Linux Virtualization - Feb 2013 - [PATCH 0/9] virtio: new API for addition of buffers, scatterlist changes

Most device drivers do not need to perform any postprocessing on the
scatterlists they receive from higher-level drivers (e.g. the block
or SCSI layer), because they translate the request metadata directly
from the various C structs into the data that is required by the device.

virtio devices however do this translation in two steps: a device-specific
step in the device driver, and generic preparation of virtio direct or
indirect buffers in virtqueue_add_buf.  Now, virtqueue_add_buf also
accepts the outcome of the first step as a struct scatterlist, hence
the drivers may need to put additional items at the front or back of
the data scatterlists.

On top of this, virtqueue_add_buf also has the limitation of not
supporting chained scatterlists: the buffers must be provided as an
array of struct scatterlist.  Chained scatterlist would at least help
in the case where you have to add additional items at the front.

Because of this, virtio-scsi has to copy each request into a scatterlist
internal to the driver.  It cannot just use the one that was prepared
by the upper SCSI layers.  (virtio-blk has a little more flexibility in
that it handles itself the preparation of the scatterlist).

This series adds a different set of APIs for adding a buffer to a
virtqueue.  The new API lets you pass the buffers piecewise, wrapping
multiple calls to virtqueue_add_sg between virtqueue_start_buf and
virtqueue_end_buf.  Letting drivers call virtqueue_add_sg multiple times
if they already have a scatterlist provided by someone else simplifies the
code and, for virtio-scsi, it saves the copying and related locking.

One major difference between virtqueue_add_buf and virtqueue_add_sg
is that the latter uses scatterlist iterators, which follow chained
scatterlist structs and stop at ending markers.  In order to avoid code
duplication, and use the new API from virtqueue_add_buf (patch 8), we need
to change all existing callers of virtqueue_add_buf to provide well-formed
scatterlists.  This is what patches 2-7 do.  For virtio-blk it is easiest
to just switch to the new API, just like for virtio-scsi.  For virtio-net
the ending marker must be reset after calling virtqueue_add_buf, in
preparation for the next usage of the scatterlist.  Other drivers are
safe already, but many can be converted to the "fast-path" function
virtqueue_add_buf_single suggested by mst; this is done in patch 8.

Compared to the RFC, I have moved the state of virtqueue_add_sg into
struct vring_desc, and I replaced virtqueue_add_sg_single with
virtqueue_add_buf_single.  I renamed the "count" and
"count_sg"
to "nents" and "nsg" following a suggestion of Stefan
Hajnoczi.
And of course the patch is now well tested (including virtio-serial,
all three virtio-net receive paths, and virtio-blk SG_IO which I hadn't
checked for the RFC); anyway this did not bring in new code changes.

Ok for 3.9?

Paolo

Paolo Bonzini (9):
  virtio: add functions for piecewise addition of buffers
  virtio-blk: reorganize virtblk_add_req
  virtio-blk: use virtqueue_start_buf on bio path
  virtio-blk: use virtqueue_start_buf on req path
  scatterlist: introduce sg_unmark_end
  virtio-net: unmark scatterlist ending after virtqueue_add_buf
  virtio-scsi: use virtqueue_start_buf
  virtio: introduce and use virtqueue_add_buf_single
  virtio: reimplement virtqueue_add_buf using new functions

 block/blk-integrity.c               |    2 +-
 block/blk-merge.c                   |    2 +-
 drivers/block/virtio_blk.c          |  165 ++++++++-------
 drivers/char/hw_random/virtio-rng.c |    2 +-
 drivers/char/virtio_console.c       |    4 +-
 drivers/net/virtio_net.c            |   21 ++-
 drivers/rpmsg/virtio_rpmsg_bus.c    |    8 +-
 drivers/scsi/virtio_scsi.c          |  106 ++++------
 drivers/virtio/virtio_balloon.c     |    6 +-
 drivers/virtio/virtio_ring.c        |  396 ++++++++++++++++++++++-------------
 include/linux/scatterlist.h         |   16 ++
 include/linux/virtio.h              |   19 ++
 tools/virtio/virtio_test.c          |    6 +-
 13 files changed, 450 insertions(+), 303 deletions(-)

virtio device drivers translate requests from higher layer in two steps:
a device-specific step in the device driver, and generic preparation
of virtio direct or indirect buffers in virtqueue_add_buf.  Because
virtqueue_add_buf also accepts the outcome of the first step as a single
struct scatterlist, drivers may need to put additional items at the
front or back of the data scatterlists before handing it to virtqueue_add_buf.
Because of this, virtio-scsi has to copy each request into a scatterlist
internal to the driver.  It cannot just use the one that was prepared
by the upper SCSI layers. 

On top of this, virtqueue_add_buf also has the limitation of not
supporting chained scatterlists: the buffers must be provided as an
array of struct scatterlist.  Chained scatterlist, though not supported
on all architectures, would help for virtio-scsi where all additional
items are placed at the front.

This patch adds a different set of APIs for adding a buffer to a virtqueue.
The new API lets you pass the buffers piecewise, wrapping multiple calls
to virtqueue_add_sg between virtqueue_start_buf and virtqueue_end_buf.
virtio-scsi can then call virtqueue_add_sg 3/4 times: for the request
header, for the write buffer (if present), for the response header, and
finally for the read buffer (again if present).  It saves the copying
and the related locking.

Signed-off-by: Paolo Bonzini <pbonzini at redhat.com>
---
 drivers/virtio/virtio_ring.c |  211 ++++++++++++++++++++++++++++++++++++++++++
 include/linux/virtio.h       |   14 +++
 2 files changed, 225 insertions(+), 0 deletions(-)

diff --git a/drivers/virtio/virtio_ring.c b/drivers/virtio/virtio_ring.c
index ffd7e7d..64184e5 100644
--- a/drivers/virtio/virtio_ring.c
+++ b/drivers/virtio/virtio_ring.c
@@ -101,6 +101,10 @@ struct vring_virtqueue
 	/* Last used index we've seen. */
 	u16 last_used_idx;
 
+	/* State between virtqueue_start_buf and virtqueue_end_buf.  */
+	int head;
+	struct vring_desc *indirect_base, *tail;
+
 	/* How to notify other side. FIXME: commonalize hcalls! */
 	void (*notify)(struct virtqueue *vq);
 
@@ -394,6 +398,213 @@ static void detach_buf(struct vring_virtqueue *vq,
unsigned int head)
 	vq->vq.num_free++;
 }
 
+/**
+ * virtqueue_start_buf - start building buffer for the other end
+ * @vq: the struct virtqueue we're talking about.
+ * @data: the token identifying the buffer.
+ * @nents: the number of buffers that will be added
+ * @nsg: the number of sg lists that will be added
+ * @gfp: how to do memory allocations (if necessary).
+ *
+ * Caller must ensure we don't call this with other virtqueue operations
+ * at the same time (except where noted), and that a successful call is
+ * followed by one or more calls to virtqueue_add_sg, and finally a call
+ * to virtqueue_end_buf.
+ *
+ * Returns zero or a negative error (ie. ENOSPC).
+ */
+int virtqueue_start_buf(struct virtqueue *_vq,
+			void *data,
+			unsigned int nents,
+			unsigned int nsg,
+			gfp_t gfp)
+{
+	struct vring_virtqueue *vq = to_vvq(_vq);
+	struct vring_desc *desc = NULL;
+	int head;
+	int ret = -ENOMEM;
+
+	START_USE(vq);
+
+	BUG_ON(data == NULL);
+
+#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(nents < nsg);
+	BUG_ON(nsg == 0);
+
+	/*
+	 * If the host supports indirect descriptor tables, and there is
+	 * no space for direct buffers or there are multi-item scatterlists,
+	 * go indirect.
+	 */
+	head = vq->free_head;
+	if (vq->indirect && (nents > nsg || vq->vq.num_free <
nents)) {
+		if (vq->vq.num_free == 0)
+			goto no_space;
+
+		desc = kmalloc(nents * sizeof(struct vring_desc), gfp);
+		if (!desc)
+			goto error;
+
+		/* We're about to use a buffer */
+		vq->vq.num_free--;
+
+		/* Use a single buffer which doesn't continue */
+		vq->vring.desc[head].flags = VRING_DESC_F_INDIRECT;
+		vq->vring.desc[head].addr = virt_to_phys(desc);
+		vq->vring.desc[head].len = nents * sizeof(struct vring_desc);
+
+		/* Update free pointer */
+		vq->free_head = vq->vring.desc[head].next;
+	}
+
+	/* Set token. */
+	vq->data[head] = data;
+
+	pr_debug("Started buffer head %i for %p\n", head, vq);
+
+	vq->indirect_base = desc;
+	vq->tail = NULL;
+	vq->head = head;
+	return 0;
+
+no_space:
+	ret = -ENOSPC;
+error:
+	pr_debug("Can't add buf (%d) - nents = %i, avail = %i\n",
+		 ret, nents, vq->vq.num_free);
+	END_USE(vq);
+	return ret;
+}
+EXPORT_SYMBOL_GPL(virtqueue_start_buf);
+
+/**
+ * virtqueue_add_sg - add sglist to buffer being built
+ * @_vq: the virtqueue for which the buffer is being built
+ * @sgl: the description of the buffer(s).
+ * @nents: the number of items to process in sgl
+ * @dir: whether the sgl is read or written (DMA_TO_DEVICE/DMA_FROM_DEVICE
only)
+ *
+ * Note that, unlike virtqueue_add_buf, this function follows chained
+ * scatterlists, and stops before the @nents-th item if a scatterlist item
+ * has a marker.
+ *
+ * Caller must ensure we don't call this with other virtqueue operations
+ * at the same time (except where noted).
+ */
+void virtqueue_add_sg(struct virtqueue *_vq,
+		      struct scatterlist sgl[],
+		      unsigned int nents,
+		      enum dma_data_direction dir)
+{
+	struct vring_virtqueue *vq = to_vvq(_vq);
+	unsigned int i, n;
+	struct scatterlist *sg;
+	struct vring_desc *tail;
+	u32 flags;
+
+#ifdef DEBUG
+	BUG_ON(!vq->in_use);
+#endif
+
+	BUG_ON(dir != DMA_FROM_DEVICE && dir != DMA_TO_DEVICE);
+	BUG_ON(nents == 0);
+
+	flags = dir == DMA_FROM_DEVICE ? VRING_DESC_F_WRITE : 0;
+	flags |= VRING_DESC_F_NEXT;
+
+	/*
+	 * If using indirect descriptor tables, fill in the buffers
+	 * at vq->indirect_base.
+	 */
+	if (vq->indirect_base) {
+		i = 0;
+		if (likely(vq->tail))
+			i = vq->tail - vq->indirect_base + 1;
+
+		for_each_sg(sgl, sg, nents, n) {
+			tail = &vq->indirect_base[i];
+			tail->flags = flags;
+			tail->addr = sg_phys(sg);
+			tail->len = sg->length;
+			tail->next = ++i;
+		}
+	} else {
+		BUG_ON(vq->vq.num_free < nents);
+
+		i = vq->free_head;
+		for_each_sg(sgl, sg, nents, n) {
+			tail = &vq->vring.desc[i];
+			tail->flags = flags;
+			tail->addr = sg_phys(sg);
+			tail->len = sg->length;
+			i = tail->next;
+			vq->vq.num_free--;
+		}
+
+		vq->free_head = i;
+	}
+	vq->tail = tail;
+}
+EXPORT_SYMBOL_GPL(virtqueue_add_sg);
+
+/**
+ * virtqueue_end_buf - expose buffer to other end
+ * @_vq: the virtqueue for which the buffer was built
+ *
+ * Caller must ensure we don't call this with other virtqueue operations
+ * at the same time (except where noted).
+ */
+void virtqueue_end_buf(struct virtqueue *_vq)
+{
+	struct vring_virtqueue *vq = to_vvq(_vq);
+	unsigned int avail;
+	int head = vq->head;
+	struct vring_desc *tail = vq->tail;
+
+#ifdef DEBUG
+	BUG_ON(!vq->in_use);
+#endif
+	BUG_ON(tail == NULL);
+
+	/* The last one does not have the next flag set.  */
+	tail->flags &= ~VRING_DESC_F_NEXT;
+
+	/*
+	 * Put entry in available array.  Descriptors and available array
+	 * need to be set before we expose the new available array entries.
+	 */
+	avail = vq->vring.avail->idx & (vq->vring.num-1);
+	vq->vring.avail->ring[avail] = head;
+	virtio_wmb(vq);
+
+	vq->vring.avail->idx++;
+	vq->num_added++;
+
+	/*
+	 * This is very unlikely, but theoretically possible.  Kick
+	 * just in case.
+	 */
+	if (unlikely(vq->num_added == (1 << 16) - 1))
+		virtqueue_kick(&vq->vq);
+
+	pr_debug("Added buffer head %i to %p\n", head, vq);
+	END_USE(vq);
+}
+EXPORT_SYMBOL_GPL(virtqueue_end_buf);
+
 static inline bool more_used(const struct vring_virtqueue *vq)
 {
 	return vq->last_used_idx != vq->vring.used->idx;
diff --git a/include/linux/virtio.h b/include/linux/virtio.h
index cf8adb1..43d6bc3 100644
--- a/include/linux/virtio.h
+++ b/include/linux/virtio.h
@@ -7,6 +7,7 @@
 #include <linux/spinlock.h>
 #include <linux/device.h>
 #include <linux/mod_devicetable.h>
+#include <linux/dma-direction.h>
 #include <linux/gfp.h>
 
 /**
@@ -40,6 +41,19 @@ int virtqueue_add_buf(struct virtqueue *vq,
 		      void *data,
 		      gfp_t gfp);
 
+int virtqueue_start_buf(struct virtqueue *_vq,
+			void *data,
+			unsigned int nents,
+			unsigned int nsg,
+			gfp_t gfp);
+
+void virtqueue_add_sg(struct virtqueue *_vq,
+		      struct scatterlist sgl[],
+		      unsigned int nents,
+		      enum dma_data_direction dir);
+
+void virtqueue_end_buf(struct virtqueue *_vq);
+
 void virtqueue_kick(struct virtqueue *vq);
 
 bool virtqueue_kick_prepare(struct virtqueue *vq);
-- 
1.7.1

Right now, both virtblk_add_req and virtblk_add_req_wait call
virtqueue_add_buf.  To prepare for the next patches, abstract the call
to virtqueue_add_buf into a new function __virtblk_add_req, and include
the waiting logic directly in virtblk_add_req.

Signed-off-by: Paolo Bonzini <pbonzini at redhat.com>
---
 drivers/block/virtio_blk.c |   55 ++++++++++++++++----------------------------
 1 files changed, 20 insertions(+), 35 deletions(-)

diff --git a/drivers/block/virtio_blk.c b/drivers/block/virtio_blk.c
index 8ad21a2..fd8a689 100644
--- a/drivers/block/virtio_blk.c
+++ b/drivers/block/virtio_blk.c
@@ -100,50 +100,39 @@ static inline struct virtblk_req *virtblk_alloc_req(struct
virtio_blk *vblk,
 	return vbr;
 }
 
-static void virtblk_add_buf_wait(struct virtio_blk *vblk,
-				 struct virtblk_req *vbr,
-				 unsigned long out,
-				 unsigned long in)
+static inline int __virtblk_add_req(struct virtqueue *vq,
+			     struct virtblk_req *vbr,
+			     unsigned long out,
+			     unsigned long in)
 {
+	return virtqueue_add_buf(vq, vbr->sg, out, in, vbr, GFP_ATOMIC);
+}
+
+static void virtblk_add_req(struct virtblk_req *vbr,
+			    unsigned int out, unsigned int in)
+{
+	struct virtio_blk *vblk = vbr->vblk;
 	DEFINE_WAIT(wait);
+	int ret;
 
-	for (;;) {
+	spin_lock_irq(vblk->disk->queue->queue_lock);
+	while (unlikely((ret = __virtblk_add_req(vblk->vq, vbr,
+						 out, in)) < 0)) {
 		prepare_to_wait_exclusive(&vblk->queue_wait, &wait,
 					  TASK_UNINTERRUPTIBLE);
 
+		spin_unlock_irq(vblk->disk->queue->queue_lock);
+		io_schedule();
 		spin_lock_irq(vblk->disk->queue->queue_lock);
-		if (virtqueue_add_buf(vblk->vq, vbr->sg, out, in, vbr,
-				      GFP_ATOMIC) < 0) {
-			spin_unlock_irq(vblk->disk->queue->queue_lock);
-			io_schedule();
-		} else {
-			virtqueue_kick(vblk->vq);
-			spin_unlock_irq(vblk->disk->queue->queue_lock);
-			break;
-		}
 
+		finish_wait(&vblk->queue_wait, &wait);
 	}
 
-	finish_wait(&vblk->queue_wait, &wait);
-}
-
-static inline void virtblk_add_req(struct virtblk_req *vbr,
-				   unsigned int out, unsigned int in)
-{
-	struct virtio_blk *vblk = vbr->vblk;
-
-	spin_lock_irq(vblk->disk->queue->queue_lock);
-	if (unlikely(virtqueue_add_buf(vblk->vq, vbr->sg, out, in, vbr,
-					GFP_ATOMIC) < 0)) {
-		spin_unlock_irq(vblk->disk->queue->queue_lock);
-		virtblk_add_buf_wait(vblk, vbr, out, in);
-		return;
-	}
 	virtqueue_kick(vblk->vq);
 	spin_unlock_irq(vblk->disk->queue->queue_lock);
 }
 
-static int virtblk_bio_send_flush(struct virtblk_req *vbr)
+static void virtblk_bio_send_flush(struct virtblk_req *vbr)
 {
 	unsigned int out = 0, in = 0;
 
@@ -155,11 +144,9 @@ static int virtblk_bio_send_flush(struct virtblk_req *vbr)
 	sg_set_buf(&vbr->sg[out + in++], &vbr->status,
sizeof(vbr->status));
 
 	virtblk_add_req(vbr, out, in);
-
-	return 0;
 }
 
-static int virtblk_bio_send_data(struct virtblk_req *vbr)
+static void virtblk_bio_send_data(struct virtblk_req *vbr)
 {
 	struct virtio_blk *vblk = vbr->vblk;
 	unsigned int num, out = 0, in = 0;
@@ -188,8 +175,6 @@ static int virtblk_bio_send_data(struct virtblk_req *vbr)
 	}
 
 	virtblk_add_req(vbr, out, in);
-
-	return 0;
 }
 
 static void virtblk_bio_send_data_work(struct work_struct *work)
-- 
1.7.1

Move the creation of the request header and response footer to
__virtblk_add_req.  vbr->sg only contains the data scatterlist,
the header/footer are added separately using the new piecewise
API for building virtqueue buffers.

With this change, virtio-blk (with use_bio) is not relying anymore on
the virtio functions ignoring the end markers in a scatterlist.
The next patch will do the same for the other path.

Signed-off-by: Paolo Bonzini <pbonzini at redhat.com>
---
 drivers/block/virtio_blk.c |   74 ++++++++++++++++++++++++++-----------------
 1 files changed, 45 insertions(+), 29 deletions(-)

diff --git a/drivers/block/virtio_blk.c b/drivers/block/virtio_blk.c
index fd8a689..4a31fcc 100644
--- a/drivers/block/virtio_blk.c
+++ b/drivers/block/virtio_blk.c
@@ -62,6 +62,7 @@ struct virtblk_req
 	struct virtio_blk *vblk;
 	int flags;
 	u8 status;
+	int nents;
 	struct scatterlist sg[];
 };
 
@@ -100,24 +101,52 @@ static inline struct virtblk_req *virtblk_alloc_req(struct
virtio_blk *vblk,
 	return vbr;
 }
 
-static inline int __virtblk_add_req(struct virtqueue *vq,
-			     struct virtblk_req *vbr,
-			     unsigned long out,
-			     unsigned long in)
+static int __virtblk_add_req(struct virtqueue *vq,
+			     struct virtblk_req *vbr)
 {
-	return virtqueue_add_buf(vq, vbr->sg, out, in, vbr, GFP_ATOMIC);
+	struct scatterlist sg;
+	enum dma_data_direction dir;
+	int ret;
+
+	unsigned int nents = 2;
+	unsigned int nsg = 2;
+
+	if (vbr->nents) {
+		nsg++;
+		nents += vbr->nents;
+	}
+
+	ret = virtqueue_start_buf(vq, vbr, nents, nsg, GFP_ATOMIC);
+	if (ret < 0)
+		return ret;
+
+	dir = DMA_TO_DEVICE;
+	sg_init_one(&sg, &vbr->out_hdr, sizeof(vbr->out_hdr));
+	virtqueue_add_sg(vq, &sg, 1, dir);
+
+	if (vbr->nents) {
+		if ((vbr->out_hdr.type & VIRTIO_BLK_T_OUT) == 0)
+			dir = DMA_FROM_DEVICE;
+
+		virtqueue_add_sg(vq, vbr->sg, vbr->nents, dir);
+	}
+
+	dir = DMA_FROM_DEVICE;
+	sg_init_one(&sg, &vbr->status, sizeof(vbr->status));
+	virtqueue_add_sg(vq, &sg, 1, dir);
+
+	virtqueue_end_buf(vq);
+	return 0;
 }
 
-static void virtblk_add_req(struct virtblk_req *vbr,
-			    unsigned int out, unsigned int in)
+static void virtblk_add_req(struct virtblk_req *vbr)
 {
 	struct virtio_blk *vblk = vbr->vblk;
 	DEFINE_WAIT(wait);
 	int ret;
 
 	spin_lock_irq(vblk->disk->queue->queue_lock);
-	while (unlikely((ret = __virtblk_add_req(vblk->vq, vbr,
-						 out, in)) < 0)) {
+	while (unlikely((ret = __virtblk_add_req(vblk->vq, vbr)) < 0)) {
 		prepare_to_wait_exclusive(&vblk->queue_wait, &wait,
 					  TASK_UNINTERRUPTIBLE);
 
@@ -134,22 +163,18 @@ static void virtblk_add_req(struct virtblk_req *vbr,
 
 static void virtblk_bio_send_flush(struct virtblk_req *vbr)
 {
-	unsigned int out = 0, in = 0;
-
 	vbr->flags |= VBLK_IS_FLUSH;
 	vbr->out_hdr.type = VIRTIO_BLK_T_FLUSH;
 	vbr->out_hdr.sector = 0;
 	vbr->out_hdr.ioprio = 0;
-	sg_set_buf(&vbr->sg[out++], &vbr->out_hdr,
sizeof(vbr->out_hdr));
-	sg_set_buf(&vbr->sg[out + in++], &vbr->status,
sizeof(vbr->status));
+	vbr->nents = 0;
 
-	virtblk_add_req(vbr, out, in);
+	virtblk_add_req(vbr);
 }
 
 static void virtblk_bio_send_data(struct virtblk_req *vbr)
 {
 	struct virtio_blk *vblk = vbr->vblk;
-	unsigned int num, out = 0, in = 0;
 	struct bio *bio = vbr->bio;
 
 	vbr->flags &= ~VBLK_IS_FLUSH;
@@ -157,24 +182,15 @@ static void virtblk_bio_send_data(struct virtblk_req *vbr)
 	vbr->out_hdr.sector = bio->bi_sector;
 	vbr->out_hdr.ioprio = bio_prio(bio);
 
-	sg_set_buf(&vbr->sg[out++], &vbr->out_hdr,
sizeof(vbr->out_hdr));
-
-	num = blk_bio_map_sg(vblk->disk->queue, bio, vbr->sg + out);
-
-	sg_set_buf(&vbr->sg[num + out + in++], &vbr->status,
-		   sizeof(vbr->status));
-
-	if (num) {
-		if (bio->bi_rw & REQ_WRITE) {
+	vbr->nents = blk_bio_map_sg(vblk->disk->queue, bio, vbr->sg);
+	if (vbr->nents) {
+		if (bio->bi_rw & REQ_WRITE)
 			vbr->out_hdr.type |= VIRTIO_BLK_T_OUT;
-			out += num;
-		} else {
+		else
 			vbr->out_hdr.type |= VIRTIO_BLK_T_IN;
-			in += num;
-		}
 	}
 
-	virtblk_add_req(vbr, out, in);
+	virtblk_add_req(vbr);
 }
 
 static void virtblk_bio_send_data_work(struct work_struct *work)
-- 
1.7.1

This is similar to the previous patch, but a bit more radical
because the bio and req paths now share the buffer construction
code.  Because the req path doesn't use vbr->sg, however, we
need to add a couple of arguments to __virtblk_add_req.

We also need to teach __virtblk_add_req how to build SCSI command
requests.

Signed-off-by: Paolo Bonzini <pbonzini at redhat.com>
---
 drivers/block/virtio_blk.c |   74 ++++++++++++++++++++++---------------------
 1 files changed, 38 insertions(+), 36 deletions(-)

diff --git a/drivers/block/virtio_blk.c b/drivers/block/virtio_blk.c
index 4a31fcc..22deb65 100644
--- a/drivers/block/virtio_blk.c
+++ b/drivers/block/virtio_blk.c
@@ -102,18 +102,26 @@ static inline struct virtblk_req *virtblk_alloc_req(struct
virtio_blk *vblk,
 }
 
 static int __virtblk_add_req(struct virtqueue *vq,
-			     struct virtblk_req *vbr)
+			     struct virtblk_req *vbr,
+			     struct scatterlist *data_sg,
+			     unsigned data_nents)
 {
 	struct scatterlist sg;
 	enum dma_data_direction dir;
 	int ret;
 
+	int type = vbr->out_hdr.type & ~VIRTIO_BLK_T_OUT;
 	unsigned int nents = 2;
 	unsigned int nsg = 2;
 
-	if (vbr->nents) {
+	if (type == VIRTIO_BLK_T_SCSI_CMD) {
+		BUG_ON(use_bio);
+		nsg += 3;
+		nents += 3;
+	}
+	if (data_nents) {
 		nsg++;
-		nents += vbr->nents;
+		nents += data_nents;
 	}
 
 	ret = virtqueue_start_buf(vq, vbr, nents, nsg, GFP_ATOMIC);
@@ -124,14 +132,32 @@ static int __virtblk_add_req(struct virtqueue *vq,
 	sg_init_one(&sg, &vbr->out_hdr, sizeof(vbr->out_hdr));
 	virtqueue_add_sg(vq, &sg, 1, dir);
 
-	if (vbr->nents) {
+	/*
+	 * If this is a packet command we need a couple of additional headers.
+	 * Behind the normal outhdr we put a segment with the scsi command
+	 * block, and before the normal inhdr we put the sense data and the
+	 * inhdr with additional status information.
+	 */
+	if (type == VIRTIO_BLK_T_SCSI_CMD) {
+		sg_init_one(&sg, vbr->req->cmd, vbr->req->cmd_len);
+		virtqueue_add_sg(vq, &sg, 1, dir);
+	}
+
+	if (data_nents) {
 		if ((vbr->out_hdr.type & VIRTIO_BLK_T_OUT) == 0)
 			dir = DMA_FROM_DEVICE;
 
-		virtqueue_add_sg(vq, vbr->sg, vbr->nents, dir);
+		virtqueue_add_sg(vq, data_sg, data_nents, dir);
 	}
 
 	dir = DMA_FROM_DEVICE;
+	if (type == VIRTIO_BLK_T_SCSI_CMD) {
+		sg_init_one(&sg, vbr->req->sense, SCSI_SENSE_BUFFERSIZE);
+		virtqueue_add_sg(vq, &sg, 1, dir);
+		sg_init_one(&sg, &vbr->in_hdr, sizeof(vbr->in_hdr));
+		virtqueue_add_sg(vq, &sg, 1, dir);
+	}
+
 	sg_init_one(&sg, &vbr->status, sizeof(vbr->status));
 	virtqueue_add_sg(vq, &sg, 1, dir);
 
@@ -146,7 +172,8 @@ static void virtblk_add_req(struct virtblk_req *vbr)
 	int ret;
 
 	spin_lock_irq(vblk->disk->queue->queue_lock);
-	while (unlikely((ret = __virtblk_add_req(vblk->vq, vbr)) < 0)) {
+	while (unlikely((ret = __virtblk_add_req(vblk->vq, vbr, vbr->sg,
+						 vbr->nents)) < 0)) {
 		prepare_to_wait_exclusive(&vblk->queue_wait, &wait,
 					  TASK_UNINTERRUPTIBLE);
 
@@ -299,7 +326,7 @@ static void virtblk_done(struct virtqueue *vq)
 static bool do_req(struct request_queue *q, struct virtio_blk *vblk,
 		   struct request *req)
 {
-	unsigned long num, out = 0, in = 0;
+	unsigned int num;
 	struct virtblk_req *vbr;
 
 	vbr = virtblk_alloc_req(vblk, GFP_ATOMIC);
@@ -336,40 +363,15 @@ static bool do_req(struct request_queue *q, struct
virtio_blk *vblk,
 		}
 	}
 
-	sg_set_buf(&vblk->sg[out++], &vbr->out_hdr,
sizeof(vbr->out_hdr));
-
-	/*
-	 * If this is a packet command we need a couple of additional headers.
-	 * Behind the normal outhdr we put a segment with the scsi command
-	 * block, and before the normal inhdr we put the sense data and the
-	 * inhdr with additional status information before the normal inhdr.
-	 */
-	if (vbr->req->cmd_type == REQ_TYPE_BLOCK_PC)
-		sg_set_buf(&vblk->sg[out++], vbr->req->cmd,
vbr->req->cmd_len);
-
-	num = blk_rq_map_sg(q, vbr->req, vblk->sg + out);
-
-	if (vbr->req->cmd_type == REQ_TYPE_BLOCK_PC) {
-		sg_set_buf(&vblk->sg[num + out + in++], vbr->req->sense,
SCSI_SENSE_BUFFERSIZE);
-		sg_set_buf(&vblk->sg[num + out + in++], &vbr->in_hdr,
-			   sizeof(vbr->in_hdr));
-	}
-
-	sg_set_buf(&vblk->sg[num + out + in++], &vbr->status,
-		   sizeof(vbr->status));
-
+	num = blk_rq_map_sg(q, vbr->req, vblk->sg);
 	if (num) {
-		if (rq_data_dir(vbr->req) == WRITE) {
+		if (rq_data_dir(vbr->req) == WRITE)
 			vbr->out_hdr.type |= VIRTIO_BLK_T_OUT;
-			out += num;
-		} else {
+		else
 			vbr->out_hdr.type |= VIRTIO_BLK_T_IN;
-			in += num;
-		}
 	}
 
-	if (virtqueue_add_buf(vblk->vq, vblk->sg, out, in, vbr,
-			      GFP_ATOMIC) < 0) {
+	if (__virtblk_add_req(vblk->vq, vbr, vblk->sg, num) < 0) {
 		mempool_free(vbr, vblk->pool);
 		return false;
 	}
-- 
1.7.1

This is useful in places that recycle the same scatterlist multiple
times, and do not want to incur the cost of sg_init_table every
time in hot paths.

Acked-by: Jens Axboe <axboe at kernel.dk>
Signed-off-by: Paolo Bonzini <pbonzini at redhat.com>
---
 block/blk-integrity.c       |    2 +-
 block/blk-merge.c           |    2 +-
 include/linux/scatterlist.h |   16 ++++++++++++++++
 3 files changed, 18 insertions(+), 2 deletions(-)

diff --git a/block/blk-integrity.c b/block/blk-integrity.c
index da2a818..0f5004f 100644
--- a/block/blk-integrity.c
+++ b/block/blk-integrity.c
@@ -110,7 +110,7 @@ new_segment:
 			if (!sg)
 				sg = sglist;
 			else {
-				sg->page_link &= ~0x02;
+				sg_unmark_end(sg);
 				sg = sg_next(sg);
 			}
 
diff --git a/block/blk-merge.c b/block/blk-merge.c
index 936a110..5f24482 100644
--- a/block/blk-merge.c
+++ b/block/blk-merge.c
@@ -143,7 +143,7 @@ new_segment:
 			 * termination bit to avoid doing a full
 			 * sg_init_table() in drivers for each command.
 			 */
-			(*sg)->page_link &= ~0x02;
+			sg_unmark_end(*sg);
 			*sg = sg_next(*sg);
 		}
 
diff --git a/include/linux/scatterlist.h b/include/linux/scatterlist.h
index 4bd6c06..9b83784 100644
--- a/include/linux/scatterlist.h
+++ b/include/linux/scatterlist.h
@@ -172,6 +172,22 @@ static inline void sg_mark_end(struct scatterlist *sg)
 }
 
 /**
+ * sg_unmark_end - Undo setting the end of the scatterlist
+ * @sg:		 SG entryScatterlist
+ *
+ * Description:
+ *   Removes the termination marker from the given entry of the scatterlist.
+ *
+ **/
+static inline void sg_unmark_end(struct scatterlist *sg)
+{
+#ifdef CONFIG_DEBUG_SG
+	BUG_ON(sg->sg_magic != SG_MAGIC);
+#endif
+	sg->page_link &= ~0x02;
+}
+
+/**
  * sg_phys - Return physical address of an sg entry
  * @sg:	     SG entry
  *
-- 
1.7.1

Prepare for when virtqueue_add_buf will use sg_next instead of
ignoring ending markers.

Note that for_each_sg (and thus virtqueue_add_sg) allows you
to pass a "truncated" scatterlist that does not have a marker
on the last item.  We rely on this in add_recvbuf_mergeable.

Signed-off-by: Paolo Bonzini <pbonzini at redhat.com>
---
 drivers/net/virtio_net.c |   19 ++++++++++++++-----
 1 files changed, 14 insertions(+), 5 deletions(-)

diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
index 35c00c5..78b6f51 100644
--- a/drivers/net/virtio_net.c
+++ b/drivers/net/virtio_net.c
@@ -440,13 +440,16 @@ static int add_recvbuf_small(struct receive_queue *rq,
gfp_t gfp)
 
 	hdr = skb_vnet_hdr(skb);
 	sg_set_buf(rq->sg, &hdr->hdr, sizeof hdr->hdr);
-
 	skb_to_sgvec(skb, rq->sg + 1, 0, skb->len);
 
 	err = virtqueue_add_buf(rq->vq, rq->sg, 0, 2, skb, gfp);
 	if (err < 0)
 		dev_kfree_skb(skb);
 
+	/* An optimization: clear the end bit set by skb_to_sgvec, so
+	 * we can simply re-use rq->sg[] next time.
+	 */
+	sg_unmark_end(rq->sg + 1);
 	return err;
 }
 
@@ -505,8 +508,7 @@ static int add_recvbuf_mergeable(struct receive_queue *rq,
gfp_t gfp)
 	if (!page)
 		return -ENOMEM;
 
-	sg_init_one(rq->sg, page_address(page), PAGE_SIZE);
-
+	sg_set_page(rq->sg, page, PAGE_SIZE, 0);
 	err = virtqueue_add_buf(rq->vq, rq->sg, 0, 1, page, gfp);
 	if (err < 0)
 		give_pages(rq, page);
@@ -671,6 +673,7 @@ static int xmit_skb(struct send_queue *sq, struct sk_buff
*skb)
 	const unsigned char *dest = ((struct ethhdr *)skb->data)->h_dest;
 	struct virtnet_info *vi = sq->vq->vdev->priv;
 	unsigned num_sg;
+	int ret;
 
 	pr_debug("%s: xmit %p %pM\n", vi->dev->name, skb, dest);
 
@@ -710,8 +713,14 @@ static int xmit_skb(struct send_queue *sq, struct sk_buff
*skb)
 		sg_set_buf(sq->sg, &hdr->hdr, sizeof hdr->hdr);
 
 	num_sg = skb_to_sgvec(skb, sq->sg + 1, 0, skb->len) + 1;
-	return virtqueue_add_buf(sq->vq, sq->sg, num_sg,
-				 0, skb, GFP_ATOMIC);
+	ret = virtqueue_add_buf(sq->vq, sq->sg, num_sg,
+				0, skb, GFP_ATOMIC);
+
+	/* An optimization: clear the end bit set by skb_to_sgvec, so
+	 * we can simply re-use sq->sg[] next time.
+	 */
+	sg_unmark_end(&sq->sg[num_sg-1]);
+	return ret;
 }
 
 static netdev_tx_t start_xmit(struct sk_buff *skb, struct net_device *dev)
-- 
1.7.1

Using the new virtio_scsi_add_sg function lets us simplify the queueing
path.  In particular, all data protected by the tgt_lock is just gone
(multiqueue will find a new use for the lock).

The speedup is relatively small (2-4%) but it is worthwhile because of
the code simplification it enables.  And it will be particularly useful
when adding multiqueue supoprt, because it keeps the tgt_lock critical
sections very slim while avoiding a proliferation of locks.

Signed-off-by: Wanlong Gao <gaowanlong at cn.fujitsu.com>
Signed-off-by: Paolo Bonzini <pbonzini at redhat.com>
---
 drivers/scsi/virtio_scsi.c |  102 +++++++++++++++++++-------------------------
 1 files changed, 44 insertions(+), 58 deletions(-)

diff --git a/drivers/scsi/virtio_scsi.c b/drivers/scsi/virtio_scsi.c
index 3449a1f..6b752f7 100644
--- a/drivers/scsi/virtio_scsi.c
+++ b/drivers/scsi/virtio_scsi.c
@@ -59,11 +59,8 @@ struct virtio_scsi_vq {
 
 /* Per-target queue state */
 struct virtio_scsi_target_state {
-	/* Protects sg.  Lock hierarchy is tgt_lock -> vq_lock.  */
+	/* Never held at the same time as vq_lock.  */
 	spinlock_t tgt_lock;
-
-	/* For sglist construction when adding commands to the virtqueue.  */
-	struct scatterlist sg[];
 };
 
 /* Driver instance state */
@@ -351,89 +348,82 @@ static void virtscsi_event_done(struct virtqueue *vq)
 	spin_unlock_irqrestore(&vscsi->event_vq.vq_lock, flags);
 };
 
-static void virtscsi_map_sgl(struct scatterlist *sg, unsigned int *p_idx,
-			     struct scsi_data_buffer *sdb)
-{
-	struct sg_table *table = &sdb->table;
-	struct scatterlist *sg_elem;
-	unsigned int idx = *p_idx;
-	int i;
-
-	for_each_sg(table->sgl, sg_elem, table->nents, i)
-		sg[idx++] = *sg_elem;
-
-	*p_idx = idx;
-}
-
 /**
- * virtscsi_map_cmd - map a scsi_cmd to a virtqueue scatterlist
+ * virtscsi_add_cmd - add a virtio_scsi_cmd to a virtqueue
  * @vscsi	: virtio_scsi state
  * @cmd		: command structure
- * @out_num	: number of read-only elements
- * @in_num	: number of write-only elements
  * @req_size	: size of the request buffer
  * @resp_size	: size of the response buffer
- *
- * Called with tgt_lock held.
+ * @gfp	: flags to use for memory allocations
  */
-static void virtscsi_map_cmd(struct virtio_scsi_target_state *tgt,
-			     struct virtio_scsi_cmd *cmd,
-			     unsigned *out_num, unsigned *in_num,
-			     size_t req_size, size_t resp_size)
+static int virtscsi_add_cmd(struct virtqueue *vq,
+			    struct virtio_scsi_cmd *cmd,
+			    size_t req_size, size_t resp_size, gfp_t gfp)
 {
 	struct scsi_cmnd *sc = cmd->sc;
-	struct scatterlist *sg = tgt->sg;
-	unsigned int idx = 0;
+	struct scatterlist sg;
+	unsigned int nents, nsg;
+	struct sg_table *out, *in;
+	int ret;
+
+	out = in = NULL;
+
+	if (sc && sc->sc_data_direction != DMA_NONE) {
+		if (sc->sc_data_direction != DMA_FROM_DEVICE)
+			out = &scsi_out(sc)->table;
+		if (sc->sc_data_direction != DMA_TO_DEVICE)
+			in = &scsi_in(sc)->table;
+	}
+
+	nsg   = 2 + (out ? 1 : 0)          + (in ? 1 : 0);
+	nents = 2 + (out ? out->nents : 0) + (in ? in->nents : 0);
+	ret = virtqueue_start_buf(vq, cmd, nents, nsg, gfp);
+	if (ret < 0)
+		return ret;
 
 	/* Request header.  */
-	sg_set_buf(&sg[idx++], &cmd->req, req_size);
+	sg_init_one(&sg, &cmd->req, req_size);
+	virtqueue_add_sg(vq, &sg, 1, DMA_TO_DEVICE);
 
 	/* Data-out buffer.  */
-	if (sc && sc->sc_data_direction != DMA_FROM_DEVICE)
-		virtscsi_map_sgl(sg, &idx, scsi_out(sc));
-
-	*out_num = idx;
+	if (out)
+		virtqueue_add_sg(vq, out->sgl, out->nents, DMA_TO_DEVICE);
 
 	/* Response header.  */
-	sg_set_buf(&sg[idx++], &cmd->resp, resp_size);
+	sg_init_one(&sg, &cmd->resp, resp_size);
+	virtqueue_add_sg(vq, &sg, 1, DMA_FROM_DEVICE);
 
 	/* Data-in buffer */
-	if (sc && sc->sc_data_direction != DMA_TO_DEVICE)
-		virtscsi_map_sgl(sg, &idx, scsi_in(sc));
+	if (in)
+		virtqueue_add_sg(vq, in->sgl, in->nents, DMA_FROM_DEVICE);
 
-	*in_num = idx - *out_num;
+	virtqueue_end_buf(vq);
+	return 0;
 }
 
-static int virtscsi_kick_cmd(struct virtio_scsi_target_state *tgt,
-			     struct virtio_scsi_vq *vq,
+static int virtscsi_kick_cmd(struct virtio_scsi_vq *vq,
 			     struct virtio_scsi_cmd *cmd,
 			     size_t req_size, size_t resp_size, gfp_t gfp)
 {
-	unsigned int out_num, in_num;
 	unsigned long flags;
-	int err;
+	int ret;
 	bool needs_kick = false;
 
-	spin_lock_irqsave(&tgt->tgt_lock, flags);
-	virtscsi_map_cmd(tgt, cmd, &out_num, &in_num, req_size, resp_size);
-
-	spin_lock(&vq->vq_lock);
-	err = virtqueue_add_buf(vq->vq, tgt->sg, out_num, in_num, cmd, gfp);
-	spin_unlock(&tgt->tgt_lock);
-	if (!err)
+	spin_lock_irqsave(&vq->vq_lock, flags);
+	ret = virtscsi_add_cmd(vq->vq, cmd, req_size, resp_size, gfp);
+	if (!ret)
 		needs_kick = virtqueue_kick_prepare(vq->vq);
 
 	spin_unlock_irqrestore(&vq->vq_lock, flags);
 
 	if (needs_kick)
 		virtqueue_notify(vq->vq);
-	return err;
+	return ret;
 }
 
 static int virtscsi_queuecommand(struct Scsi_Host *sh, struct scsi_cmnd *sc)
 {
 	struct virtio_scsi *vscsi = shost_priv(sh);
-	struct virtio_scsi_target_state *tgt = vscsi->tgt[sc->device->id];
 	struct virtio_scsi_cmd *cmd;
 	int ret;
 
@@ -467,7 +457,7 @@ static int virtscsi_queuecommand(struct Scsi_Host *sh,
struct scsi_cmnd *sc)
 	BUG_ON(sc->cmd_len > VIRTIO_SCSI_CDB_SIZE);
 	memcpy(cmd->req.cmd.cdb, sc->cmnd, sc->cmd_len);
 
-	if (virtscsi_kick_cmd(tgt, &vscsi->req_vq, cmd,
+	if (virtscsi_kick_cmd(&vscsi->req_vq, cmd,
 			      sizeof cmd->req.cmd, sizeof cmd->resp.cmd,
 			      GFP_ATOMIC) == 0)
 		ret = 0;
@@ -481,11 +471,10 @@ out:
 static int virtscsi_tmf(struct virtio_scsi *vscsi, struct virtio_scsi_cmd *cmd)
 {
 	DECLARE_COMPLETION_ONSTACK(comp);
-	struct virtio_scsi_target_state *tgt =
vscsi->tgt[cmd->sc->device->id];
 	int ret = FAILED;
 
 	cmd->comp = &comp;
-	if (virtscsi_kick_cmd(tgt, &vscsi->ctrl_vq, cmd,
+	if (virtscsi_kick_cmd(&vscsi->ctrl_vq, cmd,
 			      sizeof cmd->req.tmf, sizeof cmd->resp.tmf,
 			      GFP_NOIO) < 0)
 		goto out;
@@ -592,14 +581,11 @@ static struct virtio_scsi_target_state
*virtscsi_alloc_tgt(
 	gfp_t gfp_mask = GFP_KERNEL;
 
 	/* We need extra sg elements at head and tail.  */
-	tgt = kmalloc(sizeof(*tgt) + sizeof(tgt->sg[0]) * (sg_elems + 2),
-		      gfp_mask);
-
+	tgt = kmalloc(sizeof(*tgt), gfp_mask);
 	if (!tgt)
 		return NULL;
 
 	spin_lock_init(&tgt->tgt_lock);
-	sg_init_table(tgt->sg, sg_elems + 2);
 	return tgt;
 }
 
-- 
1.7.1

Adding a single direct buffer is a very common case.  Introduce
an optimized function for that.

Signed-off-by: Paolo Bonzini <pbonzini at redhat.com>
---
 drivers/char/hw_random/virtio-rng.c |    2 +-
 drivers/char/virtio_console.c       |    4 +-
 drivers/net/virtio_net.c            |    2 +-
 drivers/rpmsg/virtio_rpmsg_bus.c    |    8 +++---
 drivers/scsi/virtio_scsi.c          |    4 +-
 drivers/virtio/virtio_balloon.c     |    6 ++--
 drivers/virtio/virtio_ring.c        |   40 +++++++++++++++++++++++++++++++++++
 include/linux/virtio.h              |    5 ++++
 tools/virtio/virtio_test.c          |    6 ++--
 9 files changed, 61 insertions(+), 16 deletions(-)

diff --git a/drivers/char/hw_random/virtio-rng.c
b/drivers/char/hw_random/virtio-rng.c
index b65c103..8b851ed 100644
--- a/drivers/char/hw_random/virtio-rng.c
+++ b/drivers/char/hw_random/virtio-rng.c
@@ -47,7 +47,7 @@ static void register_buffer(u8 *buf, size_t size)
 	sg_init_one(&sg, buf, size);
 
 	/* There should always be room for one buffer. */
-	if (virtqueue_add_buf(vq, &sg, 0, 1, buf, GFP_KERNEL) < 0)
+	if (virtqueue_add_buf_single(vq, &sg, DMA_FROM_DEVICE, buf) < 0)
 		BUG();
 
 	virtqueue_kick(vq);
diff --git a/drivers/char/virtio_console.c b/drivers/char/virtio_console.c
index 684b0d5..62bd945 100644
--- a/drivers/char/virtio_console.c
+++ b/drivers/char/virtio_console.c
@@ -508,7 +508,7 @@ static int add_inbuf(struct virtqueue *vq, struct
port_buffer *buf)
 
 	sg_init_one(sg, buf->buf, buf->size);
 
-	ret = virtqueue_add_buf(vq, sg, 0, 1, buf, GFP_ATOMIC);
+	ret = virtqueue_add_buf_single(vq, sg, DMA_FROM_DEVICE, buf);
 	virtqueue_kick(vq);
 	if (!ret)
 		ret = vq->num_free;
@@ -575,7 +575,7 @@ static ssize_t __send_control_msg(struct ports_device
*portdev, u32 port_id,
 	vq = portdev->c_ovq;
 
 	sg_init_one(sg, &cpkt, sizeof(cpkt));
-	if (virtqueue_add_buf(vq, sg, 1, 0, &cpkt, GFP_ATOMIC) == 0) {
+	if (virtqueue_add_buf_single(vq, sg, DMA_TO_DEVICE, &cpkt) == 0) {
 		virtqueue_kick(vq);
 		while (!virtqueue_get_buf(vq, &len))
 			cpu_relax();
diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
index 78b6f51..1a5186d 100644
--- a/drivers/net/virtio_net.c
+++ b/drivers/net/virtio_net.c
@@ -509,7 +509,7 @@ static int add_recvbuf_mergeable(struct receive_queue *rq,
gfp_t gfp)
 		return -ENOMEM;
 
 	sg_set_page(rq->sg, page, PAGE_SIZE, 0);
-	err = virtqueue_add_buf(rq->vq, rq->sg, 0, 1, page, gfp);
+	err = virtqueue_add_buf_single(rq->vq, rq->sg, DMA_FROM_DEVICE, page);
 	if (err < 0)
 		give_pages(rq, page);
 
diff --git a/drivers/rpmsg/virtio_rpmsg_bus.c b/drivers/rpmsg/virtio_rpmsg_bus.c
index f1e3239..3008987 100644
--- a/drivers/rpmsg/virtio_rpmsg_bus.c
+++ b/drivers/rpmsg/virtio_rpmsg_bus.c
@@ -763,7 +763,7 @@ int rpmsg_send_offchannel_raw(struct rpmsg_channel *rpdev,
u32 src, u32 dst,
 	mutex_lock(&vrp->tx_lock);
 
 	/* add message to the remote processor's virtqueue */
-	err = virtqueue_add_buf(vrp->svq, &sg, 1, 0, msg, GFP_KERNEL);
+	err = virtqueue_add_buf_single(vrp->svq, &sg, DMA_TO_DEVICE, msg);
 	if (err) {
 		/*
 		 * need to reclaim the buffer here, otherwise it's lost
@@ -845,7 +845,7 @@ static void rpmsg_recv_done(struct virtqueue *rvq)
 	sg_init_one(&sg, msg, RPMSG_BUF_SIZE);
 
 	/* add the buffer back to the remote processor's virtqueue */
-	err = virtqueue_add_buf(vrp->rvq, &sg, 0, 1, msg, GFP_KERNEL);
+	err = virtqueue_add_buf_single(vrp->rvq, &sg, DMA_FROM_DEVICE, msg);
 	if (err < 0) {
 		dev_err(dev, "failed to add a virtqueue buffer: %d\n", err);
 		return;
@@ -976,8 +976,8 @@ static int rpmsg_probe(struct virtio_device *vdev)
 
 		sg_init_one(&sg, cpu_addr, RPMSG_BUF_SIZE);
 
-		err = virtqueue_add_buf(vrp->rvq, &sg, 0, 1, cpu_addr,
-								GFP_KERNEL);
+		err = virtqueue_add_buf_single(vrp->rvq, &sg, DMA_FROM_DEVICE,
+					       cpu_addr);
 		WARN_ON(err); /* sanity check; this can't really happen */
 	}
 
diff --git a/drivers/scsi/virtio_scsi.c b/drivers/scsi/virtio_scsi.c
index 6b752f7..887902e 100644
--- a/drivers/scsi/virtio_scsi.c
+++ b/drivers/scsi/virtio_scsi.c
@@ -220,8 +220,8 @@ static int virtscsi_kick_event(struct virtio_scsi *vscsi,
 
 	spin_lock_irqsave(&vscsi->event_vq.vq_lock, flags);
 
-	err = virtqueue_add_buf(vscsi->event_vq.vq, &sg, 0, 1, event_node,
-				GFP_ATOMIC);
+	err = virtqueue_add_buf_single(vscsi->event_vq.vq, &sg,
+				       DMA_FROM_DEVICE, event_node);
 	if (!err)
 		virtqueue_kick(vscsi->event_vq.vq);
 
diff --git a/drivers/virtio/virtio_balloon.c b/drivers/virtio/virtio_balloon.c
index 797e1c7..c280948 100644
--- a/drivers/virtio/virtio_balloon.c
+++ b/drivers/virtio/virtio_balloon.c
@@ -108,7 +108,7 @@ static void tell_host(struct virtio_balloon *vb, struct
virtqueue *vq)
 	sg_init_one(&sg, vb->pfns, sizeof(vb->pfns[0]) * vb->num_pfns);
 
 	/* We should always be able to add one buffer to an empty queue. */
-	if (virtqueue_add_buf(vq, &sg, 1, 0, vb, GFP_KERNEL) < 0)
+	if (virtqueue_add_buf_single(vq, &sg, DMA_TO_DEVICE, vb) < 0)
 		BUG();
 	virtqueue_kick(vq);
 
@@ -256,7 +256,7 @@ static void stats_handle_request(struct virtio_balloon *vb)
 	if (!virtqueue_get_buf(vq, &len))
 		return;
 	sg_init_one(&sg, vb->stats, sizeof(vb->stats));
-	if (virtqueue_add_buf(vq, &sg, 1, 0, vb, GFP_KERNEL) < 0)
+	if (virtqueue_add_buf_single(vq, &sg, DMA_TO_DEVICE, vb) < 0)
 		BUG();
 	virtqueue_kick(vq);
 }
@@ -341,7 +341,7 @@ static int init_vqs(struct virtio_balloon *vb)
 		 * use it to signal us later.
 		 */
 		sg_init_one(&sg, vb->stats, sizeof vb->stats);
-		if (virtqueue_add_buf(vb->stats_vq, &sg, 1, 0, vb, GFP_KERNEL)
+		if (virtqueue_add_buf_single(vb->stats_vq, &sg, DMA_TO_DEVICE, vb)
 		    < 0)
 			BUG();
 		virtqueue_kick(vb->stats_vq);
diff --git a/drivers/virtio/virtio_ring.c b/drivers/virtio/virtio_ring.c
index 64184e5..b803cf7 100644
--- a/drivers/virtio/virtio_ring.c
+++ b/drivers/virtio/virtio_ring.c
@@ -181,6 +181,48 @@ static int vring_add_indirect(struct vring_virtqueue *vq,
 }
 
 /**
+ * virtqueue_add_buf_single - expose a single scatterlist entry to other end
+ * @vq: the struct virtqueue we're talking about.
+ * @sg: the description of the buffer.
+ * @dir: whether the buffer will be written or read by the device
+ * @data: the token identifying the buffer.
+ *
+ * Caller must ensure we don't call this with other virtqueue operations
+ * at the same time (except where noted).  No allocations are performed.
+ *
+ * Returns zero or a negative error (ENOSPC).
+ */
+int virtqueue_add_buf_single(struct virtqueue *_vq,
+			     struct scatterlist *sg,
+			     enum dma_data_direction dir,
+			     void *data)
+{
+	struct vring_virtqueue *vq = to_vvq(_vq);
+	struct vring_desc *tail;
+	int ret;
+
+	/* Always direct, the gfp argument is not used.  */
+	ret = virtqueue_start_buf(_vq, data, 1, 1, GFP_ATOMIC);
+	if (ret < 0)
+		return ret;
+
+	BUG_ON(vq->indirect_base);
+	BUG_ON(dir != DMA_FROM_DEVICE && dir != DMA_TO_DEVICE);
+
+	/* The direct case of virtqueue_add_sg, inlined.  */
+	tail = vq->tail = &vq->vring.desc[vq->free_head];
+	tail->flags = dir == DMA_FROM_DEVICE ? VRING_DESC_F_WRITE : 0;
+	tail->addr = sg_phys(sg);
+	tail->len = sg->length;
+	vq->free_head = tail->next;
+	vq->vq.num_free--;
+
+	virtqueue_end_buf(_vq);
+	return 0;
+}
+EXPORT_SYMBOL_GPL(virtqueue_add_buf_single);
+
+/**
  * virtqueue_add_buf - expose buffer to other end
  * @vq: the struct virtqueue we're talking about.
  * @sg: the description of the buffer(s).
diff --git a/include/linux/virtio.h b/include/linux/virtio.h
index 43d6bc3..4245bec 100644
--- a/include/linux/virtio.h
+++ b/include/linux/virtio.h
@@ -41,6 +41,11 @@ int virtqueue_add_buf(struct virtqueue *vq,
 		      void *data,
 		      gfp_t gfp);
 
+int virtqueue_add_buf_single(struct virtqueue *vq,
+			     struct scatterlist *sg,
+			     enum dma_data_direction dir,
+			     void *data);
+
 int virtqueue_start_buf(struct virtqueue *_vq,
 			void *data,
 			unsigned int nents,
diff --git a/tools/virtio/virtio_test.c b/tools/virtio/virtio_test.c
index fcc9aa2..727b6a1 100644
--- a/tools/virtio/virtio_test.c
+++ b/tools/virtio/virtio_test.c
@@ -161,9 +161,9 @@ static void run_test(struct vdev_info *dev, struct vq_info
*vq,
 		do {
 			if (started < bufs) {
 				sg_init_one(&sl, dev->buf, dev->buf_size);
-				r = virtqueue_add_buf(vq->vq, &sl, 1, 0,
-						      dev->buf + started,
-						      GFP_ATOMIC);
+				r = virtqueue_add_buf_single(vq->vq, &sl,
+						      DMA_TO_DEVICE,
+						      dev->buf + started);
 				if (likely(r == 0)) {
 					++started;
 					virtqueue_kick(vq->vq);
-- 
1.7.1

Eliminate the code duplication between virtqueue_add_buf and
virtqueue_add_sg.  That's safe to do now that no devices will
pass scatterlists with a termination marker in the middle.

Signed-off-by: Paolo Bonzini <pbonzini at redhat.com>
---
 drivers/virtio/virtio_ring.c |  159 +++---------------------------------------
 1 files changed, 11 insertions(+), 148 deletions(-)

diff --git a/drivers/virtio/virtio_ring.c b/drivers/virtio/virtio_ring.c
index b803cf7..71488c5 100644
--- a/drivers/virtio/virtio_ring.c
+++ b/drivers/virtio/virtio_ring.c
@@ -123,63 +123,6 @@ struct vring_virtqueue
 
 #define to_vvq(_vq) container_of(_vq, struct vring_virtqueue, vq)
 
-/* Set up an indirect table of descriptors and add it to the queue. */
-static int vring_add_indirect(struct vring_virtqueue *vq,
-			      struct scatterlist sg[],
-			      unsigned int out,
-			      unsigned int in,
-			      gfp_t gfp)
-{
-	struct vring_desc *desc;
-	unsigned head;
-	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 | __GFP_HIGH);
-
-	desc = kmalloc((out + in) * sizeof(struct vring_desc), gfp);
-	if (!desc)
-		return -ENOMEM;
-
-	/* Transfer entries from the sg list into the indirect page */
-	for (i = 0; i < out; i++) {
-		desc[i].flags = VRING_DESC_F_NEXT;
-		desc[i].addr = sg_phys(sg);
-		desc[i].len = sg->length;
-		desc[i].next = i+1;
-		sg++;
-	}
-	for (; i < (out + in); i++) {
-		desc[i].flags = VRING_DESC_F_NEXT|VRING_DESC_F_WRITE;
-		desc[i].addr = sg_phys(sg);
-		desc[i].len = sg->length;
-		desc[i].next = i+1;
-		sg++;
-	}
-
-	/* Last one doesn't continue. */
-	desc[i-1].flags &= ~VRING_DESC_F_NEXT;
-	desc[i-1].next = 0;
-
-	/* We're about to use a buffer */
-	vq->vq.num_free--;
-
-	/* Use a single buffer which doesn't continue */
-	head = vq->free_head;
-	vq->vring.desc[head].flags = VRING_DESC_F_INDIRECT;
-	vq->vring.desc[head].addr = virt_to_phys(desc);
-	vq->vring.desc[head].len = i * sizeof(struct vring_desc);
-
-	/* Update free pointer */
-	vq->free_head = vq->vring.desc[head].next;
-
-	return head;
-}
-
 /**
  * virtqueue_add_buf_single - expose a single scatterlist entry to other end
  * @vq: the struct virtqueue we're talking about.
@@ -234,104 +177,25 @@ EXPORT_SYMBOL_GPL(virtqueue_add_buf_single);
  *
  * Returns zero or a negative error (ie. ENOSPC, ENOMEM).
  */
-int virtqueue_add_buf(struct virtqueue *_vq,
+int virtqueue_add_buf(struct virtqueue *vq,
 		      struct scatterlist sg[],
 		      unsigned int out,
 		      unsigned int in,
 		      void *data,
 		      gfp_t gfp)
 {
-	struct vring_virtqueue *vq = to_vvq(_vq);
-	unsigned int i, avail, uninitialized_var(prev);
-	int head;
-
-	START_USE(vq);
-
-	BUG_ON(data == NULL);
-
-#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
-
-	/* If the host supports indirect descriptor tables, and we have multiple
-	 * buffers, then go indirect. FIXME: tune this threshold */
-	if (vq->indirect && (out + in) > 1 &&
vq->vq.num_free) {
-		head = vring_add_indirect(vq, sg, out, in, gfp);
-		if (likely(head >= 0))
-			goto add_head;
-	}
-
-	BUG_ON(out + in > vq->vring.num);
-	BUG_ON(out + in == 0);
-
-	if (vq->vq.num_free < out + in) {
-		pr_debug("Can't add buf len %i - avail = %i\n",
-			 out + in, 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)
-			vq->notify(&vq->vq);
-		END_USE(vq);
-		return -ENOSPC;
-	}
-
-	/* We're about to use some buffers from the free list. */
-	vq->vq.num_free -= out + in;
-
-	head = vq->free_head;
-	for (i = vq->free_head; out; i = vq->vring.desc[i].next, out--) {
-		vq->vring.desc[i].flags = VRING_DESC_F_NEXT;
-		vq->vring.desc[i].addr = sg_phys(sg);
-		vq->vring.desc[i].len = sg->length;
-		prev = i;
-		sg++;
-	}
-	for (; in; i = vq->vring.desc[i].next, in--) {
-		vq->vring.desc[i].flags = VRING_DESC_F_NEXT|VRING_DESC_F_WRITE;
-		vq->vring.desc[i].addr = sg_phys(sg);
-		vq->vring.desc[i].len = sg->length;
-		prev = i;
-		sg++;
-	}
-	/* Last one doesn't continue. */
-	vq->vring.desc[prev].flags &= ~VRING_DESC_F_NEXT;
-
-	/* Update free pointer */
-	vq->free_head = i;
-
-add_head:
-	/* Set token. */
-	vq->data[head] = data;
-
-	/* Put entry in available array (but don't update avail->idx until they
-	 * do sync). */
-	avail = (vq->vring.avail->idx & (vq->vring.num-1));
-	vq->vring.avail->ring[avail] = head;
-
-	/* Descriptors and available array need to be set before we expose the
-	 * new available array entries. */
-	virtio_wmb(vq);
-	vq->vring.avail->idx++;
-	vq->num_added++;
+	int ret;
 
-	/* This is very unlikely, but theoretically possible.  Kick
-	 * just in case. */
-	if (unlikely(vq->num_added == (1 << 16) - 1))
-		virtqueue_kick(_vq);
+	ret = virtqueue_start_buf(vq, data, out + in, !!out + !!in, gfp);
+	if (ret < 0)
+		return ret;
 
-	pr_debug("Added buffer head %i to %p\n", head, vq);
-	END_USE(vq);
+	if (out)
+		virtqueue_add_sg(vq, sg, out, DMA_TO_DEVICE);
+	if (in)
+		virtqueue_add_sg(vq, sg + out, in, DMA_FROM_DEVICE);
 
+	virtqueue_end_buf(vq);
 	return 0;
 }
 EXPORT_SYMBOL_GPL(virtqueue_add_buf);
@@ -537,8 +401,7 @@ EXPORT_SYMBOL_GPL(virtqueue_start_buf);
  * @nents: the number of items to process in sgl
  * @dir: whether the sgl is read or written (DMA_TO_DEVICE/DMA_FROM_DEVICE
only)
  *
- * Note that, unlike virtqueue_add_buf, this function follows chained
- * scatterlists, and stops before the @nents-th item if a scatterlist item
+ * This function will stop before the @nents-th item if a scatterlist item
  * has a marker.
  *
  * Caller must ensure we don't call this with other virtqueue operations
-- 
1.7.1

Paolo Bonzini <pbonzini at redhat.com> writes:
> This series adds a different set of APIs for adding a buffer to a
> virtqueue.  The new API lets you pass the buffers piecewise, wrapping
> multiple calls to virtqueue_add_sg between virtqueue_start_buf and
> virtqueue_end_buf.  Letting drivers call virtqueue_add_sg multiple times
> if they already have a scatterlist provided by someone else simplifies the
> code and, for virtio-scsi, it saves the copying and related locking.
They are ugly though.  It's convoluted because we do actually know all
the buffers at once, we don't need a piecemeal API.

As a result, you now have arbitrary changes to the indirect heuristic,
because the API is now piecemeal.

How about this as a first step?

virtio_ring: virtqueue_add_sgs, to add multiple sgs.

virtio_scsi and virtio_blk can really use these, to avoid their current
hack of copying the whole sg array.

Signed-off-by: Ruty Russell <rusty at rustcorp.com.au> 

diff --git a/drivers/virtio/virtio_ring.c b/drivers/virtio/virtio_ring.c
index ffd7e7d..c5afc5d 100644
--- a/drivers/virtio/virtio_ring.c
+++ b/drivers/virtio/virtio_ring.c
@@ -121,14 +121,16 @@ struct vring_virtqueue
 
 /* Set up an indirect table of descriptors and add it to the queue. */
 static int vring_add_indirect(struct vring_virtqueue *vq,
-			      struct scatterlist sg[],
-			      unsigned int out,
-			      unsigned int in,
+			      struct scatterlist *sgs[],
+			      unsigned int total_sg,
+			      unsigned int out_sgs,
+			      unsigned int in_sgs,
 			      gfp_t gfp)
 {
 	struct vring_desc *desc;
 	unsigned head;
-	int i;
+	struct scatterlist *sg;
+	int i, n;
 
 	/*
 	 * We require lowmem mappings for the descriptors because
@@ -137,26 +139,31 @@ static int vring_add_indirect(struct vring_virtqueue *vq,
 	 */
 	gfp &= ~(__GFP_HIGHMEM | __GFP_HIGH);
 
-	desc = kmalloc((out + in) * sizeof(struct vring_desc), gfp);
+	desc = kmalloc(total_sg * sizeof(struct vring_desc), gfp);
 	if (!desc)
 		return -ENOMEM;
 
-	/* Transfer entries from the sg list into the indirect page */
-	for (i = 0; i < out; i++) {
-		desc[i].flags = VRING_DESC_F_NEXT;
-		desc[i].addr = sg_phys(sg);
-		desc[i].len = sg->length;
-		desc[i].next = i+1;
-		sg++;
+	/* Transfer entries from the sg lists into the indirect page */
+	i = 0;
+	for (n = 0; n < out_sgs; n++) {
+		for (sg = sgs[n]; sg; sg = sg_next(sg)) {
+			desc[i].flags = VRING_DESC_F_NEXT;
+			desc[i].addr = sg_phys(sg);
+			desc[i].len = sg->length;
+			desc[i].next = i+1;
+		}
 	}
-	for (; i < (out + in); i++) {
-		desc[i].flags = VRING_DESC_F_NEXT|VRING_DESC_F_WRITE;
-		desc[i].addr = sg_phys(sg);
-		desc[i].len = sg->length;
-		desc[i].next = i+1;
-		sg++;
+	for (; n < (out_sgs + in_sgs); n++) {
+		for (sg = sgs[n]; sg; sg = sg_next(sg)) {
+			desc[i].flags = VRING_DESC_F_NEXT|VRING_DESC_F_WRITE;
+			desc[i].addr = sg_phys(sg);
+			desc[i].len = sg->length;
+			desc[i].next = i+1;
+		}
 	}
 
+	BUG_ON(i != total_sg);
+
 	/* Last one doesn't continue. */
 	desc[i-1].flags &= ~VRING_DESC_F_NEXT;
 	desc[i-1].next = 0;
@@ -176,6 +183,15 @@ static int vring_add_indirect(struct vring_virtqueue *vq,
 	return head;
 }
 
+/* FIXME */
+static inline void sg_unmark_end(struct scatterlist *sg)
+{
+#ifdef CONFIG_DEBUG_SG
+	BUG_ON(sg->sg_magic != SG_MAGIC);
+#endif
+	sg->page_link &= ~0x02;
+}
+
 /**
  * virtqueue_add_buf - expose buffer to other end
  * @vq: the struct virtqueue we're talking about.
@@ -197,8 +213,47 @@ int virtqueue_add_buf(struct virtqueue *_vq,
 		      void *data,
 		      gfp_t gfp)
 {
+	struct scatterlist *sgs[2];
+	unsigned int i;
+
+	sgs[0] = sg;
+	sgs[1] = sg + out;
+
+	/* Workaround until callers pass well-formed sgs. */
+	for (i = 0; i < out + in; i++)
+		sg_unmark_end(sg + i);
+
+	sg_unmark_end(sg + out + in);
+	if (out && in)
+		sg_unmark_end(sg + out);
+
+	return virtqueue_add_sgs(_vq, sgs, out ? 1 : 0, in ? 1 : 0, data, gfp);
+}
+
+/**
+ * virtqueue_add_sgs - expose buffers to other end
+ * @vq: the struct virtqueue we're talking about.
+ * @sgs: array of terminated scatterlists.
+ * @out_num: the number of scatterlists readable by other side
+ * @in_num: the number of scatterlists which are writable (after readable ones)
+ * @data: the token identifying the buffer.
+ * @gfp: how to do memory allocations (if necessary).
+ *
+ * Caller must ensure we don't call this with other virtqueue operations
+ * at the same time (except where noted).
+ *
+ * Returns zero or a negative error (ie. ENOSPC, ENOMEM).
+ */
+int virtqueue_add_sgs(struct virtqueue *_vq,
+		      struct scatterlist *sgs[],
+		      unsigned int out_sgs,
+		      unsigned int in_sgs,
+		      void *data,
+		      gfp_t gfp)
+{
 	struct vring_virtqueue *vq = to_vvq(_vq);
-	unsigned int i, avail, uninitialized_var(prev);
+	struct scatterlist *sg;
+	unsigned int i, n, avail, uninitialized_var(prev), total_sg;
 	int head;
 
 	START_USE(vq);
@@ -218,46 +273,59 @@ int virtqueue_add_buf(struct virtqueue *_vq,
 	}
 #endif
 
+	/* Count them first. */
+	for (i = total_sg = 0; i < out_sgs + in_sgs; i++) {
+		struct scatterlist *sg;
+		for (sg = sgs[i]; sg; sg = sg_next(sg))
+			total_sg++;
+	}
+
+
 	/* If the host supports indirect descriptor tables, and we have multiple
 	 * buffers, then go indirect. FIXME: tune this threshold */
-	if (vq->indirect && (out + in) > 1 &&
vq->vq.num_free) {
-		head = vring_add_indirect(vq, sg, out, in, gfp);
+	if (vq->indirect && total_sg > 1 && vq->vq.num_free)
{
+		head = vring_add_indirect(vq, sgs, total_sg, out_sgs, in_sgs,
+					  gfp);
 		if (likely(head >= 0))
 			goto add_head;
 	}
 
-	BUG_ON(out + in > vq->vring.num);
-	BUG_ON(out + in == 0);
+	BUG_ON(total_sg > vq->vring.num);
+	BUG_ON(total_sg == 0);
 
-	if (vq->vq.num_free < out + in) {
+	if (vq->vq.num_free < total_sg) {
 		pr_debug("Can't add buf len %i - avail = %i\n",
-			 out + in, vq->vq.num_free);
+			 total_sg, 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)
+		if (out_sgs)
 			vq->notify(&vq->vq);
 		END_USE(vq);
 		return -ENOSPC;
 	}
 
 	/* We're about to use some buffers from the free list. */
-	vq->vq.num_free -= out + in;
-
-	head = vq->free_head;
-	for (i = vq->free_head; out; i = vq->vring.desc[i].next, out--) {
-		vq->vring.desc[i].flags = VRING_DESC_F_NEXT;
-		vq->vring.desc[i].addr = sg_phys(sg);
-		vq->vring.desc[i].len = sg->length;
-		prev = i;
-		sg++;
+	vq->vq.num_free -= total_sg;
+
+	head = i = vq->free_head;
+	for (n = 0; n < out_sgs; n++) {
+		for (sg = sgs[n]; sg; sg = sg_next(sg)) {
+			vq->vring.desc[i].flags = VRING_DESC_F_NEXT;
+			vq->vring.desc[i].addr = sg_phys(sg);
+			vq->vring.desc[i].len = sg->length;
+			prev = i;
+			i = vq->vring.desc[i].next;
+		}
 	}
-	for (; in; i = vq->vring.desc[i].next, in--) {
-		vq->vring.desc[i].flags = VRING_DESC_F_NEXT|VRING_DESC_F_WRITE;
-		vq->vring.desc[i].addr = sg_phys(sg);
-		vq->vring.desc[i].len = sg->length;
-		prev = i;
-		sg++;
+	for (; n < (out_sgs + in_sgs); n++) {
+		for (sg = sgs[n]; sg; sg = sg_next(sg)) {
+			vq->vring.desc[i].flags = VRING_DESC_F_NEXT|VRING_DESC_F_WRITE;
+			vq->vring.desc[i].addr = sg_phys(sg);
+			vq->vring.desc[i].len = sg->length;
+			prev = i;
+			i = vq->vring.desc[i].next;
+		}
 	}
 	/* Last one doesn't continue. */
 	vq->vring.desc[prev].flags &= ~VRING_DESC_F_NEXT;
diff --git a/include/linux/virtio.h b/include/linux/virtio.h
index ff6714e..6eff15b 100644
--- a/include/linux/virtio.h
+++ b/include/linux/virtio.h
@@ -40,6 +40,13 @@ int virtqueue_add_buf(struct virtqueue *vq,
 		      void *data,
 		      gfp_t gfp);
 
+int virtqueue_add_sgs(struct virtqueue *vq,
+		      struct scatterlist *sgs[],
+		      unsigned int out_sgs,
+		      unsigned int in_sgs,
+		      void *data,
+		      gfp_t gfp);
+
 void virtqueue_kick(struct virtqueue *vq);
 
 bool virtqueue_kick_prepare(struct virtqueue *vq);

On 02/12/2013 08:23 PM, Paolo Bonzini wrote:
> Right now, both virtblk_add_req and virtblk_add_req_wait call
> virtqueue_add_buf.  To prepare for the next patches, abstract the call
> to virtqueue_add_buf into a new function __virtblk_add_req, and include
> the waiting logic directly in virtblk_add_req.
> 
> Signed-off-by: Paolo Bonzini <pbonzini at redhat.com>
Reviewed-by: Asias He <asias at redhat.com>
> ---
>  drivers/block/virtio_blk.c |   55
++++++++++++++++----------------------------
>  1 files changed, 20 insertions(+), 35 deletions(-)
> 
> diff --git a/drivers/block/virtio_blk.c b/drivers/block/virtio_blk.c
> index 8ad21a2..fd8a689 100644
> --- a/drivers/block/virtio_blk.c
> +++ b/drivers/block/virtio_blk.c
> @@ -100,50 +100,39 @@ static inline struct virtblk_req
*virtblk_alloc_req(struct virtio_blk *vblk,
>  	return vbr;
>  }
>  
> -static void virtblk_add_buf_wait(struct virtio_blk *vblk,
> -				 struct virtblk_req *vbr,
> -				 unsigned long out,
> -				 unsigned long in)
> +static inline int __virtblk_add_req(struct virtqueue *vq,
> +			     struct virtblk_req *vbr,
> +			     unsigned long out,
> +			     unsigned long in)
>  {
> +	return virtqueue_add_buf(vq, vbr->sg, out, in, vbr, GFP_ATOMIC);
> +}
> +
> +static void virtblk_add_req(struct virtblk_req *vbr,
> +			    unsigned int out, unsigned int in)
> +{
> +	struct virtio_blk *vblk = vbr->vblk;
>  	DEFINE_WAIT(wait);
> +	int ret;
>  
> -	for (;;) {
> +	spin_lock_irq(vblk->disk->queue->queue_lock);
> +	while (unlikely((ret = __virtblk_add_req(vblk->vq, vbr,
> +						 out, in)) < 0)) {
>  		prepare_to_wait_exclusive(&vblk->queue_wait, &wait,
>  					  TASK_UNINTERRUPTIBLE);
>  
> +		spin_unlock_irq(vblk->disk->queue->queue_lock);
> +		io_schedule();
>  		spin_lock_irq(vblk->disk->queue->queue_lock);
> -		if (virtqueue_add_buf(vblk->vq, vbr->sg, out, in, vbr,
> -				      GFP_ATOMIC) < 0) {
> -			spin_unlock_irq(vblk->disk->queue->queue_lock);
> -			io_schedule();
> -		} else {
> -			virtqueue_kick(vblk->vq);
> -			spin_unlock_irq(vblk->disk->queue->queue_lock);
> -			break;
> -		}
>  
> +		finish_wait(&vblk->queue_wait, &wait);
>  	}
>  
> -	finish_wait(&vblk->queue_wait, &wait);
> -}
> -
> -static inline void virtblk_add_req(struct virtblk_req *vbr,
> -				   unsigned int out, unsigned int in)
> -{
> -	struct virtio_blk *vblk = vbr->vblk;
> -
> -	spin_lock_irq(vblk->disk->queue->queue_lock);
> -	if (unlikely(virtqueue_add_buf(vblk->vq, vbr->sg, out, in, vbr,
> -					GFP_ATOMIC) < 0)) {
> -		spin_unlock_irq(vblk->disk->queue->queue_lock);
> -		virtblk_add_buf_wait(vblk, vbr, out, in);
> -		return;
> -	}
>  	virtqueue_kick(vblk->vq);
>  	spin_unlock_irq(vblk->disk->queue->queue_lock);
>  }
>  
> -static int virtblk_bio_send_flush(struct virtblk_req *vbr)
> +static void virtblk_bio_send_flush(struct virtblk_req *vbr)
>  {
>  	unsigned int out = 0, in = 0;
>  
> @@ -155,11 +144,9 @@ static int virtblk_bio_send_flush(struct virtblk_req
*vbr)
>  	sg_set_buf(&vbr->sg[out + in++], &vbr->status,
sizeof(vbr->status));
>  
>  	virtblk_add_req(vbr, out, in);
> -
> -	return 0;
>  }
>  
> -static int virtblk_bio_send_data(struct virtblk_req *vbr)
> +static void virtblk_bio_send_data(struct virtblk_req *vbr)
>  {
>  	struct virtio_blk *vblk = vbr->vblk;
>  	unsigned int num, out = 0, in = 0;
> @@ -188,8 +175,6 @@ static int virtblk_bio_send_data(struct virtblk_req
*vbr)
>  	}
>  
>  	virtblk_add_req(vbr, out, in);
> -
> -	return 0;
>  }
>  
>  static void virtblk_bio_send_data_work(struct work_struct *work)
> 

-- 
Asias

On 02/12/2013 08:23 PM, Paolo Bonzini wrote:
> Move the creation of the request header and response footer to
> __virtblk_add_req.  vbr->sg only contains the data scatterlist,
> the header/footer are added separately using the new piecewise
> API for building virtqueue buffers.
> 
> With this change, virtio-blk (with use_bio) is not relying anymore on
> the virtio functions ignoring the end markers in a scatterlist.
> The next patch will do the same for the other path.
> 
> Signed-off-by: Paolo Bonzini <pbonzini at redhat.com>
Reviewed-by: Asias He <asias at redhat.com>
> ---
>  drivers/block/virtio_blk.c |   74
++++++++++++++++++++++++++-----------------
>  1 files changed, 45 insertions(+), 29 deletions(-)
> 
> diff --git a/drivers/block/virtio_blk.c b/drivers/block/virtio_blk.c
> index fd8a689..4a31fcc 100644
> --- a/drivers/block/virtio_blk.c
> +++ b/drivers/block/virtio_blk.c
> @@ -62,6 +62,7 @@ struct virtblk_req
>  	struct virtio_blk *vblk;
>  	int flags;
>  	u8 status;
> +	int nents;
>  	struct scatterlist sg[];
>  };
>  
> @@ -100,24 +101,52 @@ static inline struct virtblk_req
*virtblk_alloc_req(struct virtio_blk *vblk,
>  	return vbr;
>  }
>  
> -static inline int __virtblk_add_req(struct virtqueue *vq,
> -			     struct virtblk_req *vbr,
> -			     unsigned long out,
> -			     unsigned long in)
> +static int __virtblk_add_req(struct virtqueue *vq,
> +			     struct virtblk_req *vbr)
>  {
> -	return virtqueue_add_buf(vq, vbr->sg, out, in, vbr, GFP_ATOMIC);
> +	struct scatterlist sg;
> +	enum dma_data_direction dir;
> +	int ret;
> +
> +	unsigned int nents = 2;
> +	unsigned int nsg = 2;
> +
> +	if (vbr->nents) {
> +		nsg++;
> +		nents += vbr->nents;
> +	}
> +
> +	ret = virtqueue_start_buf(vq, vbr, nents, nsg, GFP_ATOMIC);
> +	if (ret < 0)
> +		return ret;
> +
> +	dir = DMA_TO_DEVICE;
> +	sg_init_one(&sg, &vbr->out_hdr, sizeof(vbr->out_hdr));
> +	virtqueue_add_sg(vq, &sg, 1, dir);
> +
> +	if (vbr->nents) {
> +		if ((vbr->out_hdr.type & VIRTIO_BLK_T_OUT) == 0)
> +			dir = DMA_FROM_DEVICE;
> +
> +		virtqueue_add_sg(vq, vbr->sg, vbr->nents, dir);
> +	}
> +
> +	dir = DMA_FROM_DEVICE;
> +	sg_init_one(&sg, &vbr->status, sizeof(vbr->status));
> +	virtqueue_add_sg(vq, &sg, 1, dir);
> +
> +	virtqueue_end_buf(vq);
> +	return 0;
>  }
>  
> -static void virtblk_add_req(struct virtblk_req *vbr,
> -			    unsigned int out, unsigned int in)
> +static void virtblk_add_req(struct virtblk_req *vbr)
>  {
>  	struct virtio_blk *vblk = vbr->vblk;
>  	DEFINE_WAIT(wait);
>  	int ret;
>  
>  	spin_lock_irq(vblk->disk->queue->queue_lock);
> -	while (unlikely((ret = __virtblk_add_req(vblk->vq, vbr,
> -						 out, in)) < 0)) {
> +	while (unlikely((ret = __virtblk_add_req(vblk->vq, vbr)) < 0)) {
>  		prepare_to_wait_exclusive(&vblk->queue_wait, &wait,
>  					  TASK_UNINTERRUPTIBLE);
>  
> @@ -134,22 +163,18 @@ static void virtblk_add_req(struct virtblk_req *vbr,
>  
>  static void virtblk_bio_send_flush(struct virtblk_req *vbr)
>  {
> -	unsigned int out = 0, in = 0;
> -
>  	vbr->flags |= VBLK_IS_FLUSH;
>  	vbr->out_hdr.type = VIRTIO_BLK_T_FLUSH;
>  	vbr->out_hdr.sector = 0;
>  	vbr->out_hdr.ioprio = 0;
> -	sg_set_buf(&vbr->sg[out++], &vbr->out_hdr,
sizeof(vbr->out_hdr));
> -	sg_set_buf(&vbr->sg[out + in++], &vbr->status,
sizeof(vbr->status));
> +	vbr->nents = 0;
>  
> -	virtblk_add_req(vbr, out, in);
> +	virtblk_add_req(vbr);
>  }
>  
>  static void virtblk_bio_send_data(struct virtblk_req *vbr)
>  {
>  	struct virtio_blk *vblk = vbr->vblk;
> -	unsigned int num, out = 0, in = 0;
>  	struct bio *bio = vbr->bio;
>  
>  	vbr->flags &= ~VBLK_IS_FLUSH;
> @@ -157,24 +182,15 @@ static void virtblk_bio_send_data(struct virtblk_req
*vbr)
>  	vbr->out_hdr.sector = bio->bi_sector;
>  	vbr->out_hdr.ioprio = bio_prio(bio);
>  
> -	sg_set_buf(&vbr->sg[out++], &vbr->out_hdr,
sizeof(vbr->out_hdr));
> -
> -	num = blk_bio_map_sg(vblk->disk->queue, bio, vbr->sg + out);
> -
> -	sg_set_buf(&vbr->sg[num + out + in++], &vbr->status,
> -		   sizeof(vbr->status));
> -
> -	if (num) {
> -		if (bio->bi_rw & REQ_WRITE) {
> +	vbr->nents = blk_bio_map_sg(vblk->disk->queue, bio, vbr->sg);
> +	if (vbr->nents) {
> +		if (bio->bi_rw & REQ_WRITE)
>  			vbr->out_hdr.type |= VIRTIO_BLK_T_OUT;
> -			out += num;
> -		} else {
> +		else
>  			vbr->out_hdr.type |= VIRTIO_BLK_T_IN;
> -			in += num;
> -		}
>  	}
>  
> -	virtblk_add_req(vbr, out, in);
> +	virtblk_add_req(vbr);
>  }
>  
>  static void virtblk_bio_send_data_work(struct work_struct *work)
> 

-- 
Asias