> +	avail = flags & VRING_DESC_F_AVAIL(1);
> +	used = flags & VRING_DESC_F_USED(1);
> +	return avail == used;
> +}
> +
>   /**
>    * virtqueue_poll - query pending used buffers
>    * @vq: the struct virtqueue we're talking about.
> @@ -829,8 +1379,8 @@ bool virtqueue_poll(struct virtqueue *_vq, unsigned
last_used_idx)
>   {
>   	struct vring_virtqueue *vq = to_vvq(_vq);
>   
> -	virtio_mb(vq->weak_barriers);
> -	return (u16)last_used_idx != virtio16_to_cpu(_vq->vdev,
vq->vring.used->idx);
> +	return vq->packed ? virtqueue_poll_packed(_vq, last_used_idx) :
> +			    virtqueue_poll_split(_vq, last_used_idx);
>   }
>   EXPORT_SYMBOL_GPL(virtqueue_poll);
>   
> @@ -852,6 +1402,83 @@ bool virtqueue_enable_cb(struct virtqueue *_vq)
>   }
>   EXPORT_SYMBOL_GPL(virtqueue_enable_cb);
>   
> +static bool virtqueue_enable_cb_delayed_split(struct virtqueue *_vq)
> +{
> +	struct vring_virtqueue *vq = to_vvq(_vq);
> +	u16 bufs;
> +
> +	START_USE(vq);
> +
> +	/* We optimistically turn back on interrupts, then check if there was
> +	 * more to do. */
> +	/* Depending on the VIRTIO_RING_F_USED_EVENT_IDX feature, we need to
> +	 * either clear the flags bit or point the event index at the next
> +	 * entry. Always update the event index to keep code simple. */
> +	if (vq->avail_flags_shadow & VRING_AVAIL_F_NO_INTERRUPT) {
> +		vq->avail_flags_shadow &= ~VRING_AVAIL_F_NO_INTERRUPT;
> +		if (!vq->event)
> +			vq->vring.avail->flags = cpu_to_virtio16(_vq->vdev,
vq->avail_flags_shadow);
> +	}
> +	/* TODO: tune this threshold */
> +	bufs = (u16)(vq->avail_idx_shadow - vq->last_used_idx) * 3 / 4;
> +
> +	virtio_store_mb(vq->weak_barriers,
> +			&vring_used_event(&vq->vring),
> +			cpu_to_virtio16(_vq->vdev, vq->last_used_idx + bufs));
> +
> +	if (unlikely((u16)(virtio16_to_cpu(_vq->vdev,
vq->vring.used->idx) - vq->last_used_idx) > bufs)) {
> +		END_USE(vq);
> +		return false;
> +	}
> +
> +	END_USE(vq);
> +	return true;
> +}
> +
> +static bool virtqueue_enable_cb_delayed_packed(struct virtqueue *_vq)
> +{
> +	struct vring_virtqueue *vq = to_vvq(_vq);
> +	u16 bufs, off_wrap, used_idx, wrap_counter;
> +
> +	START_USE(vq);
> +
> +	/* We optimistically turn back on interrupts, then check if there was
> +	 * more to do. */
> +	/* Depending on the VIRTIO_RING_F_USED_EVENT_IDX feature, we need to
> +	 * either clear the flags bit or point the event index at the next
> +	 * entry. Always update the event index to keep code simple. */
> +	if (vq->event_flags_shadow == VRING_EVENT_F_DISABLE) {
> +		vq->event_flags_shadow = vq->event ? VRING_EVENT_F_DESC:
> +						     VRING_EVENT_F_ENABLE;
> +		vq->vring_packed.driver->flags = cpu_to_virtio16(_vq->vdev,
> +							vq->event_flags_shadow);
> +	}
> +
> +	/* TODO: tune this threshold */
> +	bufs = (u16)(vq->next_avail_idx - vq->last_used_idx) * 3 / 4;
> +
> +	used_idx = vq->last_used_idx + bufs;
> +	if (used_idx >= vq->vring_packed.num)
> +		used_idx -= vq->vring_packed.num;
> +
> +	wrap_counter = vq->wrap_counter;
> +	if (used_idx > vq->next_avail_idx)
> +		wrap_counter ^= 1;
> +
> +	off_wrap = used_idx | (wrap_counter << 15);
> +
> +	virtio_store_mb(vq->weak_barriers,
&vq->vring_packed.driver->off_wrap,
> +			cpu_to_virtio16(_vq->vdev, off_wrap));
> +
> +	if (more_used_packed(vq)) {
> +		END_USE(vq);
> +		return false;
> +	}
> +
> +	END_USE(vq);
> +	return true;
> +}
> +
>   /**
>    * virtqueue_enable_cb_delayed - restart callbacks after disable_cb.
>    * @vq: the struct virtqueue we're talking about.
> @@ -868,37 +1495,69 @@ EXPORT_SYMBOL_GPL(virtqueue_enable_cb);
>   bool virtqueue_enable_cb_delayed(struct virtqueue *_vq)
>   {
>   	struct vring_virtqueue *vq = to_vvq(_vq);
> -	u16 bufs;
>   
> -	START_USE(vq);
> -
> -	/* We optimistically turn back on interrupts, then check if there was
> -	 * more to do. */
> -	/* Depending on the VIRTIO_RING_F_USED_EVENT_IDX feature, we need to
> -	 * either clear the flags bit or point the event index at the next
> -	 * entry. Always update the event index to keep code simple. */
> -	if (vq->avail_flags_shadow & VRING_AVAIL_F_NO_INTERRUPT) {
> -		vq->avail_flags_shadow &= ~VRING_AVAIL_F_NO_INTERRUPT;
> -		if (!vq->event)
> -			vq->vring.avail->flags = cpu_to_virtio16(_vq->vdev,
vq->avail_flags_shadow);
> -	}
> -	/* TODO: tune this threshold */
> -	bufs = (u16)(vq->avail_idx_shadow - vq->last_used_idx) * 3 / 4;
> -
> -	virtio_store_mb(vq->weak_barriers,
> -			&vring_used_event(&vq->vring),
> -			cpu_to_virtio16(_vq->vdev, vq->last_used_idx + bufs));
> -
> -	if (unlikely((u16)(virtio16_to_cpu(_vq->vdev,
vq->vring.used->idx) - vq->last_used_idx) > bufs)) {
> -		END_USE(vq);
> -		return false;
> -	}
> -
> -	END_USE(vq);
> -	return true;
> +	return vq->packed ? virtqueue_enable_cb_delayed_packed(_vq) :
> +			    virtqueue_enable_cb_delayed_split(_vq);
>   }
>   EXPORT_SYMBOL_GPL(virtqueue_enable_cb_delayed);
>   
> +static void *virtqueue_detach_unused_buf_split(struct virtqueue *_vq)
> +{
> +	struct vring_virtqueue *vq = to_vvq(_vq);
> +	unsigned int i;
> +	void *buf;
> +
> +	START_USE(vq);
> +
> +	for (i = 0; i < vq->vring.num; i++) {
> +		if (!vq->desc_state[i].data)
> +			continue;
> +		/* detach_buf clears data, so grab it now. */
> +		buf = vq->desc_state[i].data;
> +		detach_buf_split(vq, i, NULL);
> +		vq->avail_idx_shadow--;
> +		vq->vring.avail->idx = cpu_to_virtio16(_vq->vdev,
vq->avail_idx_shadow);
> +		END_USE(vq);
> +		return buf;
> +	}
> +	/* That should have freed everything. */
> +	BUG_ON(vq->vq.num_free != vq->vring.num);
> +
> +	END_USE(vq);
> +	return NULL;
> +}
> +
> +static void *virtqueue_detach_unused_buf_packed(struct virtqueue *_vq)
> +{
> +	struct vring_virtqueue *vq = to_vvq(_vq);
> +	unsigned int i, num;
> +	void *buf;
> +
> +	START_USE(vq);
> +
> +	for (i = 0; i < vq->vring_packed.num; i++) {
> +		if (!vq->desc_state[i].data)
> +			continue;
> +		/* detach_buf clears data, so grab it now. */
> +		buf = vq->desc_state[i].data;
> +		num = detach_buf_packed(vq, i, NULL);
> +		if (vq->next_avail_idx < num) {
> +			vq->next_avail_idx = vq->vring_packed.num -
> +					(num - vq->next_avail_idx);
> +			vq->wrap_counter ^= 1;
> +		} else {
> +			vq->next_avail_idx -= num;
> +		}
> +		END_USE(vq);
> +		return buf;
> +	}
> +	/* That should have freed everything. */
> +	BUG_ON(vq->vq.num_free != vq->vring_packed.num);
> +
> +	END_USE(vq);
> +	return NULL;
> +}
> +
>   /**
>    * virtqueue_detach_unused_buf - detach first unused buffer
>    * @vq: the struct virtqueue we're talking about.
> @@ -910,27 +1569,9 @@ EXPORT_SYMBOL_GPL(virtqueue_enable_cb_delayed);
>   void *virtqueue_detach_unused_buf(struct virtqueue *_vq)
>   {
>   	struct vring_virtqueue *vq = to_vvq(_vq);
> -	unsigned int i;
> -	void *buf;
>   
> -	START_USE(vq);
> -
> -	for (i = 0; i < vq->vring.num; i++) {
> -		if (!vq->desc_state[i].data)
> -			continue;
> -		/* detach_buf clears data, so grab it now. */
> -		buf = vq->desc_state[i].data;
> -		detach_buf(vq, i, NULL);
> -		vq->avail_idx_shadow--;
> -		vq->vring.avail->idx = cpu_to_virtio16(_vq->vdev,
vq->avail_idx_shadow);
> -		END_USE(vq);
> -		return buf;
> -	}
> -	/* That should have freed everything. */
> -	BUG_ON(vq->vq.num_free != vq->vring.num);
> -
> -	END_USE(vq);
> -	return NULL;
> +	return vq->packed ? virtqueue_detach_unused_buf_packed(_vq) :
> +			    virtqueue_detach_unused_buf_split(_vq);
>   }
>   EXPORT_SYMBOL_GPL(virtqueue_detach_unused_buf);
>   
> @@ -955,7 +1596,8 @@ irqreturn_t vring_interrupt(int irq, void *_vq)
>   EXPORT_SYMBOL_GPL(vring_interrupt);
>   
>   struct virtqueue *__vring_new_virtqueue(unsigned int index,
> -					struct vring vring,
> +					union vring_union vring,
> +					bool packed,
>   					struct virtio_device *vdev,
>   					bool weak_barriers,
>   					bool context,
> @@ -963,19 +1605,20 @@ struct virtqueue *__vring_new_virtqueue(unsigned int
index,
>   					void (*callback)(struct virtqueue *),
>   					const char *name)
>   {
> -	unsigned int i;
> +	unsigned int num, i;
>   	struct vring_virtqueue *vq;
>   
> -	vq = kmalloc(sizeof(*vq) + vring.num * sizeof(struct vring_desc_state),
> +	num = packed ? vring.vring_packed.num : vring.vring_split.num;
> +
> +	vq = kmalloc(sizeof(*vq) + num * sizeof(struct vring_desc_state),
>   		     GFP_KERNEL);
>   	if (!vq)
>   		return NULL;
>   
> -	vq->vring = vring;
>   	vq->vq.callback = callback;
>   	vq->vq.vdev = vdev;
>   	vq->vq.name = name;
> -	vq->vq.num_free = vring.num;
> +	vq->vq.num_free = num;
>   	vq->vq.index = index;
>   	vq->we_own_ring = false;
>   	vq->queue_dma_addr = 0;
> @@ -984,9 +1627,8 @@ struct virtqueue *__vring_new_virtqueue(unsigned int
index,
>   	vq->weak_barriers = weak_barriers;
>   	vq->broken = false;
>   	vq->last_used_idx = 0;
> -	vq->avail_flags_shadow = 0;
> -	vq->avail_idx_shadow = 0;
>   	vq->num_added = 0;
> +	vq->packed = packed;
>   	list_add_tail(&vq->vq.list, &vdev->vqs);
>   #ifdef DEBUG
>   	vq->in_use = false;
> @@ -997,18 +1639,37 @@ struct virtqueue *__vring_new_virtqueue(unsigned int
index,
>   		!context;
>   	vq->event = virtio_has_feature(vdev, VIRTIO_RING_F_EVENT_IDX);
>   
> +	if (vq->packed) {
> +		vq->vring_packed = vring.vring_packed;
> +		vq->next_avail_idx = 0;
> +		vq->wrap_counter = 1;
> +		vq->event_flags_shadow = 0;
> +	} else {
> +		vq->vring = vring.vring_split;
> +		vq->avail_flags_shadow = 0;
> +		vq->avail_idx_shadow = 0;
> +
> +		/* Put everything in free lists. */
> +		vq->free_head = 0;
> +		for (i = 0; i < num-1; i++)
> +			vq->vring.desc[i].next = cpu_to_virtio16(vdev, i + 1);
> +	}
> +
>   	/* No callback?  Tell other side not to bother us. */
>   	if (!callback) {
> -		vq->avail_flags_shadow |= VRING_AVAIL_F_NO_INTERRUPT;
> -		if (!vq->event)
> -			vq->vring.avail->flags = cpu_to_virtio16(vdev,
vq->avail_flags_shadow);
> +		if (packed) {
> +			vq->event_flags_shadow = VRING_EVENT_F_DISABLE;
> +			vq->vring_packed.driver->flags = cpu_to_virtio16(vdev,
> +						vq->event_flags_shadow);
> +		} else {
> +			vq->avail_flags_shadow |= VRING_AVAIL_F_NO_INTERRUPT;
> +			if (!vq->event)
> +				vq->vring.avail->flags = cpu_to_virtio16(vdev,
> +						vq->avail_flags_shadow);
> +		}
>   	}
>   
> -	/* Put everything in free lists. */
> -	vq->free_head = 0;
> -	for (i = 0; i < vring.num-1; i++)
> -		vq->vring.desc[i].next = cpu_to_virtio16(vdev, i + 1);
> -	memset(vq->desc_state, 0, vring.num * sizeof(struct
vring_desc_state));
> +	memset(vq->desc_state, 0, num * sizeof(struct vring_desc_state));
>   
>   	return &vq->vq;
>   }
> @@ -1056,6 +1717,22 @@ static void vring_free_queue(struct virtio_device
*vdev, size_t size,
>   	}
>   }
>   
> +static inline int
> +__vring_size(unsigned int num, unsigned long align, bool packed)
> +{
> +	return packed ? vring_packed_size(num, align) : vring_size(num, align);
> +}
> +
> +static inline void vring_packed_init(struct vring_packed *vr, unsigned int
num,
> +				     void *p, unsigned long align)
> +{
> +	vr->num = num;
> +	vr->desc = p;
> +	vr->driver = (void *)(((uintptr_t)p + sizeof(struct vring_packed_desc)
> +		* num + align - 1) & ~(align - 1));
> +	vr->device = vr->driver + 1;
> +}
> +
>   struct virtqueue *vring_create_virtqueue(
>   	unsigned int index,
>   	unsigned int num,
> @@ -1072,7 +1749,8 @@ struct virtqueue *vring_create_virtqueue(
>   	void *queue = NULL;
>   	dma_addr_t dma_addr;
>   	size_t queue_size_in_bytes;
> -	struct vring vring;
> +	union vring_union vring;
> +	bool packed;
>   
>   	/* We assume num is a power of 2. */
>   	if (num & (num - 1)) {
> @@ -1080,9 +1758,13 @@ struct virtqueue *vring_create_virtqueue(
>   		return NULL;
>   	}
>   
> +	packed = virtio_has_feature(vdev, VIRTIO_F_RING_PACKED);
> +
>   	/* TODO: allocate each queue chunk individually */
> -	for (; num && vring_size(num, vring_align) > PAGE_SIZE; num /=
2) {
> -		queue = vring_alloc_queue(vdev, vring_size(num, vring_align),
> +	for (; num && __vring_size(num, vring_align, packed) >
PAGE_SIZE;
> +			num /= 2) {
> +		queue = vring_alloc_queue(vdev, __vring_size(num, vring_align,
> +							     packed),
>   					  &dma_addr,
>   					  GFP_KERNEL|__GFP_NOWARN|__GFP_ZERO);
>   		if (queue)
> @@ -1094,17 +1776,21 @@ struct virtqueue *vring_create_virtqueue(
>   
>   	if (!queue) {
>   		/* Try to get a single page. You are my only hope! */
> -		queue = vring_alloc_queue(vdev, vring_size(num, vring_align),
> +		queue = vring_alloc_queue(vdev, __vring_size(num, vring_align,
> +							     packed),
>   					  &dma_addr, GFP_KERNEL|__GFP_ZERO);
>   	}
>   	if (!queue)
>   		return NULL;
>   
> -	queue_size_in_bytes = vring_size(num, vring_align);
> -	vring_init(&vring, num, queue, vring_align);
> +	queue_size_in_bytes = __vring_size(num, vring_align, packed);
> +	if (packed)
> +		vring_packed_init(&vring.vring_packed, num, queue, vring_align);
> +	else
> +		vring_init(&vring.vring_split, num, queue, vring_align);
>   
> -	vq = __vring_new_virtqueue(index, vring, vdev, weak_barriers, context,
> -				   notify, callback, name);
> +	vq = __vring_new_virtqueue(index, vring, packed, vdev, weak_barriers,
> +				   context, notify, callback, name);
>   	if (!vq) {
>   		vring_free_queue(vdev, queue_size_in_bytes, queue,
>   				 dma_addr);
> @@ -1130,10 +1816,17 @@ struct virtqueue *vring_new_virtqueue(unsigned int
index,
>   				      void (*callback)(struct virtqueue *vq),
>   				      const char *name)
>   {
> -	struct vring vring;
> -	vring_init(&vring, num, pages, vring_align);
> -	return __vring_new_virtqueue(index, vring, vdev, weak_barriers, context,
> -				     notify, callback, name);
> +	union vring_union vring;
> +	bool packed;
> +
> +	packed = virtio_has_feature(vdev, VIRTIO_F_RING_PACKED);
> +	if (packed)
> +		vring_packed_init(&vring.vring_packed, num, pages, vring_align);
> +	else
> +		vring_init(&vring.vring_split, num, pages, vring_align);
> +
> +	return __vring_new_virtqueue(index, vring, packed, vdev, weak_barriers,
> +				     context, notify, callback, name);
>   }
>   EXPORT_SYMBOL_GPL(vring_new_virtqueue);
>   
> @@ -1143,7 +1836,9 @@ void vring_del_virtqueue(struct virtqueue *_vq)
>   
>   	if (vq->we_own_ring) {
>   		vring_free_queue(vq->vq.vdev, vq->queue_size_in_bytes,
> -				 vq->vring.desc, vq->queue_dma_addr);
> +				 vq->packed ? (void *)vq->vring_packed.desc :
> +					      (void *)vq->vring.desc,
> +				 vq->queue_dma_addr);
>   	}
>   	list_del(&_vq->list);
>   	kfree(vq);
> @@ -1157,14 +1852,18 @@ void vring_transport_features(struct virtio_device
*vdev)
>   
>   	for (i = VIRTIO_TRANSPORT_F_START; i < VIRTIO_TRANSPORT_F_END; i++) {
>   		switch (i) {
> -		case VIRTIO_RING_F_INDIRECT_DESC:
> +#if 0
> +		case VIRTIO_RING_F_INDIRECT_DESC: // FIXME not tested yet.
>   			break;
> -		case VIRTIO_RING_F_EVENT_IDX:
> +		case VIRTIO_RING_F_EVENT_IDX: // FIXME probably not work.
>   			break;
> +#endif
>   		case VIRTIO_F_VERSION_1:
>   			break;
>   		case VIRTIO_F_IOMMU_PLATFORM:
>   			break;
> +		case VIRTIO_F_RING_PACKED:
> +			break;
>   		default:
>   			/* We don't understand this bit. */
>   			__virtio_clear_bit(vdev, i);
> @@ -1185,7 +1884,7 @@ unsigned int virtqueue_get_vring_size(struct
virtqueue *_vq)
>   
>   	struct vring_virtqueue *vq = to_vvq(_vq);
>   
> -	return vq->vring.num;
> +	return vq->packed ? vq->vring_packed.num : vq->vring.num;
>   }
>   EXPORT_SYMBOL_GPL(virtqueue_get_vring_size);
>   
> @@ -1228,6 +1927,10 @@ dma_addr_t virtqueue_get_avail_addr(struct virtqueue
*_vq)
>   
>   	BUG_ON(!vq->we_own_ring);
>   
> +	if (vq->packed)
> +		return vq->queue_dma_addr + ((char *)vq->vring_packed.driver -
> +				(char *)vq->vring_packed.desc);
> +
>   	return vq->queue_dma_addr +
>   		((char *)vq->vring.avail - (char *)vq->vring.desc);
>   }
> @@ -1239,11 +1942,16 @@ dma_addr_t virtqueue_get_used_addr(struct virtqueue
*_vq)
>   
>   	BUG_ON(!vq->we_own_ring);
>   
> +	if (vq->packed)
> +		return vq->queue_dma_addr + ((char *)vq->vring_packed.device -
> +				(char *)vq->vring_packed.desc);
> +
>   	return vq->queue_dma_addr +
>   		((char *)vq->vring.used - (char *)vq->vring.desc);
>   }
>   EXPORT_SYMBOL_GPL(virtqueue_get_used_addr);
>   
> +/* Only available for split ring */
>   const struct vring *virtqueue_get_vring(struct virtqueue *vq)
>   {
>   	return &to_vvq(vq)->vring;
> diff --git a/include/linux/virtio_ring.h b/include/linux/virtio_ring.h
> index bbf32524ab27..a0075894ad16 100644
> --- a/include/linux/virtio_ring.h
> +++ b/include/linux/virtio_ring.h
> @@ -60,6 +60,11 @@ static inline void virtio_store_mb(bool weak_barriers,
>   struct virtio_device;
>   struct virtqueue;
>   
> +union vring_union {
> +	struct vring vring_split;
> +	struct vring_packed vring_packed;
> +};
> +
>   /*
>    * Creates a virtqueue and allocates the descriptor ring.  If
>    * may_reduce_num is set, then this may allocate a smaller ring than
> @@ -79,7 +84,8 @@ struct virtqueue *vring_create_virtqueue(unsigned int
index,
>   
>   /* Creates a virtqueue with a custom layout. */
>   struct virtqueue *__vring_new_virtqueue(unsigned int index,
> -					struct vring vring,
> +					union vring_union vring,
> +					bool packed,
>   					struct virtio_device *vdev,
>   					bool weak_barriers,
>   					bool ctx,
> diff --git a/include/uapi/linux/virtio_config.h
b/include/uapi/linux/virtio_config.h
> index 308e2096291f..a6e392325e3a 100644
> --- a/include/uapi/linux/virtio_config.h
> +++ b/include/uapi/linux/virtio_config.h
> @@ -49,7 +49,7 @@
>    * transport being used (eg. virtio_ring), the rest are per-device
feature
>    * bits. */
>   #define VIRTIO_TRANSPORT_F_START	28
> -#define VIRTIO_TRANSPORT_F_END		34
> +#define VIRTIO_TRANSPORT_F_END		36
>   
>   #ifndef VIRTIO_CONFIG_NO_LEGACY
>   /* Do we get callbacks when the ring is completely used, even if
we've
> @@ -71,4 +71,14 @@
>    * this is for compatibility with legacy systems.
>    */
>   #define VIRTIO_F_IOMMU_PLATFORM		33
> +
> +/* This feature indicates support for the packed virtqueue layout. */
> +#define VIRTIO_F_RING_PACKED		34
> +
> +/*
> + * This feature indicates that all buffers are used by the device
> + * in the same order in which they have been made available.
> + */
> +#define VIRTIO_F_IN_ORDER		35
> +
>   #endif /* _UAPI_LINUX_VIRTIO_CONFIG_H */
> diff --git a/include/uapi/linux/virtio_ring.h
b/include/uapi/linux/virtio_ring.h
> index 6d5d5faa989b..735d4207c988 100644
> --- a/include/uapi/linux/virtio_ring.h
> +++ b/include/uapi/linux/virtio_ring.h
> @@ -44,6 +44,9 @@
>   /* This means the buffer contains a list of buffer descriptors. */
>   #define VRING_DESC_F_INDIRECT	4
>   
> +#define VRING_DESC_F_AVAIL(b)	((b) << 7)
> +#define VRING_DESC_F_USED(b)	((b) << 15)
> +
>   /* The Host uses this in used->flags to advise the Guest: don't
kick me when
>    * you add a buffer.  It's unreliable, so it's simply an
optimization.  Guest
>    * will still kick if it's out of buffers. */
> @@ -53,6 +56,10 @@
>    * optimization.  */
>   #define VRING_AVAIL_F_NO_INTERRUPT	1
>   
> +#define VRING_EVENT_F_ENABLE	0x0
> +#define VRING_EVENT_F_DISABLE	0x1
> +#define VRING_EVENT_F_DESC	0x2
> +
>   /* We support indirect buffer descriptors */
>   #define VIRTIO_RING_F_INDIRECT_DESC	28
>   
> @@ -171,4 +178,58 @@ static inline int vring_need_event(__u16 event_idx,
__u16 new_idx, __u16 old)
>   	return (__u16)(new_idx - event_idx - 1) < (__u16)(new_idx - old);
>   }
>   
> +struct vring_packed_desc_event {
> +	/* __virtio16 off  : 15; // Descriptor Event Offset
> +	 * __virtio16 wrap : 1;  // Descriptor Event Wrap Counter */
> +	__virtio16 off_wrap;
> +	/* __virtio16 flags : 2; // Descriptor Event Flags */
> +	__virtio16 flags;
> +};
> +
> +struct vring_packed_desc {
> +	/* Buffer Address. */
> +	__virtio64 addr;
> +	/* Buffer Length. */
> +	__virtio32 len;
> +	/* Buffer ID. */
> +	__virtio16 id;
> +	/* The flags depending on descriptor type. */
> +	__virtio16 flags;
> +};
> +
> +struct vring_packed {
> +	unsigned int num;
> +
> +	struct vring_packed_desc *desc;
> +
> +	struct vring_packed_desc_event *driver;
> +
> +	struct vring_packed_desc_event *device;
> +};
> +
> +/* The standard layout for the packed ring is a continuous chunk of memory
> + * which looks like this.
> + *
> + * struct vring_packed
> + * {
> + *	// The actual descriptors (16 bytes each)
> + *	struct vring_packed_desc desc[num];
> + *
> + *	// Padding to the next align boundary.
> + *	char pad[];
> + *
> + *	// Driver Event Suppression
> + *	struct vring_packed_desc_event driver;
> + *
> + *	// Device Event Suppression
> + *	struct vring_packed_desc_event device;
> + * };
> + */
> +
> +static inline unsigned vring_packed_size(unsigned int num, unsigned long
align)
> +{
> +	return ((sizeof(struct vring_packed_desc) * num + align - 1)
> +		& ~(align - 1)) + sizeof(struct vring_packed_desc_event) * 2;
> +}
> +
>   #endif /* _UAPI_LINUX_VIRTIO_RING_H */

On Tue, Apr 10, 2018 at 10:55:25AM +0800, Jason Wang
wrote:
> On 2018?04?01? 22:12, Tiwei Bie wrote:
> > Hello everyone,
> > 
> > This RFC implements packed ring support for virtio driver.
> > 
> > The code was tested with DPDK vhost (testpmd/vhost-PMD) implemented
> > by Jens at http://dpdk.org/ml/archives/dev/2018-January/089417.html
> > Minor changes are needed for the vhost code, e.g. to kick the guest.
> > 
> > TODO:
> > - Refinements and bug fixes;
> > - Split into small patches;
> > - Test indirect descriptor support;
> > - Test/fix event suppression support;
> > - Test devices other than net;
> > 
> > RFC v1 -> RFC v2:
> > - Add indirect descriptor support - compile test only;
> > - Add event suppression supprt - compile test only;
> > - Move vring_packed_init() out of uapi (Jason, MST);
> > - Merge two loops into one in virtqueue_add_packed() (Jason);
> > - Split vring_unmap_one() for packed ring and split ring (Jason);
> > - Avoid using '%' operator (Jason);
> > - Rename free_head -> next_avail_idx (Jason);
> > - Add comments for virtio_wmb() in virtqueue_add_packed() (Jason);
> > - Some other refinements and bug fixes;
> > 
> > Thanks!
> 
> Will try to review this later.
> 
> But it would be better if you can split it (more than 1000 lines is too big
> to be reviewed easily). E.g you can at least split it into three patches,
> new structures, datapath, and event suppression.
> 
No problem! It's on my TODO list. I'll get it done in the next version.

Thanks!

On 2018?04?01? 22:12, Tiwei Bie wrote:
> Hello everyone,
>
> This RFC implements packed ring support for virtio driver.
>
> The code was tested with DPDK vhost (testpmd/vhost-PMD) implemented
> by Jens at http://dpdk.org/ml/archives/dev/2018-January/089417.html
> Minor changes are needed for the vhost code, e.g. to kick the guest.
>
> TODO:
> - Refinements and bug fixes;
> - Split into small patches;
> - Test indirect descriptor support;
> - Test/fix event suppression support;
> - Test devices other than net;
>
> RFC v1 -> RFC v2:
> - Add indirect descriptor support - compile test only;
> - Add event suppression supprt - compile test only;
> - Move vring_packed_init() out of uapi (Jason, MST);
> - Merge two loops into one in virtqueue_add_packed() (Jason);
> - Split vring_unmap_one() for packed ring and split ring (Jason);
> - Avoid using '%' operator (Jason);
> - Rename free_head -> next_avail_idx (Jason);
> - Add comments for virtio_wmb() in virtqueue_add_packed() (Jason);
> - Some other refinements and bug fixes;
>
> Thanks!
>
> Signed-off-by: Tiwei Bie <tiwei.bie at intel.com>
> ---
>   drivers/virtio/virtio_ring.c       | 1094
+++++++++++++++++++++++++++++-------
>   include/linux/virtio_ring.h        |    8 +-
>   include/uapi/linux/virtio_config.h |   12 +-
>   include/uapi/linux/virtio_ring.h   |   61 ++
>   4 files changed, 980 insertions(+), 195 deletions(-)
>
> diff --git a/drivers/virtio/virtio_ring.c b/drivers/virtio/virtio_ring.c
> index 71458f493cf8..0515dca34d77 100644
> --- a/drivers/virtio/virtio_ring.c
> +++ b/drivers/virtio/virtio_ring.c
> @@ -58,14 +58,15 @@
>   
>   struct vring_desc_state {
>   	void *data;			/* Data for callback. */
> -	struct vring_desc *indir_desc;	/* Indirect descriptor, if any. */
> +	void *indir_desc;		/* Indirect descriptor, if any. */
> +	int num;			/* Descriptor list length. */
>   };
>   
>   struct vring_virtqueue {
>   	struct virtqueue vq;
>   
> -	/* Actual memory layout for this queue */
> -	struct vring vring;
> +	/* Is this a packed ring? */
> +	bool packed;
>   
>   	/* Can we use weak barriers? */
>   	bool weak_barriers;
> @@ -79,19 +80,45 @@ struct vring_virtqueue {
>   	/* Host publishes avail event idx */
>   	bool event;
>   
> -	/* Head of free buffer list. */
> -	unsigned int free_head;
>   	/* Number we've added since last sync. */
>   	unsigned int num_added;
>   
>   	/* Last used index we've seen. */
>   	u16 last_used_idx;
>   
> -	/* Last written value to avail->flags */
> -	u16 avail_flags_shadow;
> +	union {
> +		/* Available for split ring */
> +		struct {
> +			/* Actual memory layout for this queue. */
> +			struct vring vring;
>   
> -	/* Last written value to avail->idx in guest byte order */
> -	u16 avail_idx_shadow;
> +			/* Head of free buffer list. */
> +			unsigned int free_head;
> +
> +			/* Last written value to avail->flags */
> +			u16 avail_flags_shadow;
> +
> +			/* Last written value to avail->idx in
> +			 * guest byte order. */
> +			u16 avail_idx_shadow;
> +		};
> +
> +		/* Available for packed ring */
> +		struct {
> +			/* Actual memory layout for this queue. */
> +			struct vring_packed vring_packed;
> +
> +			/* Driver ring wrap counter. */
> +			u8 wrap_counter;
> +
> +			/* Index of the next avail descriptor. */
> +			unsigned int next_avail_idx;
> +
> +			/* Last written value to driver->flags in
> +			 * guest byte order. */
> +			u16 event_flags_shadow;
> +		};
> +	};
>   
>   	/* How to notify other side. FIXME: commonalize hcalls! */
>   	bool (*notify)(struct virtqueue *vq);
> @@ -201,8 +228,33 @@ static dma_addr_t vring_map_single(const struct
vring_virtqueue *vq,
>   			      cpu_addr, size, direction);
>   }
>   
> -static void vring_unmap_one(const struct vring_virtqueue *vq,
> -			    struct vring_desc *desc)
> +static void vring_unmap_one_split(const struct vring_virtqueue *vq,
> +				  struct vring_desc *desc)
> +{
> +	u16 flags;
> +
> +	if (!vring_use_dma_api(vq->vq.vdev))
> +		return;
> +
> +	flags = virtio16_to_cpu(vq->vq.vdev, desc->flags);
> +
> +	if (flags & VRING_DESC_F_INDIRECT) {
> +		dma_unmap_single(vring_dma_dev(vq),
> +				 virtio64_to_cpu(vq->vq.vdev, desc->addr),
> +				 virtio32_to_cpu(vq->vq.vdev, desc->len),
> +				 (flags & VRING_DESC_F_WRITE) ?
> +				 DMA_FROM_DEVICE : DMA_TO_DEVICE);
> +	} else {
> +		dma_unmap_page(vring_dma_dev(vq),
> +			       virtio64_to_cpu(vq->vq.vdev, desc->addr),
> +			       virtio32_to_cpu(vq->vq.vdev, desc->len),
> +			       (flags & VRING_DESC_F_WRITE) ?
> +			       DMA_FROM_DEVICE : DMA_TO_DEVICE);
> +	}
> +}
> +
> +static void vring_unmap_one_packed(const struct vring_virtqueue *vq,
> +				   struct vring_packed_desc *desc)
>   {
>   	u16 flags;
>   
> @@ -235,8 +287,9 @@ static int vring_mapping_error(const struct
vring_virtqueue *vq,
>   	return dma_mapping_error(vring_dma_dev(vq), addr);
>   }
>   
> -static struct vring_desc *alloc_indirect(struct virtqueue *_vq,
> -					 unsigned int total_sg, gfp_t gfp)
> +static struct vring_desc *alloc_indirect_split(struct virtqueue *_vq,
> +					       unsigned int total_sg,
> +					       gfp_t gfp)
>   {
>   	struct vring_desc *desc;
>   	unsigned int i;
> @@ -257,14 +310,32 @@ static struct vring_desc *alloc_indirect(struct
virtqueue *_vq,
>   	return desc;
>   }
>   
> -static inline int virtqueue_add(struct virtqueue *_vq,
> -				struct scatterlist *sgs[],
> -				unsigned int total_sg,
> -				unsigned int out_sgs,
> -				unsigned int in_sgs,
> -				void *data,
> -				void *ctx,
> -				gfp_t gfp)
> +static struct vring_packed_desc *alloc_indirect_packed(struct virtqueue
*_vq,
> +						       unsigned int total_sg,
> +						       gfp_t gfp)
> +{
> +	struct vring_packed_desc *desc;
> +
> +	/*
> +	 * We require lowmem mappings for the descriptors because
> +	 * otherwise virt_to_phys will give us bogus addresses in the
> +	 * virtqueue.
> +	 */
> +	gfp &= ~__GFP_HIGHMEM;
> +
> +	desc = kmalloc(total_sg * sizeof(struct vring_packed_desc), gfp);
Can we simply check vq->packed here to avoid duplicating helpers?
> +
> +	return desc;
> +}
> +
> +static inline int virtqueue_add_split(struct virtqueue *_vq,
> +				      struct scatterlist *sgs[],
> +				      unsigned int total_sg,
> +				      unsigned int out_sgs,
> +				      unsigned int in_sgs,
> +				      void *data,
> +				      void *ctx,
> +				      gfp_t gfp)
>   {
>   	struct vring_virtqueue *vq = to_vvq(_vq);
>   	struct scatterlist *sg;
> @@ -303,7 +374,7 @@ static inline int virtqueue_add(struct virtqueue *_vq,
>   	/* If the host supports indirect descriptor tables, and we have multiple
>   	 * buffers, then go indirect. FIXME: tune this threshold */
>   	if (vq->indirect && total_sg > 1 &&
vq->vq.num_free)
> -		desc = alloc_indirect(_vq, total_sg, gfp);
> +		desc = alloc_indirect_split(_vq, total_sg, gfp);
>   	else {
>   		desc = NULL;
>   		WARN_ON_ONCE(total_sg > vq->vring.num &&
!vq->indirect);
> @@ -424,7 +495,7 @@ static inline int virtqueue_add(struct virtqueue *_vq,
>   	for (n = 0; n < total_sg; n++) {
>   		if (i == err_idx)
>   			break;
> -		vring_unmap_one(vq, &desc[i]);
> +		vring_unmap_one_split(vq, &desc[i]);
>   		i = virtio16_to_cpu(_vq->vdev, vq->vring.desc[i].next);
>   	}
>   
> @@ -435,6 +506,210 @@ static inline int virtqueue_add(struct virtqueue
*_vq,
>   	return -EIO;
>   }
>   
> +static inline int virtqueue_add_packed(struct virtqueue *_vq,
> +				       struct scatterlist *sgs[],
> +				       unsigned int total_sg,
> +				       unsigned int out_sgs,
> +				       unsigned int in_sgs,
> +				       void *data,
> +				       void *ctx,
> +				       gfp_t gfp)
> +{
> +	struct vring_virtqueue *vq = to_vvq(_vq);
> +	struct vring_packed_desc *desc;
> +	struct scatterlist *sg;
> +	unsigned int i, n, descs_used, uninitialized_var(prev), err_idx;
> +	__virtio16 uninitialized_var(head_flags), flags;
> +	int head, wrap_counter;
> +	bool indirect;
> +
> +	START_USE(vq);
> +
> +	BUG_ON(data == NULL);
> +	BUG_ON(ctx && vq->indirect);
> +
> +	if (unlikely(vq->broken)) {
> +		END_USE(vq);
> +		return -EIO;
> +	}
> +
> +#ifdef DEBUG
> +	{
> +		ktime_t now = ktime_get();
> +
> +		/* No kick or get, with .1 second between?  Warn. */
> +		if (vq->last_add_time_valid)
> +			WARN_ON(ktime_to_ms(ktime_sub(now, vq->last_add_time))
> +					    > 100);
> +		vq->last_add_time = now;
> +		vq->last_add_time_valid = true;
> +	}
> +#endif
> +
> +	BUG_ON(total_sg == 0);
> +
> +	head = vq->next_avail_idx;
> +	wrap_counter = vq->wrap_counter;
> +
> +	/* If the host supports indirect descriptor tables, and we have multiple
> +	 * buffers, then go indirect. FIXME: tune this threshold */
> +	if (vq->indirect && total_sg > 1 &&
vq->vq.num_free)
Let's introduce a helper like virtqueue_need_indirect() to avoid 
duplicating codes and FIXME.
> +		desc = alloc_indirect_packed(_vq, total_sg, gfp);
> +	else {
> +		desc = NULL;
> +		WARN_ON_ONCE(total_sg > vq->vring_packed.num &&
!vq->indirect);
> +	}
> +
> +	if (desc) {
> +		/* Use a single buffer which doesn't continue */
> +		indirect = true;
> +		/* Set up rest to use this indirect table. */
> +		i = 0;
> +		descs_used = 1;
> +	} else {
> +		indirect = false;
> +		desc = vq->vring_packed.desc;
> +		i = head;
> +		descs_used = total_sg;
> +	}
> +
> +	if (vq->vq.num_free < descs_used) {
> +		pr_debug("Can't add buf len %i - avail = %i\n",
> +			 descs_used, vq->vq.num_free);
> +		/* FIXME: for historical reasons, we force a notify here if
> +		 * there are outgoing parts to the buffer.  Presumably the
> +		 * host should service the ring ASAP. */
> +		if (out_sgs)
> +			vq->notify(&vq->vq);
> +		if (indirect)
> +			kfree(desc);
> +		END_USE(vq);
> +		return -ENOSPC;
> +	}
> +
> +	for (n = 0; n < out_sgs + in_sgs; n++) {
> +		for (sg = sgs[n]; sg; sg = sg_next(sg)) {
> +			dma_addr_t addr = vring_map_one_sg(vq, sg, n < out_sgs ?
> +						DMA_TO_DEVICE : DMA_FROM_DEVICE);
> +			if (vring_mapping_error(vq, addr))
> +				goto unmap_release;
> +
> +			flags = cpu_to_virtio16(_vq->vdev, VRING_DESC_F_NEXT |
> +					(n < out_sgs ? 0 : VRING_DESC_F_WRITE) |
> +					VRING_DESC_F_AVAIL(vq->wrap_counter) |
> +					VRING_DESC_F_USED(!vq->wrap_counter));
> +			if (!indirect && i == head)
> +				head_flags = flags;
> +			else
> +				desc[i].flags = flags;
> +
> +			desc[i].addr = cpu_to_virtio64(_vq->vdev, addr);
> +			desc[i].len = cpu_to_virtio32(_vq->vdev, sg->length);
> +			desc[i].id = cpu_to_virtio32(_vq->vdev, head);
Similar to V1, we only need this for the last descriptor.
> +			prev = i;
It looks to me there's no need to track prev inside the loop here.
> +			i++;
> +			if (!indirect && i >= vq->vring_packed.num) {
> +				i = 0;
> +				vq->wrap_counter ^= 1;
> +			}
> +		}
> +	}
> +	/* Last one doesn't continue. */
> +	if (total_sg == 1)
> +		head_flags &= cpu_to_virtio16(_vq->vdev, ~VRING_DESC_F_NEXT);
> +	else
> +		desc[prev].flags &= cpu_to_virtio16(_vq->vdev,
~VRING_DESC_F_NEXT);
The only case when prev != i - 1 is i == 0, we can add a if here.
> +
> +	if (indirect) {
> +		/* Now that the indirect table is filled in, map it. */
> +		dma_addr_t addr = vring_map_single(
> +			vq, desc, total_sg * sizeof(struct vring_packed_desc),
> +			DMA_TO_DEVICE);
> +		if (vring_mapping_error(vq, addr))
> +			goto unmap_release;
> +
> +		head_flags = cpu_to_virtio16(_vq->vdev, VRING_DESC_F_INDIRECT |
> +					     VRING_DESC_F_AVAIL(wrap_counter) |
> +					     VRING_DESC_F_USED(!wrap_counter));
> +		vq->vring_packed.desc[head].addr = cpu_to_virtio64(_vq->vdev,
addr);
> +		vq->vring_packed.desc[head].len = cpu_to_virtio32(_vq->vdev,
> +				total_sg * sizeof(struct vring_packed_desc));
> +		vq->vring_packed.desc[head].id = cpu_to_virtio32(_vq->vdev, head);
> +	}
> +
> +	/* We're using some buffers from the free list. */
> +	vq->vq.num_free -= descs_used;
> +
> +	/* Update free pointer */
> +	if (indirect) {
> +		n = head + 1;
> +		if (n >= vq->vring_packed.num) {
> +			n = 0;
> +			vq->wrap_counter ^= 1;
> +		}
> +		vq->next_avail_idx = n;
> +	} else
> +		vq->next_avail_idx = i;
> +
> +	/* Store token and indirect buffer state. */
> +	vq->desc_state[head].num = descs_used;
> +	vq->desc_state[head].data = data;
> +	if (indirect)
> +		vq->desc_state[head].indir_desc = desc;
> +	else
> +		vq->desc_state[head].indir_desc = ctx;
> +
> +	/* A driver MUST NOT make the first descriptor in the list
> +	 * available before all subsequent descriptors comprising
> +	 * the list are made available. */
> +	virtio_wmb(vq->weak_barriers);
> +	vq->vring_packed.desc[head].flags = head_flags;
> +	vq->num_added++;
> +
> +	pr_debug("Added buffer head %i to %p\n", head, vq);
> +	END_USE(vq);
> +
> +	return 0;
> +
> +unmap_release:
> +	err_idx = i;
> +	i = head;
> +
> +	for (n = 0; n < total_sg; n++) {
> +		if (i == err_idx)
> +			break;
> +		vring_unmap_one_packed(vq, &desc[i]);
> +		i++;
> +		if (!indirect && i >= vq->vring_packed.num)
> +			i = 0;
> +	}
> +
> +	vq->wrap_counter = wrap_counter;
> +
> +	if (indirect)
> +		kfree(desc);
> +
> +	END_USE(vq);
> +	return -EIO;
> +}
> +
> +static inline int virtqueue_add(struct virtqueue *_vq,
> +				struct scatterlist *sgs[],
> +				unsigned int total_sg,
> +				unsigned int out_sgs,
> +				unsigned int in_sgs,
> +				void *data,
> +				void *ctx,
> +				gfp_t gfp)
> +{
> +	struct vring_virtqueue *vq = to_vvq(_vq);
> +
> +	return vq->packed ? virtqueue_add_packed(_vq, sgs, total_sg, out_sgs,
> +						 in_sgs, data, ctx, gfp) :
> +			    virtqueue_add_split(_vq, sgs, total_sg, out_sgs,
> +						in_sgs, data, ctx, gfp);
> +}
> +
>   /**
>    * virtqueue_add_sgs - expose buffers to other end
>    * @vq: the struct virtqueue we're talking about.
> @@ -537,18 +812,7 @@ int virtqueue_add_inbuf_ctx(struct virtqueue *vq,
>   }
>   EXPORT_SYMBOL_GPL(virtqueue_add_inbuf_ctx);
>   
> -/**
> - * virtqueue_kick_prepare - first half of split virtqueue_kick call.
> - * @vq: the struct virtqueue
> - *
> - * Instead of virtqueue_kick(), you can do:
> - *	if (virtqueue_kick_prepare(vq))
> - *		virtqueue_notify(vq);
> - *
> - * This is sometimes useful because the virtqueue_kick_prepare() needs
> - * to be serialized, but the actual virtqueue_notify() call does not.
> - */
> -bool virtqueue_kick_prepare(struct virtqueue *_vq)
> +static bool virtqueue_kick_prepare_split(struct virtqueue *_vq)
>   {
>   	struct vring_virtqueue *vq = to_vvq(_vq);
>   	u16 new, old;
> @@ -580,6 +844,62 @@ bool virtqueue_kick_prepare(struct virtqueue *_vq)
>   	END_USE(vq);
>   	return needs_kick;
>   }
> +
> +static bool virtqueue_kick_prepare_packed(struct virtqueue *_vq)
> +{
> +	struct vring_virtqueue *vq = to_vvq(_vq);
> +	u16 new, old, off_wrap;
> +	bool needs_kick;
> +
> +	START_USE(vq);
> +	/* We need to expose the new flags value before checking notification
> +	 * suppressions. */
> +	virtio_mb(vq->weak_barriers);
> +
> +	old = vq->next_avail_idx - vq->num_added;
> +	new = vq->next_avail_idx;
> +	vq->num_added = 0;
> +
> +#ifdef DEBUG
> +	if (vq->last_add_time_valid) {
> +		WARN_ON(ktime_to_ms(ktime_sub(ktime_get(),
> +					      vq->last_add_time)) > 100);
> +	}
> +	vq->last_add_time_valid = false;
> +#endif
> +
> +	off_wrap = virtio16_to_cpu(_vq->vdev,
vq->vring_packed.device->off_wrap);
> +
> +	if (vq->event) {
It looks to me we should examine RING_EVENT_FLAGS_DESC in 
desc_event_flags instead of vq->event here. Spec does not forces to use 
evenf_off and event_wrap if event index is enabled.
> +		// FIXME: fix this!
> +		needs_kick = ((off_wrap >> 15) == vq->wrap_counter) &&
> +			     vring_need_event(off_wrap & ~(1<<15), new, old);
Why need a & here?
> +	} else {
Need a smp_rmb() to make sure desc_event_flags was checked before flags.
> +		needs_kick = (vq->vring_packed.device->flags !> +			     
cpu_to_virtio16(_vq->vdev, VRING_EVENT_F_DISABLE));
> +	}
> +	END_USE(vq);
> +	return needs_kick;
> +}
> +
> +/**
> + * virtqueue_kick_prepare - first half of split virtqueue_kick call.
> + * @vq: the struct virtqueue
> + *
> + * Instead of virtqueue_kick(), you can do:
> + *	if (virtqueue_kick_prepare(vq))
> + *		virtqueue_notify(vq);
> + *
> + * This is sometimes useful because the virtqueue_kick_prepare() needs
> + * to be serialized, but the actual virtqueue_notify() call does not.
> + */
> +bool virtqueue_kick_prepare(struct virtqueue *_vq)
> +{
> +	struct vring_virtqueue *vq = to_vvq(_vq);
> +
> +	return vq->packed ? virtqueue_kick_prepare_packed(_vq) :
> +			    virtqueue_kick_prepare_split(_vq);
> +}
>   EXPORT_SYMBOL_GPL(virtqueue_kick_prepare);
>   
>   /**
> @@ -626,8 +946,8 @@ bool virtqueue_kick(struct virtqueue *vq)
>   }
>   EXPORT_SYMBOL_GPL(virtqueue_kick);
>   
> -static void detach_buf(struct vring_virtqueue *vq, unsigned int head,
> -		       void **ctx)
> +static void detach_buf_split(struct vring_virtqueue *vq, unsigned int
head,
> +			     void **ctx)
>   {
>   	unsigned int i, j;
>   	__virtio16 nextflag = cpu_to_virtio16(vq->vq.vdev,
VRING_DESC_F_NEXT);
> @@ -639,12 +959,12 @@ static void detach_buf(struct vring_virtqueue *vq,
unsigned int head,
>   	i = head;
>   
>   	while (vq->vring.desc[i].flags & nextflag) {
> -		vring_unmap_one(vq, &vq->vring.desc[i]);
> +		vring_unmap_one_split(vq, &vq->vring.desc[i]);
>   		i = virtio16_to_cpu(vq->vq.vdev, vq->vring.desc[i].next);
>   		vq->vq.num_free++;
>   	}
>   
> -	vring_unmap_one(vq, &vq->vring.desc[i]);
> +	vring_unmap_one_split(vq, &vq->vring.desc[i]);
>   	vq->vring.desc[i].next = cpu_to_virtio16(vq->vq.vdev,
vq->free_head);
>   	vq->free_head = head;
>   
> @@ -666,7 +986,7 @@ static void detach_buf(struct vring_virtqueue *vq,
unsigned int head,
>   		BUG_ON(len == 0 || len % sizeof(struct vring_desc));
>   
>   		for (j = 0; j < len / sizeof(struct vring_desc); j++)
> -			vring_unmap_one(vq, &indir_desc[j]);
> +			vring_unmap_one_split(vq, &indir_desc[j]);
>   
>   		kfree(indir_desc);
>   		vq->desc_state[head].indir_desc = NULL;
> @@ -675,11 +995,207 @@ static void detach_buf(struct vring_virtqueue *vq,
unsigned int head,
>   	}
>   }
>   
> -static inline bool more_used(const struct vring_virtqueue *vq)
> +static int detach_buf_packed(struct vring_virtqueue *vq, unsigned int
head,
> +			      void **ctx)
> +{
> +	struct vring_packed_desc *desc;
> +	unsigned int i, j;
> +
> +	/* Clear data ptr. */
> +	vq->desc_state[head].data = NULL;
> +
> +	i = head;
> +
> +	for (j = 0; j < vq->desc_state[head].num; j++) {
> +		desc = &vq->vring_packed.desc[i];
> +		vring_unmap_one_packed(vq, desc);
> +		desc->flags = 0x0;
Looks like this is unnecessary.
> +		i++;
> +		if (i >= vq->vring_packed.num)
> +			i = 0;
> +	}
> +
> +	vq->vq.num_free += vq->desc_state[head].num;
> +
> +	if (vq->indirect) {
> +		u32 len;
> +
> +		desc = vq->desc_state[head].indir_desc;
> +		/* Free the indirect table, if any, now that it's unmapped. */
> +		if (!desc)
> +			goto out;
> +
> +		len = virtio32_to_cpu(vq->vq.vdev,
> +				      vq->vring_packed.desc[head].len);
> +
> +		BUG_ON(!(vq->vring_packed.desc[head].flags &
> +			 cpu_to_virtio16(vq->vq.vdev, VRING_DESC_F_INDIRECT)));
> +		BUG_ON(len == 0 || len % sizeof(struct vring_packed_desc));
> +
> +		for (j = 0; j < len / sizeof(struct vring_packed_desc); j++)
> +			vring_unmap_one_packed(vq, &desc[j]);
> +
> +		kfree(desc);
> +		vq->desc_state[head].indir_desc = NULL;
> +	} else if (ctx) {
> +		*ctx = vq->desc_state[head].indir_desc;
> +	}
> +
> +out:
> +	return vq->desc_state[head].num;
> +}
> +
> +static inline bool more_used_split(const struct vring_virtqueue *vq)
>   {
>   	return vq->last_used_idx != virtio16_to_cpu(vq->vq.vdev,
vq->vring.used->idx);
>   }
>   
> +static inline bool more_used_packed(const struct vring_virtqueue *vq)
> +{
> +	u16 last_used, flags;
> +	bool avail, used;
> +
> +	if (vq->vq.num_free == vq->vring_packed.num)
> +		return false;
> +
> +	last_used = vq->last_used_idx;
> +	flags = virtio16_to_cpu(vq->vq.vdev,
> +				vq->vring_packed.desc[last_used].flags);
> +	avail = flags & VRING_DESC_F_AVAIL(1);
> +	used = flags & VRING_DESC_F_USED(1);
> +
> +	return avail == used;
> +}
> +
> +static inline bool more_used(const struct vring_virtqueue *vq)
> +{
> +	return vq->packed ? more_used_packed(vq) : more_used_split(vq);
> +}
> +
> +void *virtqueue_get_buf_ctx_split(struct virtqueue *_vq, unsigned int
*len,
> +				  void **ctx)
> +{
> +	struct vring_virtqueue *vq = to_vvq(_vq);
> +	void *ret;
> +	unsigned int i;
> +	u16 last_used;
> +
> +	START_USE(vq);
> +
> +	if (unlikely(vq->broken)) {
> +		END_USE(vq);
> +		return NULL;
> +	}
> +
> +	if (!more_used(vq)) {
> +		pr_debug("No more buffers in queue\n");
> +		END_USE(vq);
> +		return NULL;
> +	}
> +
> +	/* Only get used array entries after they have been exposed by host. */
> +	virtio_rmb(vq->weak_barriers);
> +
> +	last_used = (vq->last_used_idx & (vq->vring.num - 1));
> +	i = virtio32_to_cpu(_vq->vdev,
vq->vring.used->ring[last_used].id);
> +	*len = virtio32_to_cpu(_vq->vdev,
vq->vring.used->ring[last_used].len);
> +
> +	if (unlikely(i >= vq->vring.num)) {
> +		BAD_RING(vq, "id %u out of range\n", i);
> +		return NULL;
> +	}
> +	if (unlikely(!vq->desc_state[i].data)) {
> +		BAD_RING(vq, "id %u is not a head!\n", i);
> +		return NULL;
> +	}
> +
> +	/* detach_buf_split clears data, so grab it now. */
> +	ret = vq->desc_state[i].data;
> +	detach_buf_split(vq, i, ctx);
> +	vq->last_used_idx++;
> +	/* If we expect an interrupt for the next entry, tell host
> +	 * by writing event index and flush out the write before
> +	 * the read in the next get_buf call. */
> +	if (!(vq->avail_flags_shadow & VRING_AVAIL_F_NO_INTERRUPT))
> +		virtio_store_mb(vq->weak_barriers,
> +				&vring_used_event(&vq->vring),
> +				cpu_to_virtio16(_vq->vdev, vq->last_used_idx));
> +
> +#ifdef DEBUG
> +	vq->last_add_time_valid = false;
> +#endif
> +
> +	END_USE(vq);
> +	return ret;
> +}
> +
> +void *virtqueue_get_buf_ctx_packed(struct virtqueue *_vq, unsigned int
*len,
> +				   void **ctx)
> +{
> +	struct vring_virtqueue *vq = to_vvq(_vq);
> +	uint16_t wrap_counter;
> +	void *ret;
> +	unsigned int i;
> +	u16 last_used;
> +
> +	START_USE(vq);
> +
> +	if (unlikely(vq->broken)) {
> +		END_USE(vq);
> +		return NULL;
> +	}
> +
> +	if (!more_used(vq)) {
> +		pr_debug("No more buffers in queue\n");
> +		END_USE(vq);
> +		return NULL;
> +	}
> +
> +	/* Only get used elements after they have been exposed by host. */
> +	virtio_rmb(vq->weak_barriers);
> +
> +	last_used = vq->last_used_idx;
> +	i = virtio32_to_cpu(_vq->vdev,
vq->vring_packed.desc[last_used].id);
> +	*len = virtio32_to_cpu(_vq->vdev,
vq->vring_packed.desc[last_used].len);
> +
> +	if (unlikely(i >= vq->vring_packed.num)) {
> +		BAD_RING(vq, "id %u out of range\n", i);
> +		return NULL;
> +	}
> +	if (unlikely(!vq->desc_state[i].data)) {
> +		BAD_RING(vq, "id %u is not a head!\n", i);
> +		return NULL;
> +	}
> +
> +	/* detach_buf_packed clears data, so grab it now. */
> +	ret = vq->desc_state[i].data;
> +	detach_buf_packed(vq, i, ctx);
> +
> +	vq->last_used_idx += vq->desc_state[i].num;
> +	if (vq->last_used_idx >= vq->vring_packed.num)
> +		vq->last_used_idx -= vq->vring_packed.num;
> +
> +	wrap_counter = vq->wrap_counter;
> +	if (vq->last_used_idx > vq->next_avail_idx)
> +		wrap_counter ^= 1;
> +
> +	/* If we expect an interrupt for the next entry, tell host
> +	 * by writing event index and flush out the write before
> +	 * the read in the next get_buf call. */
> +	if (vq->event_flags_shadow == VRING_EVENT_F_DESC)
> +		virtio_store_mb(vq->weak_barriers,
> +				&vq->vring_packed.driver->off_wrap,
> +				cpu_to_virtio16(_vq->vdev, vq->last_used_idx |
> +						wrap_counter << 15));
> +
> +#ifdef DEBUG
> +	vq->last_add_time_valid = false;
> +#endif
> +
> +	END_USE(vq);
> +	return ret;
> +}
> +
>   /**
>    * virtqueue_get_buf - get the next used buffer
>    * @vq: the struct virtqueue we're talking about.
> @@ -700,57 +1216,9 @@ void *virtqueue_get_buf_ctx(struct virtqueue *_vq,
unsigned int *len,
>   			    void **ctx)
>   {
>   	struct vring_virtqueue *vq = to_vvq(_vq);
> -	void *ret;
> -	unsigned int i;
> -	u16 last_used;
>   
> -	START_USE(vq);
> -
> -	if (unlikely(vq->broken)) {
> -		END_USE(vq);
> -		return NULL;
> -	}
> -
> -	if (!more_used(vq)) {
> -		pr_debug("No more buffers in queue\n");
> -		END_USE(vq);
> -		return NULL;
> -	}
> -
> -	/* Only get used array entries after they have been exposed by host. */
> -	virtio_rmb(vq->weak_barriers);
> -
> -	last_used = (vq->last_used_idx & (vq->vring.num - 1));
> -	i = virtio32_to_cpu(_vq->vdev,
vq->vring.used->ring[last_used].id);
> -	*len = virtio32_to_cpu(_vq->vdev,
vq->vring.used->ring[last_used].len);
> -
> -	if (unlikely(i >= vq->vring.num)) {
> -		BAD_RING(vq, "id %u out of range\n", i);
> -		return NULL;
> -	}
> -	if (unlikely(!vq->desc_state[i].data)) {
> -		BAD_RING(vq, "id %u is not a head!\n", i);
> -		return NULL;
> -	}
> -
> -	/* detach_buf clears data, so grab it now. */
> -	ret = vq->desc_state[i].data;
> -	detach_buf(vq, i, ctx);
> -	vq->last_used_idx++;
> -	/* If we expect an interrupt for the next entry, tell host
> -	 * by writing event index and flush out the write before
> -	 * the read in the next get_buf call. */
> -	if (!(vq->avail_flags_shadow & VRING_AVAIL_F_NO_INTERRUPT))
> -		virtio_store_mb(vq->weak_barriers,
> -				&vring_used_event(&vq->vring),
> -				cpu_to_virtio16(_vq->vdev, vq->last_used_idx));
> -
> -#ifdef DEBUG
> -	vq->last_add_time_valid = false;
> -#endif
> -
> -	END_USE(vq);
> -	return ret;
> +	return vq->packed ? virtqueue_get_buf_ctx_packed(_vq, len, ctx) :
> +			    virtqueue_get_buf_ctx_split(_vq, len, ctx);
>   }
>   EXPORT_SYMBOL_GPL(virtqueue_get_buf_ctx);
>   
> @@ -759,6 +1227,29 @@ void *virtqueue_get_buf(struct virtqueue *_vq,
unsigned int *len)
>   	return virtqueue_get_buf_ctx(_vq, len, NULL);
>   }
>   EXPORT_SYMBOL_GPL(virtqueue_get_buf);
> +
> +static void virtqueue_disable_cb_split(struct virtqueue *_vq)
> +{
> +	struct vring_virtqueue *vq = to_vvq(_vq);
> +
> +	if (!(vq->avail_flags_shadow & VRING_AVAIL_F_NO_INTERRUPT)) {
> +		vq->avail_flags_shadow |= VRING_AVAIL_F_NO_INTERRUPT;
> +		if (!vq->event)
> +			vq->vring.avail->flags = cpu_to_virtio16(_vq->vdev,
vq->avail_flags_shadow);
> +	}
> +}
> +
> +static void virtqueue_disable_cb_packed(struct virtqueue *_vq)
> +{
> +	struct vring_virtqueue *vq = to_vvq(_vq);
> +
> +	if (vq->event_flags_shadow != VRING_EVENT_F_DISABLE) {
> +		vq->event_flags_shadow = VRING_EVENT_F_DISABLE;
> +		vq->vring_packed.driver->flags = cpu_to_virtio16(_vq->vdev,
> +							vq->event_flags_shadow);
> +	}
> +}
> +
>   /**
>    * virtqueue_disable_cb - disable callbacks
>    * @vq: the struct virtqueue we're talking about.
> @@ -772,15 +1263,66 @@ void virtqueue_disable_cb(struct virtqueue *_vq)
>   {
>   	struct vring_virtqueue *vq = to_vvq(_vq);
>   
> -	if (!(vq->avail_flags_shadow & VRING_AVAIL_F_NO_INTERRUPT)) {
> -		vq->avail_flags_shadow |= VRING_AVAIL_F_NO_INTERRUPT;
> -		if (!vq->event)
> -			vq->vring.avail->flags = cpu_to_virtio16(_vq->vdev,
vq->avail_flags_shadow);
> -	}
> -
> +	if (vq->packed)
> +		virtqueue_disable_cb_packed(_vq);
> +	else
> +		virtqueue_disable_cb_split(_vq);
>   }
>   EXPORT_SYMBOL_GPL(virtqueue_disable_cb);
>   
> +static unsigned virtqueue_enable_cb_prepare_split(struct virtqueue *_vq)
> +{
> +	struct vring_virtqueue *vq = to_vvq(_vq);
> +	u16 last_used_idx;
> +
> +	START_USE(vq);
> +
> +	/* We optimistically turn back on interrupts, then check if there was
> +	 * more to do. */
> +	/* Depending on the VIRTIO_RING_F_EVENT_IDX feature, we need to
> +	 * either clear the flags bit or point the event index at the next
> +	 * entry. Always do both to keep code simple. */
> +	if (vq->avail_flags_shadow & VRING_AVAIL_F_NO_INTERRUPT) {
> +		vq->avail_flags_shadow &= ~VRING_AVAIL_F_NO_INTERRUPT;
> +		if (!vq->event)
> +			vq->vring.avail->flags = cpu_to_virtio16(_vq->vdev,
vq->avail_flags_shadow);
> +	}
> +	vring_used_event(&vq->vring) = cpu_to_virtio16(_vq->vdev,
last_used_idx = vq->last_used_idx);
> +	END_USE(vq);
> +	return last_used_idx;
> +}
> +
> +static unsigned virtqueue_enable_cb_prepare_packed(struct virtqueue *_vq)
> +{
> +	struct vring_virtqueue *vq = to_vvq(_vq);
> +	u16 last_used_idx, wrap_counter, off_wrap;
> +
> +	START_USE(vq);
> +
> +	last_used_idx = vq->last_used_idx;
> +	wrap_counter = vq->wrap_counter;
> +
> +	if (last_used_idx > vq->next_avail_idx)
> +		wrap_counter ^= 1;
> +
> +	off_wrap = last_used_idx | (wrap_counter << 15);
> +
> +	/* We optimistically turn back on interrupts, then check if there was
> +	 * more to do. */
> +	/* Depending on the VIRTIO_RING_F_EVENT_IDX feature, we need to
> +	 * either clear the flags bit or point the event index at the next
> +	 * entry. Always do both to keep code simple. */
> +	if (vq->event_flags_shadow == VRING_EVENT_F_DISABLE) {
> +		vq->event_flags_shadow = vq->event ? VRING_EVENT_F_DESC:
> +						     VRING_EVENT_F_ENABLE;
> +		vq->vring_packed.driver->flags = cpu_to_virtio16(_vq->vdev,
> +							vq->event_flags_shadow);
> +	}
A smp_wmb() is missed here?
> +	vq->vring_packed.driver->off_wrap = cpu_to_virtio16(_vq->vdev,
off_wrap);
And according to the spec, it looks to me write a VRING_EVENT_F_ENABLE 
is sufficient here.
> +	END_USE(vq);
> +	return last_used_idx;
> +}
> +
>   /**
>    * virtqueue_enable_cb_prepare - restart callbacks after disable_cb
>    * @vq: the struct virtqueue we're talking about.
> @@ -796,26 +1338,34 @@ EXPORT_SYMBOL_GPL(virtqueue_disable_cb);
>   unsigned virtqueue_enable_cb_prepare(struct virtqueue *_vq)
>   {
>   	struct vring_virtqueue *vq = to_vvq(_vq);
> -	u16 last_used_idx;
>   
> -	START_USE(vq);
> -
> -	/* We optimistically turn back on interrupts, then check if there was
> -	 * more to do. */
> -	/* Depending on the VIRTIO_RING_F_EVENT_IDX feature, we need to
> -	 * either clear the flags bit or point the event index at the next
> -	 * entry. Always do both to keep code simple. */
> -	if (vq->avail_flags_shadow & VRING_AVAIL_F_NO_INTERRUPT) {
> -		vq->avail_flags_shadow &= ~VRING_AVAIL_F_NO_INTERRUPT;
> -		if (!vq->event)
> -			vq->vring.avail->flags = cpu_to_virtio16(_vq->vdev,
vq->avail_flags_shadow);
> -	}
> -	vring_used_event(&vq->vring) = cpu_to_virtio16(_vq->vdev,
last_used_idx = vq->last_used_idx);
> -	END_USE(vq);
> -	return last_used_idx;
> +	return vq->packed ? virtqueue_enable_cb_prepare_packed(_vq) :
> +			    virtqueue_enable_cb_prepare_split(_vq);
>   }
>   EXPORT_SYMBOL_GPL(virtqueue_enable_cb_prepare);
>   
> +static bool virtqueue_poll_split(struct virtqueue *_vq, unsigned
last_used_idx)
> +{
> +	struct vring_virtqueue *vq = to_vvq(_vq);
> +
> +	virtio_mb(vq->weak_barriers);
> +	return (u16)last_used_idx != virtio16_to_cpu(_vq->vdev,
vq->vring.used->idx);
> +}
> +
> +static bool virtqueue_poll_packed(struct virtqueue *_vq, unsigned
last_used_idx)
> +{
> +	struct vring_virtqueue *vq = to_vvq(_vq);
> +	bool avail, used;
> +	u16 flags;
> +
> +	virtio_mb(vq->weak_barriers);
> +	flags = virtio16_to_cpu(vq->vq.vdev,
> +			vq->vring_packed.desc[last_used_idx].flags);
> +	avail = flags & VRING_DESC_F_AVAIL(1);
> +	used = flags & VRING_DESC_F_USED(1);
> +	return avail == used;
> +}
> +
>   /**
>    * virtqueue_poll - query pending used buffers
>    * @vq: the struct virtqueue we're talking about.
> @@ -829,8 +1379,8 @@ bool virtqueue_poll(struct virtqueue *_vq, unsigned
last_used_idx)
>   {
>   	struct vring_virtqueue *vq = to_vvq(_vq);
>   
> -	virtio_mb(vq->weak_barriers);
> -	return (u16)last_used_idx != virtio16_to_cpu(_vq->vdev,
vq->vring.used->idx);
> +	return vq->packed ? virtqueue_poll_packed(_vq, last_used_idx) :
> +			    virtqueue_poll_split(_vq, last_used_idx);
>   }
>   EXPORT_SYMBOL_GPL(virtqueue_poll);
>   
> @@ -852,6 +1402,83 @@ bool virtqueue_enable_cb(struct virtqueue *_vq)
>   }
>   EXPORT_SYMBOL_GPL(virtqueue_enable_cb);
>   
> +static bool virtqueue_enable_cb_delayed_split(struct virtqueue *_vq)
> +{
> +	struct vring_virtqueue *vq = to_vvq(_vq);
> +	u16 bufs;
> +
> +	START_USE(vq);
> +
> +	/* We optimistically turn back on interrupts, then check if there was
> +	 * more to do. */
> +	/* Depending on the VIRTIO_RING_F_USED_EVENT_IDX feature, we need to
> +	 * either clear the flags bit or point the event index at the next
> +	 * entry. Always update the event index to keep code simple. */
> +	if (vq->avail_flags_shadow & VRING_AVAIL_F_NO_INTERRUPT) {
> +		vq->avail_flags_shadow &= ~VRING_AVAIL_F_NO_INTERRUPT;
> +		if (!vq->event)
> +			vq->vring.avail->flags = cpu_to_virtio16(_vq->vdev,
vq->avail_flags_shadow);
> +	}
> +	/* TODO: tune this threshold */
> +	bufs = (u16)(vq->avail_idx_shadow - vq->last_used_idx) * 3 / 4;
> +
> +	virtio_store_mb(vq->weak_barriers,
> +			&vring_used_event(&vq->vring),
> +			cpu_to_virtio16(_vq->vdev, vq->last_used_idx + bufs));
> +
> +	if (unlikely((u16)(virtio16_to_cpu(_vq->vdev,
vq->vring.used->idx) - vq->last_used_idx) > bufs)) {
> +		END_USE(vq);
> +		return false;
> +	}
> +
> +	END_USE(vq);
> +	return true;
> +}
> +
> +static bool virtqueue_enable_cb_delayed_packed(struct virtqueue *_vq)
> +{
> +	struct vring_virtqueue *vq = to_vvq(_vq);
> +	u16 bufs, off_wrap, used_idx, wrap_counter;
> +
> +	START_USE(vq);
> +
> +	/* We optimistically turn back on interrupts, then check if there was
> +	 * more to do. */
> +	/* Depending on the VIRTIO_RING_F_USED_EVENT_IDX feature, we need to
> +	 * either clear the flags bit or point the event index at the next
> +	 * entry. Always update the event index to keep code simple. */
> +	if (vq->event_flags_shadow == VRING_EVENT_F_DISABLE) {
> +		vq->event_flags_shadow = vq->event ? VRING_EVENT_F_DESC:
> +						     VRING_EVENT_F_ENABLE;
> +		vq->vring_packed.driver->flags = cpu_to_virtio16(_vq->vdev,
> +							vq->event_flags_shadow);
> +	}
> +
> +	/* TODO: tune this threshold */
> +	bufs = (u16)(vq->next_avail_idx - vq->last_used_idx) * 3 / 4;
> +
> +	used_idx = vq->last_used_idx + bufs;
> +	if (used_idx >= vq->vring_packed.num)
> +		used_idx -= vq->vring_packed.num;
> +
> +	wrap_counter = vq->wrap_counter;
> +	if (used_idx > vq->next_avail_idx)
> +		wrap_counter ^= 1;
> +
> +	off_wrap = used_idx | (wrap_counter << 15);
> +
> +	virtio_store_mb(vq->weak_barriers,
&vq->vring_packed.driver->off_wrap,
> +			cpu_to_virtio16(_vq->vdev, off_wrap));
> +
> +	if (more_used_packed(vq)) {
> +		END_USE(vq);
> +		return false;
> +	}
> +
> +	END_USE(vq);
> +	return true;
> +}
> +
>   /**
>    * virtqueue_enable_cb_delayed - restart callbacks after disable_cb.
>    * @vq: the struct virtqueue we're talking about.
> @@ -868,37 +1495,69 @@ EXPORT_SYMBOL_GPL(virtqueue_enable_cb);
>   bool virtqueue_enable_cb_delayed(struct virtqueue *_vq)
>   {
>   	struct vring_virtqueue *vq = to_vvq(_vq);
> -	u16 bufs;
>   
> -	START_USE(vq);
> -
> -	/* We optimistically turn back on interrupts, then check if there was
> -	 * more to do. */
> -	/* Depending on the VIRTIO_RING_F_USED_EVENT_IDX feature, we need to
> -	 * either clear the flags bit or point the event index at the next
> -	 * entry. Always update the event index to keep code simple. */
> -	if (vq->avail_flags_shadow & VRING_AVAIL_F_NO_INTERRUPT) {
> -		vq->avail_flags_shadow &= ~VRING_AVAIL_F_NO_INTERRUPT;
> -		if (!vq->event)
> -			vq->vring.avail->flags = cpu_to_virtio16(_vq->vdev,
vq->avail_flags_shadow);
> -	}
> -	/* TODO: tune this threshold */
> -	bufs = (u16)(vq->avail_idx_shadow - vq->last_used_idx) * 3 / 4;
> -
> -	virtio_store_mb(vq->weak_barriers,
> -			&vring_used_event(&vq->vring),
> -			cpu_to_virtio16(_vq->vdev, vq->last_used_idx + bufs));
> -
> -	if (unlikely((u16)(virtio16_to_cpu(_vq->vdev,
vq->vring.used->idx) - vq->last_used_idx) > bufs)) {
> -		END_USE(vq);
> -		return false;
> -	}
> -
> -	END_USE(vq);
> -	return true;
> +	return vq->packed ? virtqueue_enable_cb_delayed_packed(_vq) :
> +			    virtqueue_enable_cb_delayed_split(_vq);
>   }
>   EXPORT_SYMBOL_GPL(virtqueue_enable_cb_delayed);
>   
> +static void *virtqueue_detach_unused_buf_split(struct virtqueue *_vq)
> +{
> +	struct vring_virtqueue *vq = to_vvq(_vq);
> +	unsigned int i;
> +	void *buf;
> +
> +	START_USE(vq);
> +
> +	for (i = 0; i < vq->vring.num; i++) {
> +		if (!vq->desc_state[i].data)
> +			continue;
> +		/* detach_buf clears data, so grab it now. */
> +		buf = vq->desc_state[i].data;
> +		detach_buf_split(vq, i, NULL);
> +		vq->avail_idx_shadow--;
> +		vq->vring.avail->idx = cpu_to_virtio16(_vq->vdev,
vq->avail_idx_shadow);
> +		END_USE(vq);
> +		return buf;
> +	}
> +	/* That should have freed everything. */
> +	BUG_ON(vq->vq.num_free != vq->vring.num);
> +
> +	END_USE(vq);
> +	return NULL;
> +}
> +
> +static void *virtqueue_detach_unused_buf_packed(struct virtqueue *_vq)
> +{
> +	struct vring_virtqueue *vq = to_vvq(_vq);
> +	unsigned int i, num;
> +	void *buf;
> +
> +	START_USE(vq);
> +
> +	for (i = 0; i < vq->vring_packed.num; i++) {
> +		if (!vq->desc_state[i].data)
> +			continue;
> +		/* detach_buf clears data, so grab it now. */
> +		buf = vq->desc_state[i].data;
> +		num = detach_buf_packed(vq, i, NULL);
> +		if (vq->next_avail_idx < num) {
> +			vq->next_avail_idx = vq->vring_packed.num -
> +					(num - vq->next_avail_idx);
> +			vq->wrap_counter ^= 1;
> +		} else {
> +			vq->next_avail_idx -= num;
> +		}
> +		END_USE(vq);
> +		return buf;
> +	}
> +	/* That should have freed everything. */
> +	BUG_ON(vq->vq.num_free != vq->vring_packed.num);
> +
> +	END_USE(vq);
> +	return NULL;
> +}
> +
>   /**
>    * virtqueue_detach_unused_buf - detach first unused buffer
>    * @vq: the struct virtqueue we're talking about.
> @@ -910,27 +1569,9 @@ EXPORT_SYMBOL_GPL(virtqueue_enable_cb_delayed);
>   void *virtqueue_detach_unused_buf(struct virtqueue *_vq)
>   {
>   	struct vring_virtqueue *vq = to_vvq(_vq);
> -	unsigned int i;
> -	void *buf;
>   
> -	START_USE(vq);
> -
> -	for (i = 0; i < vq->vring.num; i++) {
> -		if (!vq->desc_state[i].data)
> -			continue;
> -		/* detach_buf clears data, so grab it now. */
> -		buf = vq->desc_state[i].data;
> -		detach_buf(vq, i, NULL);
> -		vq->avail_idx_shadow--;
> -		vq->vring.avail->idx = cpu_to_virtio16(_vq->vdev,
vq->avail_idx_shadow);
> -		END_USE(vq);
> -		return buf;
> -	}
> -	/* That should have freed everything. */
> -	BUG_ON(vq->vq.num_free != vq->vring.num);
> -
> -	END_USE(vq);
> -	return NULL;
> +	return vq->packed ? virtqueue_detach_unused_buf_packed(_vq) :
> +			    virtqueue_detach_unused_buf_split(_vq);
>   }
>   EXPORT_SYMBOL_GPL(virtqueue_detach_unused_buf);
>   
> @@ -955,7 +1596,8 @@ irqreturn_t vring_interrupt(int irq, void *_vq)
>   EXPORT_SYMBOL_GPL(vring_interrupt);
>   
>   struct virtqueue *__vring_new_virtqueue(unsigned int index,
> -					struct vring vring,
> +					union vring_union vring,
> +					bool packed,
>   					struct virtio_device *vdev,
>   					bool weak_barriers,
>   					bool context,
> @@ -963,19 +1605,20 @@ struct virtqueue *__vring_new_virtqueue(unsigned int
index,
>   					void (*callback)(struct virtqueue *),
>   					const char *name)
>   {
> -	unsigned int i;
> +	unsigned int num, i;
>   	struct vring_virtqueue *vq;
>   
> -	vq = kmalloc(sizeof(*vq) + vring.num * sizeof(struct vring_desc_state),
> +	num = packed ? vring.vring_packed.num : vring.vring_split.num;
> +
> +	vq = kmalloc(sizeof(*vq) + num * sizeof(struct vring_desc_state),
>   		     GFP_KERNEL);
>   	if (!vq)
>   		return NULL;
>   
> -	vq->vring = vring;
>   	vq->vq.callback = callback;
>   	vq->vq.vdev = vdev;
>   	vq->vq.name = name;
> -	vq->vq.num_free = vring.num;
> +	vq->vq.num_free = num;
>   	vq->vq.index = index;
>   	vq->we_own_ring = false;
>   	vq->queue_dma_addr = 0;
> @@ -984,9 +1627,8 @@ struct virtqueue *__vring_new_virtqueue(unsigned int
index,
>   	vq->weak_barriers = weak_barriers;
>   	vq->broken = false;
>   	vq->last_used_idx = 0;
> -	vq->avail_flags_shadow = 0;
> -	vq->avail_idx_shadow = 0;
>   	vq->num_added = 0;
> +	vq->packed = packed;
>   	list_add_tail(&vq->vq.list, &vdev->vqs);
>   #ifdef DEBUG
>   	vq->in_use = false;
> @@ -997,18 +1639,37 @@ struct virtqueue *__vring_new_virtqueue(unsigned int
index,
>   		!context;
>   	vq->event = virtio_has_feature(vdev, VIRTIO_RING_F_EVENT_IDX);
>   
> +	if (vq->packed) {
> +		vq->vring_packed = vring.vring_packed;
> +		vq->next_avail_idx = 0;
> +		vq->wrap_counter = 1;
> +		vq->event_flags_shadow = 0;
> +	} else {
> +		vq->vring = vring.vring_split;
> +		vq->avail_flags_shadow = 0;
> +		vq->avail_idx_shadow = 0;
> +
> +		/* Put everything in free lists. */
> +		vq->free_head = 0;
> +		for (i = 0; i < num-1; i++)
> +			vq->vring.desc[i].next = cpu_to_virtio16(vdev, i + 1);
> +	}
> +
>   	/* No callback?  Tell other side not to bother us. */
>   	if (!callback) {
> -		vq->avail_flags_shadow |= VRING_AVAIL_F_NO_INTERRUPT;
> -		if (!vq->event)
> -			vq->vring.avail->flags = cpu_to_virtio16(vdev,
vq->avail_flags_shadow);
> +		if (packed) {
> +			vq->event_flags_shadow = VRING_EVENT_F_DISABLE;
> +			vq->vring_packed.driver->flags = cpu_to_virtio16(vdev,
> +						vq->event_flags_shadow);
> +		} else {
> +			vq->avail_flags_shadow |= VRING_AVAIL_F_NO_INTERRUPT;
> +			if (!vq->event)
> +				vq->vring.avail->flags = cpu_to_virtio16(vdev,
> +						vq->avail_flags_shadow);
> +		}
>   	}
>   
> -	/* Put everything in free lists. */
> -	vq->free_head = 0;
> -	for (i = 0; i < vring.num-1; i++)
> -		vq->vring.desc[i].next = cpu_to_virtio16(vdev, i + 1);
> -	memset(vq->desc_state, 0, vring.num * sizeof(struct
vring_desc_state));
> +	memset(vq->desc_state, 0, num * sizeof(struct vring_desc_state));
>   
>   	return &vq->vq;
>   }
> @@ -1056,6 +1717,22 @@ static void vring_free_queue(struct virtio_device
*vdev, size_t size,
>   	}
>   }
>   
> +static inline int
> +__vring_size(unsigned int num, unsigned long align, bool packed)
> +{
> +	return packed ? vring_packed_size(num, align) : vring_size(num, align);
> +}
> +
> +static inline void vring_packed_init(struct vring_packed *vr, unsigned int
num,
> +				     void *p, unsigned long align)
> +{
> +	vr->num = num;
> +	vr->desc = p;
> +	vr->driver = (void *)(((uintptr_t)p + sizeof(struct vring_packed_desc)
> +		* num + align - 1) & ~(align - 1));
> +	vr->device = vr->driver + 1;
> +}
> +
>   struct virtqueue *vring_create_virtqueue(
>   	unsigned int index,
>   	unsigned int num,
> @@ -1072,7 +1749,8 @@ struct virtqueue *vring_create_virtqueue(
>   	void *queue = NULL;
>   	dma_addr_t dma_addr;
>   	size_t queue_size_in_bytes;
> -	struct vring vring;
> +	union vring_union vring;
> +	bool packed;
>   
>   	/* We assume num is a power of 2. */
>   	if (num & (num - 1)) {
> @@ -1080,9 +1758,13 @@ struct virtqueue *vring_create_virtqueue(
>   		return NULL;
>   	}
>   
> +	packed = virtio_has_feature(vdev, VIRTIO_F_RING_PACKED);
> +
>   	/* TODO: allocate each queue chunk individually */
> -	for (; num && vring_size(num, vring_align) > PAGE_SIZE; num /=
2) {
> -		queue = vring_alloc_queue(vdev, vring_size(num, vring_align),
> +	for (; num && __vring_size(num, vring_align, packed) >
PAGE_SIZE;
> +			num /= 2) {
> +		queue = vring_alloc_queue(vdev, __vring_size(num, vring_align,
> +							     packed),
>   					  &dma_addr,
>   					  GFP_KERNEL|__GFP_NOWARN|__GFP_ZERO);
>   		if (queue)
> @@ -1094,17 +1776,21 @@ struct virtqueue *vring_create_virtqueue(
>   
>   	if (!queue) {
>   		/* Try to get a single page. You are my only hope! */
> -		queue = vring_alloc_queue(vdev, vring_size(num, vring_align),
> +		queue = vring_alloc_queue(vdev, __vring_size(num, vring_align,
> +							     packed),
>   					  &dma_addr, GFP_KERNEL|__GFP_ZERO);
>   	}
>   	if (!queue)
>   		return NULL;
>   
> -	queue_size_in_bytes = vring_size(num, vring_align);
> -	vring_init(&vring, num, queue, vring_align);
> +	queue_size_in_bytes = __vring_size(num, vring_align, packed);
> +	if (packed)
> +		vring_packed_init(&vring.vring_packed, num, queue, vring_align);
> +	else
> +		vring_init(&vring.vring_split, num, queue, vring_align);
>   
> -	vq = __vring_new_virtqueue(index, vring, vdev, weak_barriers, context,
> -				   notify, callback, name);
> +	vq = __vring_new_virtqueue(index, vring, packed, vdev, weak_barriers,
> +				   context, notify, callback, name);
>   	if (!vq) {
>   		vring_free_queue(vdev, queue_size_in_bytes, queue,
>   				 dma_addr);
> @@ -1130,10 +1816,17 @@ struct virtqueue *vring_new_virtqueue(unsigned int
index,
>   				      void (*callback)(struct virtqueue *vq),
>   				      const char *name)
>   {
> -	struct vring vring;
> -	vring_init(&vring, num, pages, vring_align);
> -	return __vring_new_virtqueue(index, vring, vdev, weak_barriers, context,
> -				     notify, callback, name);
> +	union vring_union vring;
> +	bool packed;
> +
> +	packed = virtio_has_feature(vdev, VIRTIO_F_RING_PACKED);
> +	if (packed)
> +		vring_packed_init(&vring.vring_packed, num, pages, vring_align);
> +	else
> +		vring_init(&vring.vring_split, num, pages, vring_align);
> +
> +	return __vring_new_virtqueue(index, vring, packed, vdev, weak_barriers,
> +				     context, notify, callback, name);
>   }
>   EXPORT_SYMBOL_GPL(vring_new_virtqueue);
>   
> @@ -1143,7 +1836,9 @@ void vring_del_virtqueue(struct virtqueue *_vq)
>   
>   	if (vq->we_own_ring) {
>   		vring_free_queue(vq->vq.vdev, vq->queue_size_in_bytes,
> -				 vq->vring.desc, vq->queue_dma_addr);
> +				 vq->packed ? (void *)vq->vring_packed.desc :
> +					      (void *)vq->vring.desc,
> +				 vq->queue_dma_addr);
>   	}
>   	list_del(&_vq->list);
>   	kfree(vq);
> @@ -1157,14 +1852,18 @@ void vring_transport_features(struct virtio_device
*vdev)
>   
>   	for (i = VIRTIO_TRANSPORT_F_START; i < VIRTIO_TRANSPORT_F_END; i++) {
>   		switch (i) {
> -		case VIRTIO_RING_F_INDIRECT_DESC:
> +#if 0
> +		case VIRTIO_RING_F_INDIRECT_DESC: // FIXME not tested yet.
>   			break;
> -		case VIRTIO_RING_F_EVENT_IDX:
> +		case VIRTIO_RING_F_EVENT_IDX: // FIXME probably not work.
>   			break;
> +#endif
It would be better if you can split EVENT_IDX and INDIRECT_DESC into 
separate patches too.

Thanks
>   		case VIRTIO_F_VERSION_1:
>   			break;
>   		case VIRTIO_F_IOMMU_PLATFORM:
>   			break;
> +		case VIRTIO_F_RING_PACKED:
> +			break;
>   		default:
>   			/* We don't understand this bit. */
>   			__virtio_clear_bit(vdev, i);
> @@ -1185,7 +1884,7 @@ unsigned int virtqueue_get_vring_size(struct
virtqueue *_vq)
>   
>   	struct vring_virtqueue *vq = to_vvq(_vq);
>   
> -	return vq->vring.num;
> +	return vq->packed ? vq->vring_packed.num : vq->vring.num;
>   }
>   EXPORT_SYMBOL_GPL(virtqueue_get_vring_size);
>   
> @@ -1228,6 +1927,10 @@ dma_addr_t virtqueue_get_avail_addr(struct virtqueue
*_vq)
>   
>   	BUG_ON(!vq->we_own_ring);
>   
> +	if (vq->packed)
> +		return vq->queue_dma_addr + ((char *)vq->vring_packed.driver -
> +				(char *)vq->vring_packed.desc);
> +
>   	return vq->queue_dma_addr +
>   		((char *)vq->vring.avail - (char *)vq->vring.desc);
>   }
> @@ -1239,11 +1942,16 @@ dma_addr_t virtqueue_get_used_addr(struct virtqueue
*_vq)
>   
>   	BUG_ON(!vq->we_own_ring);
>   
> +	if (vq->packed)
> +		return vq->queue_dma_addr + ((char *)vq->vring_packed.device -
> +				(char *)vq->vring_packed.desc);
> +
>   	return vq->queue_dma_addr +
>   		((char *)vq->vring.used - (char *)vq->vring.desc);
>   }
>   EXPORT_SYMBOL_GPL(virtqueue_get_used_addr);
>   
> +/* Only available for split ring */
>   const struct vring *virtqueue_get_vring(struct virtqueue *vq)
>   {
>   	return &to_vvq(vq)->vring;
> diff --git a/include/linux/virtio_ring.h b/include/linux/virtio_ring.h
> index bbf32524ab27..a0075894ad16 100644
> --- a/include/linux/virtio_ring.h
> +++ b/include/linux/virtio_ring.h
> @@ -60,6 +60,11 @@ static inline void virtio_store_mb(bool weak_barriers,
>   struct virtio_device;
>   struct virtqueue;
>   
> +union vring_union {
> +	struct vring vring_split;
> +	struct vring_packed vring_packed;
> +};
> +
>   /*
>    * Creates a virtqueue and allocates the descriptor ring.  If
>    * may_reduce_num is set, then this may allocate a smaller ring than
> @@ -79,7 +84,8 @@ struct virtqueue *vring_create_virtqueue(unsigned int
index,
>   
>   /* Creates a virtqueue with a custom layout. */
>   struct virtqueue *__vring_new_virtqueue(unsigned int index,
> -					struct vring vring,
> +					union vring_union vring,
> +					bool packed,
>   					struct virtio_device *vdev,
>   					bool weak_barriers,
>   					bool ctx,
> diff --git a/include/uapi/linux/virtio_config.h
b/include/uapi/linux/virtio_config.h
> index 308e2096291f..a6e392325e3a 100644
> --- a/include/uapi/linux/virtio_config.h
> +++ b/include/uapi/linux/virtio_config.h
> @@ -49,7 +49,7 @@
>    * transport being used (eg. virtio_ring), the rest are per-device
feature
>    * bits. */
>   #define VIRTIO_TRANSPORT_F_START	28
> -#define VIRTIO_TRANSPORT_F_END		34
> +#define VIRTIO_TRANSPORT_F_END		36
>   
>   #ifndef VIRTIO_CONFIG_NO_LEGACY
>   /* Do we get callbacks when the ring is completely used, even if
we've
> @@ -71,4 +71,14 @@
>    * this is for compatibility with legacy systems.
>    */
>   #define VIRTIO_F_IOMMU_PLATFORM		33
> +
> +/* This feature indicates support for the packed virtqueue layout. */
> +#define VIRTIO_F_RING_PACKED		34
> +
> +/*
> + * This feature indicates that all buffers are used by the device
> + * in the same order in which they have been made available.
> + */
> +#define VIRTIO_F_IN_ORDER		35
> +
>   #endif /* _UAPI_LINUX_VIRTIO_CONFIG_H */
> diff --git a/include/uapi/linux/virtio_ring.h
b/include/uapi/linux/virtio_ring.h
> index 6d5d5faa989b..735d4207c988 100644
> --- a/include/uapi/linux/virtio_ring.h
> +++ b/include/uapi/linux/virtio_ring.h
> @@ -44,6 +44,9 @@
>   /* This means the buffer contains a list of buffer descriptors. */
>   #define VRING_DESC_F_INDIRECT	4
>   
> +#define VRING_DESC_F_AVAIL(b)	((b) << 7)
> +#define VRING_DESC_F_USED(b)	((b) << 15)
> +
>   /* The Host uses this in used->flags to advise the Guest: don't
kick me when
>    * you add a buffer.  It's unreliable, so it's simply an
optimization.  Guest
>    * will still kick if it's out of buffers. */
> @@ -53,6 +56,10 @@
>    * optimization.  */
>   #define VRING_AVAIL_F_NO_INTERRUPT	1
>   
> +#define VRING_EVENT_F_ENABLE	0x0
> +#define VRING_EVENT_F_DISABLE	0x1
> +#define VRING_EVENT_F_DESC	0x2
> +
>   /* We support indirect buffer descriptors */
>   #define VIRTIO_RING_F_INDIRECT_DESC	28
>   
> @@ -171,4 +178,58 @@ static inline int vring_need_event(__u16 event_idx,
__u16 new_idx, __u16 old)
>   	return (__u16)(new_idx - event_idx - 1) < (__u16)(new_idx - old);
>   }
>   
> +struct vring_packed_desc_event {
> +	/* __virtio16 off  : 15; // Descriptor Event Offset
> +	 * __virtio16 wrap : 1;  // Descriptor Event Wrap Counter */
> +	__virtio16 off_wrap;
> +	/* __virtio16 flags : 2; // Descriptor Event Flags */
> +	__virtio16 flags;
> +};
> +
> +struct vring_packed_desc {
> +	/* Buffer Address. */
> +	__virtio64 addr;
> +	/* Buffer Length. */
> +	__virtio32 len;
> +	/* Buffer ID. */
> +	__virtio16 id;
> +	/* The flags depending on descriptor type. */
> +	__virtio16 flags;
> +};
> +
> +struct vring_packed {
> +	unsigned int num;
> +
> +	struct vring_packed_desc *desc;
> +
> +	struct vring_packed_desc_event *driver;
> +
> +	struct vring_packed_desc_event *device;
> +};
> +
> +/* The standard layout for the packed ring is a continuous chunk of memory
> + * which looks like this.
> + *
> + * struct vring_packed
> + * {
> + *	// The actual descriptors (16 bytes each)
> + *	struct vring_packed_desc desc[num];
> + *
> + *	// Padding to the next align boundary.
> + *	char pad[];
> + *
> + *	// Driver Event Suppression
> + *	struct vring_packed_desc_event driver;
> + *
> + *	// Device Event Suppression
> + *	struct vring_packed_desc_event device;
> + * };
> + */
> +
> +static inline unsigned vring_packed_size(unsigned int num, unsigned long
align)
> +{
> +	return ((sizeof(struct vring_packed_desc) * num + align - 1)
> +		& ~(align - 1)) + sizeof(struct vring_packed_desc_event) * 2;
> +}
> +
>   #endif /* _UAPI_LINUX_VIRTIO_RING_H */

On Fri, Apr 13, 2018 at 12:30:24PM +0800, Jason Wang
wrote:
> On 2018?04?01? 22:12, Tiwei Bie wrote:
> > Hello everyone,
> > 
> > This RFC implements packed ring support for virtio driver.
> > 
> > The code was tested with DPDK vhost (testpmd/vhost-PMD) implemented
> > by Jens at http://dpdk.org/ml/archives/dev/2018-January/089417.html
> > Minor changes are needed for the vhost code, e.g. to kick the guest.
> > 
> > TODO:
> > - Refinements and bug fixes;
> > - Split into small patches;
> > - Test indirect descriptor support;
> > - Test/fix event suppression support;
> > - Test devices other than net;
> > 
> > RFC v1 -> RFC v2:
> > - Add indirect descriptor support - compile test only;
> > - Add event suppression supprt - compile test only;
> > - Move vring_packed_init() out of uapi (Jason, MST);
> > - Merge two loops into one in virtqueue_add_packed() (Jason);
> > - Split vring_unmap_one() for packed ring and split ring (Jason);
> > - Avoid using '%' operator (Jason);
> > - Rename free_head -> next_avail_idx (Jason);
> > - Add comments for virtio_wmb() in virtqueue_add_packed() (Jason);
> > - Some other refinements and bug fixes;
> > 
> > Thanks!
> > 
> > Signed-off-by: Tiwei Bie <tiwei.bie at intel.com>
> > ---
> >   drivers/virtio/virtio_ring.c       | 1094
+++++++++++++++++++++++++++++-------
> >   include/linux/virtio_ring.h        |    8 +-
> >   include/uapi/linux/virtio_config.h |   12 +-
> >   include/uapi/linux/virtio_ring.h   |   61 ++
> >   4 files changed, 980 insertions(+), 195 deletions(-)
[...]
> > +static struct vring_packed_desc *alloc_indirect_packed(struct
virtqueue *_vq,
> > +						       unsigned int total_sg,
> > +						       gfp_t gfp)
> > +{
> > +	struct vring_packed_desc *desc;
> > +
> > +	/*
> > +	 * We require lowmem mappings for the descriptors because
> > +	 * otherwise virt_to_phys will give us bogus addresses in the
> > +	 * virtqueue.
> > +	 */
> > +	gfp &= ~__GFP_HIGHMEM;
> > +
> > +	desc = kmalloc(total_sg * sizeof(struct vring_packed_desc), gfp);
> 
> Can we simply check vq->packed here to avoid duplicating helpers?
Then it would be something like this:

static void *alloc_indirect(struct virtqueue *_vq, unsigned int total_sg,
			    gfp_t gfp)
{
	struct vring_virtqueue *vq = to_vvq(_vq);
	void *data;

	/*
	 * We require lowmem mappings for the descriptors because
	 * otherwise virt_to_phys will give us bogus addresses in the
	 * virtqueue.
	 */
	gfp &= ~__GFP_HIGHMEM;

	if (vq->packed) {
		data = kmalloc(total_sg * sizeof(struct vring_packed_desc),
				gfp);
		if (!data)
			return NULL;
	} else {
		struct vring_desc *desc;
		unsigned int i;

		desc = kmalloc(total_sg * sizeof(struct vring_desc), gfp);
		if (!desc)
			return NULL;

		for (i = 0; i < total_sg; i++)
			desc[i].next = cpu_to_virtio16(_vq->vdev, i + 1);

		data = desc;
	}

	return data;
}

I would prefer to have two simpler helpers (and to the callers,
it's already very clear about which one they should call), i.e.
the current implementation:

static struct vring_packed_desc *alloc_indirect_packed(struct virtqueue *_vq,
						       unsigned int total_sg,
						       gfp_t gfp)
{
	struct vring_packed_desc *desc;

	/*
	 * We require lowmem mappings for the descriptors because
	 * otherwise virt_to_phys will give us bogus addresses in the
	 * virtqueue.
	 */
	gfp &= ~__GFP_HIGHMEM;

	desc = kmalloc(total_sg * sizeof(struct vring_packed_desc), gfp);

	return desc;
}

static struct vring_desc *alloc_indirect_split(struct virtqueue *_vq,
					       unsigned int total_sg,
					       gfp_t gfp)
{
	struct vring_desc *desc;
	unsigned int i;

	/*
	 * We require lowmem mappings for the descriptors because
	 * otherwise virt_to_phys will give us bogus addresses in the
	 * virtqueue.
	 */
	gfp &= ~__GFP_HIGHMEM;

	desc = kmalloc(total_sg * sizeof(struct vring_desc), gfp);
	if (!desc)
		return NULL;

	for (i = 0; i < total_sg; i++)
		desc[i].next = cpu_to_virtio16(_vq->vdev, i + 1);
	return desc;
}
> 
> > +
> > +	return desc;
> > +}
[...]
> > +static inline int virtqueue_add_packed(struct virtqueue *_vq,
> > +				       struct scatterlist *sgs[],
> > +				       unsigned int total_sg,
> > +				       unsigned int out_sgs,
> > +				       unsigned int in_sgs,
> > +				       void *data,
> > +				       void *ctx,
> > +				       gfp_t gfp)
> > +{
> > +	struct vring_virtqueue *vq = to_vvq(_vq);
> > +	struct vring_packed_desc *desc;
> > +	struct scatterlist *sg;
> > +	unsigned int i, n, descs_used, uninitialized_var(prev), err_idx;
> > +	__virtio16 uninitialized_var(head_flags), flags;
> > +	int head, wrap_counter;
> > +	bool indirect;
> > +
> > +	START_USE(vq);
> > +
> > +	BUG_ON(data == NULL);
> > +	BUG_ON(ctx && vq->indirect);
> > +
> > +	if (unlikely(vq->broken)) {
> > +		END_USE(vq);
> > +		return -EIO;
> > +	}
> > +
> > +#ifdef DEBUG
> > +	{
> > +		ktime_t now = ktime_get();
> > +
> > +		/* No kick or get, with .1 second between?  Warn. */
> > +		if (vq->last_add_time_valid)
> > +			WARN_ON(ktime_to_ms(ktime_sub(now, vq->last_add_time))
> > +					    > 100);
> > +		vq->last_add_time = now;
> > +		vq->last_add_time_valid = true;
> > +	}
> > +#endif
> > +
> > +	BUG_ON(total_sg == 0);
> > +
> > +	head = vq->next_avail_idx;
> > +	wrap_counter = vq->wrap_counter;
> > +
> > +	/* If the host supports indirect descriptor tables, and we have
multiple
> > +	 * buffers, then go indirect. FIXME: tune this threshold */
> > +	if (vq->indirect && total_sg > 1 &&
vq->vq.num_free)
> 
> Let's introduce a helper like virtqueue_need_indirect() to avoid
duplicating
> codes and FIXME.
Okay.
> 
> > +		desc = alloc_indirect_packed(_vq, total_sg, gfp);
> > +	else {
> > +		desc = NULL;
> > +		WARN_ON_ONCE(total_sg > vq->vring_packed.num &&
!vq->indirect);
> > +	}
> > +
> > +	if (desc) {
> > +		/* Use a single buffer which doesn't continue */
> > +		indirect = true;
> > +		/* Set up rest to use this indirect table. */
> > +		i = 0;
> > +		descs_used = 1;
> > +	} else {
> > +		indirect = false;
> > +		desc = vq->vring_packed.desc;
> > +		i = head;
> > +		descs_used = total_sg;
> > +	}
> > +
> > +	if (vq->vq.num_free < descs_used) {
> > +		pr_debug("Can't add buf len %i - avail = %i\n",
> > +			 descs_used, vq->vq.num_free);
> > +		/* FIXME: for historical reasons, we force a notify here if
> > +		 * there are outgoing parts to the buffer.  Presumably the
> > +		 * host should service the ring ASAP. */
> > +		if (out_sgs)
> > +			vq->notify(&vq->vq);
> > +		if (indirect)
> > +			kfree(desc);
> > +		END_USE(vq);
> > +		return -ENOSPC;
> > +	}
> > +
> > +	for (n = 0; n < out_sgs + in_sgs; n++) {
> > +		for (sg = sgs[n]; sg; sg = sg_next(sg)) {
> > +			dma_addr_t addr = vring_map_one_sg(vq, sg, n < out_sgs ?
> > +						DMA_TO_DEVICE : DMA_FROM_DEVICE);
> > +			if (vring_mapping_error(vq, addr))
> > +				goto unmap_release;
> > +
> > +			flags = cpu_to_virtio16(_vq->vdev, VRING_DESC_F_NEXT |
> > +					(n < out_sgs ? 0 : VRING_DESC_F_WRITE) |
> > +					VRING_DESC_F_AVAIL(vq->wrap_counter) |
> > +					VRING_DESC_F_USED(!vq->wrap_counter));
> > +			if (!indirect && i == head)
> > +				head_flags = flags;
> > +			else
> > +				desc[i].flags = flags;
> > +
> > +			desc[i].addr = cpu_to_virtio64(_vq->vdev, addr);
> > +			desc[i].len = cpu_to_virtio32(_vq->vdev, sg->length);
> > +			desc[i].id = cpu_to_virtio32(_vq->vdev, head);
> 
> Similar to V1, we only need this for the last descriptor.
Okay, will just set it for the last desc.
> 
> > +			prev = i;
> 
> It looks to me there's no need to track prev inside the loop here.
> 
> > +			i++;
> > +			if (!indirect && i >= vq->vring_packed.num) {
> > +				i = 0;
> > +				vq->wrap_counter ^= 1;
> > +			}
> > +		}
> > +	}
> > +	/* Last one doesn't continue. */
> > +	if (total_sg == 1)
> > +		head_flags &= cpu_to_virtio16(_vq->vdev,
~VRING_DESC_F_NEXT);
> > +	else
> > +		desc[prev].flags &= cpu_to_virtio16(_vq->vdev,
~VRING_DESC_F_NEXT);
> 
> The only case when prev != i - 1 is i == 0, we can add a if here.
It's just a mirror of the existing implementation in split ring.
It seems that split ring implementation needs this just because
it's much harder for it to find the prev, which is not true for
packed ring. So I'll take your suggestion. Thanks!
> 
[...]
> > +static bool virtqueue_kick_prepare_packed(struct virtqueue *_vq)
> > +{
> > +	struct vring_virtqueue *vq = to_vvq(_vq);
> > +	u16 new, old, off_wrap;
> > +	bool needs_kick;
> > +
> > +	START_USE(vq);
> > +	/* We need to expose the new flags value before checking
notification
> > +	 * suppressions. */
> > +	virtio_mb(vq->weak_barriers);
> > +
> > +	old = vq->next_avail_idx - vq->num_added;
> > +	new = vq->next_avail_idx;
> > +	vq->num_added = 0;
> > +
> > +#ifdef DEBUG
> > +	if (vq->last_add_time_valid) {
> > +		WARN_ON(ktime_to_ms(ktime_sub(ktime_get(),
> > +					      vq->last_add_time)) > 100);
> > +	}
> > +	vq->last_add_time_valid = false;
> > +#endif
> > +
> > +	off_wrap = virtio16_to_cpu(_vq->vdev,
vq->vring_packed.device->off_wrap);
> > +
> > +	if (vq->event) {
> 
> It looks to me we should examine RING_EVENT_FLAGS_DESC in desc_event_flags
> instead of vq->event here. Spec does not forces to use evenf_off and
> event_wrap if event index is enabled.
> 
> > +		// FIXME: fix this!
> > +		needs_kick = ((off_wrap >> 15) == vq->wrap_counter)
&&
> > +			     vring_need_event(off_wrap & ~(1<<15), new, old);
> 
> Why need a & here?
Because wrap_counter (the most significant bit in off_wrap)
isn't part of the index.
> 
> > +	} else {
> 
> Need a smp_rmb() to make sure desc_event_flags was checked before flags.
I don't get your point, if my understanding is correct,
desc_event_flags is vq->vring_packed.device->flags. So
what's the "flags"?
> 
> > +		needs_kick = (vq->vring_packed.device->flags !> > +			 
cpu_to_virtio16(_vq->vdev, VRING_EVENT_F_DISABLE));
> > +	}
> > +	END_USE(vq);
> > +	return needs_kick;
> > +}
[...]
> > +static int detach_buf_packed(struct vring_virtqueue *vq, unsigned int
head,
> > +			      void **ctx)
> > +{
> > +	struct vring_packed_desc *desc;
> > +	unsigned int i, j;
> > +
> > +	/* Clear data ptr. */
> > +	vq->desc_state[head].data = NULL;
> > +
> > +	i = head;
> > +
> > +	for (j = 0; j < vq->desc_state[head].num; j++) {
> > +		desc = &vq->vring_packed.desc[i];
> > +		vring_unmap_one_packed(vq, desc);
> > +		desc->flags = 0x0;
> 
> Looks like this is unnecessary.
It's safer to zero it. If we don't zero it, after we
call virtqueue_detach_unused_buf_packed() which calls
this function, the desc is still available to the
device.
> 
> > +		i++;
> > +		if (i >= vq->vring_packed.num)
> > +			i = 0;
> > +	}
[...]
> > +static unsigned virtqueue_enable_cb_prepare_packed(struct virtqueue
*_vq)
> > +{
> > +	struct vring_virtqueue *vq = to_vvq(_vq);
> > +	u16 last_used_idx, wrap_counter, off_wrap;
> > +
> > +	START_USE(vq);
> > +
> > +	last_used_idx = vq->last_used_idx;
> > +	wrap_counter = vq->wrap_counter;
> > +
> > +	if (last_used_idx > vq->next_avail_idx)
> > +		wrap_counter ^= 1;
> > +
> > +	off_wrap = last_used_idx | (wrap_counter << 15);
> > +
> > +	/* We optimistically turn back on interrupts, then check if there
was
> > +	 * more to do. */
> > +	/* Depending on the VIRTIO_RING_F_EVENT_IDX feature, we need to
> > +	 * either clear the flags bit or point the event index at the next
> > +	 * entry. Always do both to keep code simple. */
> > +	if (vq->event_flags_shadow == VRING_EVENT_F_DISABLE) {
> > +		vq->event_flags_shadow = vq->event ? VRING_EVENT_F_DESC:
> > +						     VRING_EVENT_F_ENABLE;
> > +		vq->vring_packed.driver->flags =
cpu_to_virtio16(_vq->vdev,
> > +							vq->event_flags_shadow);
> > +	}
> 
> A smp_wmb() is missed here?
> 
> > +	vq->vring_packed.driver->off_wrap =
cpu_to_virtio16(_vq->vdev, off_wrap);
> 
> And according to the spec, it looks to me write a VRING_EVENT_F_ENABLE is
> sufficient here.
I didn't think much when implementing the event suppression
for packed ring previously. After I saw your comments, I found
something new. Indeed, unlike the split ring, for the packed
ring, spec doesn't say we must use VRING_EVENT_F_DESC when
EVENT_IDX is negotiated. So do you think below thought is
right or makes sense?

- For virtqueue_enable_cb_prepare(), we just need to enable
  the ring by setting flags to VRING_EVENT_F_ENABLE in any
  case.

- We will try to use VRING_EVENT_F_DESC (if EVENT_IDX is
  negotiated) only when we want to delay the interrupts
  virtqueue_enable_cb_delayed().
> 
> > +	END_USE(vq);
> > +	return last_used_idx;
> > +}
> > +
[...]
> > @@ -1157,14 +1852,18 @@ void vring_transport_features(struct
virtio_device *vdev)
> >   	for (i = VIRTIO_TRANSPORT_F_START; i < VIRTIO_TRANSPORT_F_END;
i++) {
> >   		switch (i) {
> > -		case VIRTIO_RING_F_INDIRECT_DESC:
> > +#if 0
> > +		case VIRTIO_RING_F_INDIRECT_DESC: // FIXME not tested yet.
> >   			break;
> > -		case VIRTIO_RING_F_EVENT_IDX:
> > +		case VIRTIO_RING_F_EVENT_IDX: // FIXME probably not work.
> >   			break;
> > +#endif
> 
> It would be better if you can split EVENT_IDX and INDIRECT_DESC into
> separate patches too.
Sure. Will do it in the next version.

Thanks for the review!
> 
> Thanks
>

On Sun, Apr 01, 2018 at 10:12:16PM +0800, Tiwei Bie
wrote:
> +static inline bool more_used(const struct vring_virtqueue *vq)
> +{
> +	return vq->packed ? more_used_packed(vq) : more_used_split(vq);
> +}
> +
> +void *virtqueue_get_buf_ctx_split(struct virtqueue *_vq, unsigned int
*len,
> +				  void **ctx)
> +{
> +	struct vring_virtqueue *vq = to_vvq(_vq);
> +	void *ret;
> +	unsigned int i;
> +	u16 last_used;
> +
> +	START_USE(vq);
> +
> +	if (unlikely(vq->broken)) {
> +		END_USE(vq);
> +		return NULL;
> +	}
> +
> +	if (!more_used(vq)) {
> +		pr_debug("No more buffers in queue\n");
> +		END_USE(vq);
> +		return NULL;
> +	}
So virtqueue_get_buf_ctx_split should only call more_used_split.

to avoid such issues I think we should lay out the code like this:

XXX_split

XXX_packed

XXX wrappers
> +/* The standard layout
I'd drop standard here.
> for the packed ring is a continuous chunk of memory
> + * which looks like this.
> + *
> + * struct vring_packed
> + * {
Can the opening bracket go on the prev line pls?
> + *	// The actual descriptors (16 bytes each)
> + *	struct vring_packed_desc desc[num];
> + *
> + *	// Padding to the next align boundary.
> + *	char pad[];
> + *
> + *	// Driver Event Suppression
> + *	struct vring_packed_desc_event driver;
> + *
> + *	// Device Event Suppression
> + *	struct vring_packed_desc_event device;
Maybe that's how our driver does it but it's not based on spec
so I don't think this belongs in the header.
> + * };
> + */
> +
> +static inline unsigned vring_packed_size(unsigned int num, unsigned long
align)
> +{
> +	return ((sizeof(struct vring_packed_desc) * num + align - 1)
> +		& ~(align - 1)) + sizeof(struct vring_packed_desc_event) * 2;
> +}
> +
Cant say this API makes sense for me.

>  #endif /* _UAPI_LINUX_VIRTIO_RING_H */
> -- 
> 2.11.0

On Fri, Apr 13, 2018 at 06:22:45PM +0300, Michael S. Tsirkin
wrote:
> On Sun, Apr 01, 2018 at 10:12:16PM +0800, Tiwei Bie wrote:
> > +static inline bool more_used(const struct vring_virtqueue *vq)
> > +{
> > +	return vq->packed ? more_used_packed(vq) : more_used_split(vq);
> > +}
> > +
> > +void *virtqueue_get_buf_ctx_split(struct virtqueue *_vq, unsigned int
*len,
> > +				  void **ctx)
> > +{
> > +	struct vring_virtqueue *vq = to_vvq(_vq);
> > +	void *ret;
> > +	unsigned int i;
> > +	u16 last_used;
> > +
> > +	START_USE(vq);
> > +
> > +	if (unlikely(vq->broken)) {
> > +		END_USE(vq);
> > +		return NULL;
> > +	}
> > +
> > +	if (!more_used(vq)) {
> > +		pr_debug("No more buffers in queue\n");
> > +		END_USE(vq);
> > +		return NULL;
> > +	}
> 
> So virtqueue_get_buf_ctx_split should only call more_used_split.
Yeah, you're right! Will fix this in the next version.
> 
> to avoid such issues I think we should lay out the code like this:
> 
> XXX_split
> 
> XXX_packed
> 
> XXX wrappers
I'll do it. Thanks for the suggestion!
> 
> > +/* The standard layout
> 
> I'd drop standard here.
Got it. I'll drop the word "standard".
> 
> > for the packed ring is a continuous chunk of memory
> > + * which looks like this.
> > + *
> > + * struct vring_packed
> > + * {
> 
> Can the opening bracket go on the prev line pls?
Sure.
> 
> > + *	// The actual descriptors (16 bytes each)
> > + *	struct vring_packed_desc desc[num];
> > + *
> > + *	// Padding to the next align boundary.
> > + *	char pad[];
> > + *
> > + *	// Driver Event Suppression
> > + *	struct vring_packed_desc_event driver;
> > + *
> > + *	// Device Event Suppression
> > + *	struct vring_packed_desc_event device;
> 
> Maybe that's how our driver does it but it's not based on spec
> so I don't think this belongs in the header.
I will move it to the place where vring_packed_init()
is defined.
> 
> > + * };
> > + */
> > +
> > +static inline unsigned vring_packed_size(unsigned int num, unsigned
long align)
> > +{
> > +	return ((sizeof(struct vring_packed_desc) * num + align - 1)
> > +		& ~(align - 1)) + sizeof(struct vring_packed_desc_event) * 2;
> > +}
> > +
> 
> Cant say this API makes sense for me.
Hmm, do you have any suggestion? Also move it out of this header?

Thanks for the review! :)

Best regards,
Tiwei Bie
> 
> 
> >  #endif /* _UAPI_LINUX_VIRTIO_RING_H */
> > -- 
> > 2.11.0

On 2018?04?01? 22:12, Tiwei Bie wrote:
> Hello everyone,
>
> This RFC implements packed ring support for virtio driver.
>
> The code was tested with DPDK vhost (testpmd/vhost-PMD) implemented
> by Jens at http://dpdk.org/ml/archives/dev/2018-January/089417.html
> Minor changes are needed for the vhost code, e.g. to kick the guest.
>
> TODO:
> - Refinements and bug fixes;
> - Split into small patches;
> - Test indirect descriptor support;
> - Test/fix event suppression support;
> - Test devices other than net;
>
> RFC v1 -> RFC v2:
> - Add indirect descriptor support - compile test only;
> - Add event suppression supprt - compile test only;
> - Move vring_packed_init() out of uapi (Jason, MST);
> - Merge two loops into one in virtqueue_add_packed() (Jason);
> - Split vring_unmap_one() for packed ring and split ring (Jason);
> - Avoid using '%' operator (Jason);
> - Rename free_head -> next_avail_idx (Jason);
> - Add comments for virtio_wmb() in virtqueue_add_packed() (Jason);
> - Some other refinements and bug fixes;
>
> Thanks!
>
> Signed-off-by: Tiwei Bie <tiwei.bie at intel.com>
> ---
>   drivers/virtio/virtio_ring.c       | 1094
+++++++++++++++++++++++++++++-------
>   include/linux/virtio_ring.h        |    8 +-
>   include/uapi/linux/virtio_config.h |   12 +-
>   include/uapi/linux/virtio_ring.h   |   61 ++
>   4 files changed, 980 insertions(+), 195 deletions(-)
>
> diff --git a/drivers/virtio/virtio_ring.c b/drivers/virtio/virtio_ring.c
> index 71458f493cf8..0515dca34d77 100644
> --- a/drivers/virtio/virtio_ring.c
> +++ b/drivers/virtio/virtio_ring.c
> @@ -58,14 +58,15 @@
>   
[...]
> +
> +	if (vq->indirect) {
> +		u32 len;
> +
> +		desc = vq->desc_state[head].indir_desc;
> +		/* Free the indirect table, if any, now that it's unmapped. */
> +		if (!desc)
> +			goto out;
> +
> +		len = virtio32_to_cpu(vq->vq.vdev,
> +				      vq->vring_packed.desc[head].len);
> +
> +		BUG_ON(!(vq->vring_packed.desc[head].flags &
> +			 cpu_to_virtio16(vq->vq.vdev, VRING_DESC_F_INDIRECT)));
It looks to me spec does not force to keep VRING_DESC_F_INDIRECT here. 
So we can safely remove this BUG_ON() here.
> +		BUG_ON(len == 0 || len % sizeof(struct vring_packed_desc));
Len could be ignored for used descriptor according to the spec, so we 
need remove this BUG_ON() too.

The reason is we don't touch descriptor ring in the case of split, so 
BUG_ON()s may help there.
> +
> +		for (j = 0; j < len / sizeof(struct vring_packed_desc); j++)
> +			vring_unmap_one_packed(vq, &desc[j]);
> +
> +		kfree(desc);
> +		vq->desc_state[head].indir_desc = NULL;
> +	} else if (ctx) {
> +		*ctx = vq->desc_state[head].indir_desc;
> +	}
> +
> +out:
> +	return vq->desc_state[head].num;
> +}
> +
> +static inline bool more_used_split(const struct vring_virtqueue *vq)
>   {
>   	return vq->last_used_idx != virtio16_to_cpu(vq->vq.vdev,
vq->vring.used->idx);
>   }
>   
> +static inline bool more_used_packed(const struct vring_virtqueue *vq)
> +{
> +	u16 last_used, flags;
> +	bool avail, used;
> +
> +	if (vq->vq.num_free == vq->vring_packed.num)
> +		return false;
> +
> +	last_used = vq->last_used_idx;
> +	flags = virtio16_to_cpu(vq->vq.vdev,
> +				vq->vring_packed.desc[last_used].flags);
> +	avail = flags & VRING_DESC_F_AVAIL(1);
> +	used = flags & VRING_DESC_F_USED(1);
> +
> +	return avail == used;
> +}
This looks interesting, spec said:

"
Thus VIRTQ_DESC_F_AVAIL and VIRTQ_DESC_F_USED bits are different for an 
available descriptor and
equal for a used descriptor.
Note that this observation is mostly useful for sanity-checking as these 
are necessary but not sufficient
conditions - for example, all descriptors are zero-initialized. To 
detect used and available descriptors it is
possible for drivers and devices to keep track of the last observed 
value of VIRTQ_DESC_F_USED/VIRTQ_-
DESC_F_AVAIL. Other techniques to detect 
VIRTQ_DESC_F_AVAIL/VIRTQ_DESC_F_USED bit changes
might also be possible.
"

So it looks to me it was not sufficient, looking at the example codes in 
spec, do we need to track last seen used_wrap_counter here?

Thanks

On Mon, Apr 23, 2018 at 01:42:14PM +0800, Jason Wang
wrote:
> On 2018?04?01? 22:12, Tiwei Bie wrote:
> > Hello everyone,
> > 
> > This RFC implements packed ring support for virtio driver.
> > 
> > The code was tested with DPDK vhost (testpmd/vhost-PMD) implemented
> > by Jens at http://dpdk.org/ml/archives/dev/2018-January/089417.html
> > Minor changes are needed for the vhost code, e.g. to kick the guest.
> > 
> > TODO:
> > - Refinements and bug fixes;
> > - Split into small patches;
> > - Test indirect descriptor support;
> > - Test/fix event suppression support;
> > - Test devices other than net;
> > 
> > RFC v1 -> RFC v2:
> > - Add indirect descriptor support - compile test only;
> > - Add event suppression supprt - compile test only;
> > - Move vring_packed_init() out of uapi (Jason, MST);
> > - Merge two loops into one in virtqueue_add_packed() (Jason);
> > - Split vring_unmap_one() for packed ring and split ring (Jason);
> > - Avoid using '%' operator (Jason);
> > - Rename free_head -> next_avail_idx (Jason);
> > - Add comments for virtio_wmb() in virtqueue_add_packed() (Jason);
> > - Some other refinements and bug fixes;
> > 
> > Thanks!
> > 
> > Signed-off-by: Tiwei Bie <tiwei.bie at intel.com>
> > ---
> >   drivers/virtio/virtio_ring.c       | 1094
+++++++++++++++++++++++++++++-------
> >   include/linux/virtio_ring.h        |    8 +-
> >   include/uapi/linux/virtio_config.h |   12 +-
> >   include/uapi/linux/virtio_ring.h   |   61 ++
> >   4 files changed, 980 insertions(+), 195 deletions(-)
> > 
> > diff --git a/drivers/virtio/virtio_ring.c
b/drivers/virtio/virtio_ring.c
> > index 71458f493cf8..0515dca34d77 100644
> > --- a/drivers/virtio/virtio_ring.c
> > +++ b/drivers/virtio/virtio_ring.c
> > @@ -58,14 +58,15 @@
> 
> [...]
> 
> > +
> > +	if (vq->indirect) {
> > +		u32 len;
> > +
> > +		desc = vq->desc_state[head].indir_desc;
> > +		/* Free the indirect table, if any, now that it's unmapped. */
> > +		if (!desc)
> > +			goto out;
> > +
> > +		len = virtio32_to_cpu(vq->vq.vdev,
> > +				      vq->vring_packed.desc[head].len);
> > +
> > +		BUG_ON(!(vq->vring_packed.desc[head].flags &
> > +			 cpu_to_virtio16(vq->vq.vdev, VRING_DESC_F_INDIRECT)));
> 
> It looks to me spec does not force to keep VRING_DESC_F_INDIRECT here. So
we
> can safely remove this BUG_ON() here.
> 
> > +		BUG_ON(len == 0 || len % sizeof(struct vring_packed_desc));
> 
> Len could be ignored for used descriptor according to the spec, so we need
> remove this BUG_ON() too.
Yeah, you're right! The BUG_ON() isn't right. I'll remove it.
And I think something related to this in the spec isn't very
clear currently.

In the spec, there are below words:

https://github.com/oasis-tcs/virtio-spec/blob/d4fec517dfcf/packed-ring.tex#L272
"""
In descriptors with VIRTQ_DESC_F_INDIRECT set VIRTQ_DESC_F_WRITE
is reserved and is ignored by the device.
"""

So when device writes back an used descriptor in this case,
device may not set the VIRTQ_DESC_F_WRITE flag as the flag
is reserved and should be ignored.

https://github.com/oasis-tcs/virtio-spec/blob/d4fec517dfcf/packed-ring.tex#L170
"""
Element Length is reserved for used descriptors without the
VIRTQ_DESC_F_WRITE flag, and is ignored by drivers.
"""

And this is the way how driver ignores the `len` in an used
descriptor.

https://github.com/oasis-tcs/virtio-spec/blob/d4fec517dfcf/packed-ring.tex#L241
"""
To increase ring capacity the driver can store a (read-only
by the device) table of indirect descriptors anywhere in memory,
and insert a descriptor in the main virtqueue (with \field{Flags}
bit VIRTQ_DESC_F_INDIRECT on) that refers to a buffer element
containing this indirect descriptor table;
"""

So the indirect descriptors in the table are read-only by
the device. And the only descriptor which is writeable by
the device is the descriptor in the main virtqueue (with
Flags bit VIRTQ_DESC_F_INDIRECT on). So if we ignore the
`len` in this descriptor, we won't be able to get the
length of the data written by the device.

So I think the `len` in this descriptor will carry the
length of the data written by the device (if the buffers
are writable to the device) even if the VIRTQ_DESC_F_WRITE
isn't set by the device. How do you think?

> 
> The reason is we don't touch descriptor ring in the case of split, so
> BUG_ON()s may help there.
> 
> > +
> > +		for (j = 0; j < len / sizeof(struct vring_packed_desc); j++)
> > +			vring_unmap_one_packed(vq, &desc[j]);
> > +
> > +		kfree(desc);
> > +		vq->desc_state[head].indir_desc = NULL;
> > +	} else if (ctx) {
> > +		*ctx = vq->desc_state[head].indir_desc;
> > +	}
> > +
> > +out:
> > +	return vq->desc_state[head].num;
> > +}
> > +
> > +static inline bool more_used_split(const struct vring_virtqueue *vq)
> >   {
> >   	return vq->last_used_idx != virtio16_to_cpu(vq->vq.vdev,
vq->vring.used->idx);
> >   }
> > +static inline bool more_used_packed(const struct vring_virtqueue *vq)
> > +{
> > +	u16 last_used, flags;
> > +	bool avail, used;
> > +
> > +	if (vq->vq.num_free == vq->vring_packed.num)
> > +		return false;
> > +
> > +	last_used = vq->last_used_idx;
> > +	flags = virtio16_to_cpu(vq->vq.vdev,
> > +				vq->vring_packed.desc[last_used].flags);
> > +	avail = flags & VRING_DESC_F_AVAIL(1);
> > +	used = flags & VRING_DESC_F_USED(1);
> > +
> > +	return avail == used;
> > +}
> 
> This looks interesting, spec said:
> 
> "
> Thus VIRTQ_DESC_F_AVAIL and VIRTQ_DESC_F_USED bits are different for an
> available descriptor and
> equal for a used descriptor.
> Note that this observation is mostly useful for sanity-checking as these
are
> necessary but not sufficient
> conditions - for example, all descriptors are zero-initialized. To detect
> used and available descriptors it is
> possible for drivers and devices to keep track of the last observed value
of
> VIRTQ_DESC_F_USED/VIRTQ_-
> DESC_F_AVAIL. Other techniques to detect
> VIRTQ_DESC_F_AVAIL/VIRTQ_DESC_F_USED bit changes
> might also be possible.
> "
> 
> So it looks to me it was not sufficient, looking at the example codes in
> spec, do we need to track last seen used_wrap_counter here?
I don't think we have to track used_wrap_counter in
driver. There was a discussion on this:

https://lists.oasis-open.org/archives/virtio-dev/201802/msg00177.html

And after that, below sentence was added (it's also
in the above words you quoted):

"""
Other techniques to detect
VIRTQ_DESC_F_AVAIL/VIRTQ_DESC_F_USED bit changes
might also be possible.
"""

Best regards,
Tiwei Bie

> 
> Thanks