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Michael S. Tsirkin

2018-Nov-07 17:48 UTC

[PATCH net-next v2 3/5] virtio_ring: add packed ring support

On Wed, Jul 11, 2018 at 10:27:09AM +0800, Tiwei Bie
wrote:> This commit introduces the support (without EVENT_IDX) for
> packed ring.
> 
> Signed-off-by: Tiwei Bie <tiwei.bie at intel.com>
> ---
>  drivers/virtio/virtio_ring.c | 495 ++++++++++++++++++++++++++++++++++-
>  1 file changed, 487 insertions(+), 8 deletions(-)
> 
> diff --git a/drivers/virtio/virtio_ring.c b/drivers/virtio/virtio_ring.c
> index c4f8abc7445a..f317b485ba54 100644
> --- a/drivers/virtio/virtio_ring.c
> +++ b/drivers/virtio/virtio_ring.c
> @@ -55,12 +55,21 @@
>  #define END_USE(vq)
>  #endif
>  
> +#define _VRING_DESC_F_AVAIL(b)	((__u16)(b) << 7)
> +#define _VRING_DESC_F_USED(b)	((__u16)(b) << 15)
> +
>  struct vring_desc_state {
>  	void *data;			/* Data for callback. */
>  	struct vring_desc *indir_desc;	/* Indirect descriptor, if any. */
>  };
>  
>  struct vring_desc_state_packed {
> +	void *data;			/* Data for callback. */
> +	struct vring_packed_desc *indir_desc; /* Indirect descriptor, if any. */
> +	int num;			/* Descriptor list length. */
> +	dma_addr_t addr;		/* Buffer DMA addr. */
> +	u32 len;			/* Buffer length. */
> +	u16 flags;			/* Descriptor flags. */
>  	int next;			/* The next desc state. */
>  };
>  
> @@ -660,7 +669,6 @@ 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);
>  }
>  
> @@ -757,6 +765,72 @@ static inline unsigned vring_size_packed(unsigned int
num, unsigned long align)
>  		& ~(align - 1)) + sizeof(struct vring_packed_desc_event) * 2;
>  }
>  
> +static void vring_unmap_state_packed(const struct vring_virtqueue *vq,
> +				     struct vring_desc_state_packed *state)
> +{
> +	u16 flags;
> +
> +	if (!vring_use_dma_api(vq->vq.vdev))
> +		return;
> +
> +	flags = state->flags;
> +
> +	if (flags & VRING_DESC_F_INDIRECT) {
> +		dma_unmap_single(vring_dma_dev(vq),
> +				 state->addr, state->len,
> +				 (flags & VRING_DESC_F_WRITE) ?
> +				 DMA_FROM_DEVICE : DMA_TO_DEVICE);
> +	} else {
> +		dma_unmap_page(vring_dma_dev(vq),
> +			       state->addr, state->len,
> +			       (flags & VRING_DESC_F_WRITE) ?
> +			       DMA_FROM_DEVICE : DMA_TO_DEVICE);
> +	}
> +}
> +
> +static void vring_unmap_desc_packed(const struct vring_virtqueue *vq,
> +				   struct vring_packed_desc *desc)
> +{
> +	u16 flags;
> +
> +	if (!vring_use_dma_api(vq->vq.vdev))
> +		return;
> +
> +	flags = virtio16_to_cpu(vq->vq.vdev, desc->flags);
BTW this stuff is only used on error etc. Is there a way to
reuse vring_unmap_state_packed?
> +
> +	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 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_packed(struct virtqueue *_vq,
>  				       struct scatterlist *sgs[],
>  				       unsigned int total_sg,
> @@ -766,47 +840,449 @@ static inline int virtqueue_add_packed(struct
virtqueue *_vq,
>  				       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;
> +	u16 head, avail_wrap_counter, id, curr;
> +	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;
> +	avail_wrap_counter = vq->avail_wrap_counter;
> +
> +	if (virtqueue_use_indirect(_vq, total_sg))
> +		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. */
I don't think we have a reason to do this for packed ring.
No historical baggage there, right?
> +		if (out_sgs)
> +			vq->notify(&vq->vq);
> +		if (indirect)
> +			kfree(desc);
> +		END_USE(vq);
> +		return -ENOSPC;
> +	}
> +
> +	id = vq->free_head;
> +	BUG_ON(id == vq->vring_packed.num);
> +
> +	curr = id;
> +	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->avail_wrap_counter) |
> +				  _VRING_DESC_F_USED(!vq->avail_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);
> +			i++;
> +			if (!indirect) {
> +				if (vring_use_dma_api(_vq->vdev)) {
> +					vq->desc_state_packed[curr].addr = addr;
> +					vq->desc_state_packed[curr].len > +						sg->length;
> +					vq->desc_state_packed[curr].flags > +					
virtio16_to_cpu(_vq->vdev,
> +								flags);
> +				}
> +				curr = vq->desc_state_packed[curr].next;
> +
> +				if (i >= vq->vring_packed.num) {
> +					i = 0;
> +					vq->avail_wrap_counter ^= 1;
> +				}
> +			}
> +		}
> +	}
> +
> +	prev = (i > 0 ? i : vq->vring_packed.num) - 1;
> +	desc[prev].id = cpu_to_virtio16(_vq->vdev, id);
> +
> +	/* 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(avail_wrap_counter) |
> +				      _VRING_DESC_F_USED(!avail_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_virtio16(_vq->vdev, id);
> +
> +		if (vring_use_dma_api(_vq->vdev)) {
> +			vq->desc_state_packed[id].addr = addr;
> +			vq->desc_state_packed[id].len = total_sg *
> +					sizeof(struct vring_packed_desc);
> +			vq->desc_state_packed[id].flags > +				
virtio16_to_cpu(_vq->vdev, head_flags);
> +		}
> +	}
> +
> +	/* 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->avail_wrap_counter ^= 1;
> +		}
> +		vq->next_avail_idx = n;
> +		vq->free_head = vq->desc_state_packed[id].next;
> +	} else {
> +		vq->next_avail_idx = i;
> +		vq->free_head = curr;
> +	}
> +
> +	/* Store token and indirect buffer state. */
> +	vq->desc_state_packed[id].num = descs_used;
> +	vq->desc_state_packed[id].data = data;
> +	if (indirect)
> +		vq->desc_state_packed[id].indir_desc = desc;
> +	else
> +		vq->desc_state_packed[id].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 += descs_used;
> +
> +	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_desc_packed(vq, &desc[i]);
> +		i++;
> +		if (!indirect && i >= vq->vring_packed.num)
> +			i = 0;
> +	}
> +
> +	vq->avail_wrap_counter = avail_wrap_counter;
> +
> +	if (indirect)
> +		kfree(desc);
> +
> +	END_USE(vq);
>  	return -EIO;
>  }
>  
>  static bool virtqueue_kick_prepare_packed(struct virtqueue *_vq)
>  {
> -	return false;
> +	struct vring_virtqueue *vq = to_vvq(_vq);
> +	u16 flags;
> +	bool needs_kick;
> +	u32 snapshot;
> +
> +	START_USE(vq);
> +	/* We need to expose the new flags value before checking notification
> +	 * suppressions. */
> +	virtio_mb(vq->weak_barriers);
> +
> +	snapshot = READ_ONCE(*(u32 *)vq->vring_packed.device);
> +	flags = virtio16_to_cpu(_vq->vdev, (__virtio16)(snapshot >> 16))
& 0x3;
> +
> +#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
> +
> +	needs_kick = (flags != VRING_EVENT_F_DISABLE);
> +	END_USE(vq);
> +	return needs_kick;
> +}
> +
> +static void detach_buf_packed(struct vring_virtqueue *vq,
> +			      unsigned int id, void **ctx)
> +{
> +	struct vring_desc_state_packed *state = NULL;
> +	struct vring_packed_desc *desc;
> +	unsigned int curr, i;
> +
> +	/* Clear data ptr. */
> +	vq->desc_state_packed[id].data = NULL;
> +
> +	curr = id;
> +	for (i = 0; i < vq->desc_state_packed[id].num; i++) {
> +		state = &vq->desc_state_packed[curr];
> +		vring_unmap_state_packed(vq, state);
> +		curr = state->next;
> +	}
> +
> +	BUG_ON(state == NULL);
> +	vq->vq.num_free += vq->desc_state_packed[id].num;
> +	state->next = vq->free_head;
> +	vq->free_head = id;
> +
> +	if (vq->indirect) {
> +		u32 len;
> +
> +		/* Free the indirect table, if any, now that it's unmapped. */
> +		desc = vq->desc_state_packed[id].indir_desc;
> +		if (!desc)
> +			return;
> +
> +		if (vring_use_dma_api(vq->vq.vdev)) {
> +			len = vq->desc_state_packed[id].len;
> +			for (i = 0; i < len / sizeof(struct vring_packed_desc);
> +					i++)
> +				vring_unmap_desc_packed(vq, &desc[i]);
> +		}
> +		kfree(desc);
> +		vq->desc_state_packed[id].indir_desc = NULL;
> +	} else if (ctx) {
> +		*ctx = vq->desc_state_packed[id].indir_desc;
> +	}
> +}
> +
> +static inline bool is_used_desc_packed(const struct vring_virtqueue *vq,
> +				       u16 idx, bool used_wrap_counter)
> +{
> +	u16 flags;
> +	bool avail, used;
> +
> +	flags = virtio16_to_cpu(vq->vq.vdev,
> +				vq->vring_packed.desc[idx].flags);
> +	avail = !!(flags & VRING_DESC_F_AVAIL);
> +	used = !!(flags & VRING_DESC_F_USED);
> +
> +	return avail == used && used == used_wrap_counter;
>  }
>  
>  static inline bool more_used_packed(const struct vring_virtqueue *vq)
>  {
> -	return false;
> +	return is_used_desc_packed(vq, vq->last_used_idx,
> +			vq->used_wrap_counter);
>  }
>  
>  static void *virtqueue_get_buf_ctx_packed(struct virtqueue *_vq,
>  					  unsigned int *len,
>  					  void **ctx)
>  {
> -	return NULL;
> +	struct vring_virtqueue *vq = to_vvq(_vq);
> +	u16 last_used, id;
> +	void *ret;
> +
> +	START_USE(vq);
> +
> +	if (unlikely(vq->broken)) {
> +		END_USE(vq);
> +		return NULL;
> +	}
> +
> +	if (!more_used_packed(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;
> +	id = virtio16_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(id >= vq->vring_packed.num)) {
> +		BAD_RING(vq, "id %u out of range\n", id);
> +		return NULL;
> +	}
> +	if (unlikely(!vq->desc_state_packed[id].data)) {
> +		BAD_RING(vq, "id %u is not a head!\n", id);
> +		return NULL;
> +	}
> +
> +	vq->last_used_idx += vq->desc_state_packed[id].num;
> +	if (vq->last_used_idx >= vq->vring_packed.num) {
> +		vq->last_used_idx -= vq->vring_packed.num;
> +		vq->used_wrap_counter ^= 1;
> +	}
> +
> +	/* detach_buf_packed clears data, so grab it now. */
> +	ret = vq->desc_state_packed[id].data;
> +	detach_buf_packed(vq, id, ctx);
> +
> +#ifdef DEBUG
> +	vq->last_add_time_valid = false;
> +#endif
> +
> +	END_USE(vq);
> +	return ret;
>  }
>  
>  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);
> +	}
>  }
>  
>  static unsigned virtqueue_enable_cb_prepare_packed(struct virtqueue *_vq)
>  {
> -	return 0;
> +	struct vring_virtqueue *vq = to_vvq(_vq);
> +
> +	START_USE(vq);
> +
> +	/* We optimistically turn back on interrupts, then check if there was
> +	 * more to do. */
> +
> +	if (vq->event_flags_shadow == VRING_EVENT_F_DISABLE) {
> +		vq->event_flags_shadow = VRING_EVENT_F_ENABLE;
> +		vq->vring_packed.driver->flags = cpu_to_virtio16(_vq->vdev,
> +							vq->event_flags_shadow);
> +	}
> +
> +	END_USE(vq);
> +	return vq->last_used_idx | ((u16)vq->used_wrap_counter <<
15);
>  }
>  
> -static bool virtqueue_poll_packed(struct virtqueue *_vq, unsigned
last_used_idx)
> +static bool virtqueue_poll_packed(struct virtqueue *_vq, unsigned
off_wrap)
>  {
> -	return false;
> +	struct vring_virtqueue *vq = to_vvq(_vq);
> +	bool wrap_counter;
> +	u16 used_idx;
> +
> +	wrap_counter = off_wrap >> 15;
> +	used_idx = off_wrap & ~(1 << 15);
> +
> +	return is_used_desc_packed(vq, used_idx, wrap_counter);
>  }
>  
>  static bool virtqueue_enable_cb_delayed_packed(struct virtqueue *_vq)
>  {
> -	return false;
> +	struct vring_virtqueue *vq = to_vvq(_vq);
> +
> +	START_USE(vq);
> +
> +	/* We optimistically turn back on interrupts, then check if there was
> +	 * more to do. */
> +
> +	if (vq->event_flags_shadow == VRING_EVENT_F_DISABLE) {
> +		vq->event_flags_shadow = VRING_EVENT_F_ENABLE;
> +		vq->vring_packed.driver->flags = cpu_to_virtio16(_vq->vdev,
> +							vq->event_flags_shadow);
> +		/* We need to enable interrupts first before re-checking
> +		 * for more used buffers. */
> +		virtio_mb(vq->weak_barriers);
> +	}
> +
> +	if (more_used_packed(vq)) {
> +		END_USE(vq);
> +		return false;
> +	}
> +
> +	END_USE(vq);
> +	return true;
>  }
>  
>  static void *virtqueue_detach_unused_buf_packed(struct virtqueue *_vq)
>  {
> +	struct vring_virtqueue *vq = to_vvq(_vq);
> +	unsigned int i;
> +	void *buf;
> +
> +	START_USE(vq);
> +
> +	for (i = 0; i < vq->vring_packed.num; i++) {
> +		if (!vq->desc_state_packed[i].data)
> +			continue;
> +		/* detach_buf clears data, so grab it now. */
> +		buf = vq->desc_state_packed[i].data;
> +		detach_buf_packed(vq, i, NULL);
> +		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;
>  }
>  
> @@ -1083,6 +1559,9 @@ bool virtqueue_poll(struct virtqueue *_vq, unsigned
last_used_idx)
>  {
>  	struct vring_virtqueue *vq = to_vvq(_vq);
>  
> +	/* We need to enable interrupts first before re-checking
> +	 * for more used buffers. */
> +	virtio_mb(vq->weak_barriers);
>  	return vq->packed ? virtqueue_poll_packed(_vq, last_used_idx) :
>  			    virtqueue_poll_split(_vq, last_used_idx);
>  }
> -- 
> 2.18.0

Tiwei Bie

2018-Nov-08 01:38 UTC

head link

[PATCH net-next v2 3/5] virtio_ring: add packed ring support

On Wed, Nov 07, 2018 at 12:48:46PM -0500, Michael S. Tsirkin
wrote:> On Wed, Jul 11, 2018 at 10:27:09AM +0800, Tiwei Bie wrote:
> > This commit introduces the support (without EVENT_IDX) for
> > packed ring.
> > 
> > Signed-off-by: Tiwei Bie <tiwei.bie at intel.com>
> > ---
> >  drivers/virtio/virtio_ring.c | 495
++++++++++++++++++++++++++++++++++-
> >  1 file changed, 487 insertions(+), 8 deletions(-)
[...]> >  
> > +static void vring_unmap_state_packed(const struct vring_virtqueue
*vq,
> > +				     struct vring_desc_state_packed *state)
> > +{
> > +	u16 flags;
> > +
> > +	if (!vring_use_dma_api(vq->vq.vdev))
> > +		return;
> > +
> > +	flags = state->flags;
> > +
> > +	if (flags & VRING_DESC_F_INDIRECT) {
> > +		dma_unmap_single(vring_dma_dev(vq),
> > +				 state->addr, state->len,
> > +				 (flags & VRING_DESC_F_WRITE) ?
> > +				 DMA_FROM_DEVICE : DMA_TO_DEVICE);
> > +	} else {
> > +		dma_unmap_page(vring_dma_dev(vq),
> > +			       state->addr, state->len,
> > +			       (flags & VRING_DESC_F_WRITE) ?
> > +			       DMA_FROM_DEVICE : DMA_TO_DEVICE);
> > +	}
> > +}
> > +
> > +static void vring_unmap_desc_packed(const struct vring_virtqueue *vq,
> > +				   struct vring_packed_desc *desc)
> > +{
> > +	u16 flags;
> > +
> > +	if (!vring_use_dma_api(vq->vq.vdev))
> > +		return;
> > +
> > +	flags = virtio16_to_cpu(vq->vq.vdev, desc->flags);
> 
> BTW this stuff is only used on error etc. Is there a way to
> reuse vring_unmap_state_packed?
It's also used by the INDIRECT path. We don't allocate desc
state for INDIRECT descriptors to save DMA addr/len etc.
> 
> > +
> > +	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);
> > +	}
> > +}
[...]> > @@ -766,47 +840,449 @@ static inline int virtqueue_add_packed(struct
virtqueue *_vq,
> >  				       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;
> > +	u16 head, avail_wrap_counter, id, curr;
> > +	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;
> > +	avail_wrap_counter = vq->avail_wrap_counter;
> > +
> > +	if (virtqueue_use_indirect(_vq, total_sg))
> > +		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. */
> 
> I don't think we have a reason to do this for packed ring.
> No historical baggage there, right?
Based on the original commit log, it seems that the notify here
is just an "optimization". But I don't quite understand what does
the "the heuristics which KVM uses" refer to. If it's safe to drop
this in packed ring, I'd like to do it.

commit 44653eae1407f79dff6f52fcf594ae84cb165ec4
Author: Rusty Russell <rusty at rustcorp.com.au>
Date:   Fri Jul 25 12:06:04 2008 -0500

    virtio: don't always force a notification when ring is full
    
    We force notification when the ring is full, even if the host has
    indicated it doesn't want to know.  This seemed like a good idea at
    the time: if we fill the transmit ring, we should tell the host
    immediately.
    
    Unfortunately this logic also applies to the receiving ring, which is
    refilled constantly.  We should introduce real notification thesholds
    to replace this logic.  Meanwhile, removing the logic altogether breaks
    the heuristics which KVM uses, so we use a hack: only notify if there are
    outgoing parts of the new buffer.
    
    Here are the number of exits with lguest's crappy network
implementation:
    Before:
            network xmit 7859051 recv 236420
    After:
            network xmit 7858610 recv 118136
    
    Signed-off-by: Rusty Russell <rusty at rustcorp.com.au>

diff --git a/drivers/virtio/virtio_ring.c b/drivers/virtio/virtio_ring.c
index 72bf8bc09014..21d9a62767af 100644
--- a/drivers/virtio/virtio_ring.c
+++ b/drivers/virtio/virtio_ring.c
@@ -87,8 +87,11 @@ static int vring_add_buf(struct virtqueue *_vq,
 	if (vq->num_free < out + in) {
 		pr_debug("Can't add buf len %i - avail = %i\n",
 			 out + in, vq->num_free);
-		/* We notify *even if* VRING_USED_F_NO_NOTIFY is set here. */
-		vq->notify(&vq->vq);
+		/* 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;
 	}

> 
> > +		if (out_sgs)
> > +			vq->notify(&vq->vq);
> > +		if (indirect)
> > +			kfree(desc);
> > +		END_USE(vq);
> > +		return -ENOSPC;
> > +	}
> > +[...]

Jason Wang

2018-Nov-08 08:18 UTC

head link

[PATCH net-next v2 3/5] virtio_ring: add packed ring support

On 2018/11/8 ??9:38, Tiwei Bie wrote:>>> +
>>> +	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. */
>> I don't think we have a reason to do this for packed ring.
>> No historical baggage there, right?
> Based on the original commit log, it seems that the notify here
> is just an "optimization". But I don't quite understand what
does
> the "the heuristics which KVM uses" refer to. If it's safe to
drop
> this in packed ring, I'd like to do it.

According to the commit log, it seems like a workaround of lguest 
networking backend. I agree to drop it, we should not have such burden.

But we should notice that, with this removed, the compare between packed 
vs split is kind of unfair. Consider the removal of lguest support 
recently, maybe we can drop this for split ring as well?

Thanks

>
> commit 44653eae1407f79dff6f52fcf594ae84cb165ec4
> Author: Rusty Russell<rusty at rustcorp.com.au>
> Date:   Fri Jul 25 12:06:04 2008 -0500
>
>      virtio: don't always force a notification when ring is full
>      
>      We force notification when the ring is full, even if the host has
>      indicated it doesn't want to know.  This seemed like a good idea
at
>      the time: if we fill the transmit ring, we should tell the host
>      immediately.
>      
>      Unfortunately this logic also applies to the receiving ring, which is
>      refilled constantly.  We should introduce real notification thesholds
>      to replace this logic.  Meanwhile, removing the logic altogether
breaks
>      the heuristics which KVM uses, so we use a hack: only notify if there
are
>      outgoing parts of the new buffer.
>      
>      Here are the number of exits with lguest's crappy network
implementation:
>      Before:
>              network xmit 7859051 recv 236420
>      After:
>              network xmit 7858610 recv 118136
>      
>      Signed-off-by: Rusty Russell<rusty at rustcorp.com.au>
>
> diff --git a/drivers/virtio/virtio_ring.c b/drivers/virtio/virtio_ring.c
> index 72bf8bc09014..21d9a62767af 100644
> --- a/drivers/virtio/virtio_ring.c
> +++ b/drivers/virtio/virtio_ring.c
> @@ -87,8 +87,11 @@ static int vring_add_buf(struct virtqueue *_vq,
>   	if (vq->num_free < out + in) {
>   		pr_debug("Can't add buf len %i - avail = %i\n",
>   			 out + in, vq->num_free);
> -		/* We notify*even if*  VRING_USED_F_NO_NOTIFY is set here. */
> -		vq->notify(&vq->vq);
> +		/* 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;
>   	}
>
>

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