Message-ID: <2000f635-bc34-71ff-ff51-a711c2e9726d@redhat.com>
Date:   Wed, 16 May 2018 19:50:16 +0800
From:   Jason Wang <jasowang@...hat.com>
To:     Tiwei Bie <tiwei.bie@...el.com>, mst@...hat.com,
        virtualization@...ts.linux-foundation.org,
        linux-kernel@...r.kernel.org, netdev@...r.kernel.org
Cc:     wexu@...hat.com, jfreimann@...hat.com
Subject: Re: [RFC v4 3/5] virtio_ring: add packed ring support



On 2018年05月16日 16:37, Tiwei Bie wrote:
> This commit introduces the basic support (without EVENT_IDX)
> for packed ring.
>
> Signed-off-by: Tiwei Bie <tiwei.bie@...el.com>
> ---
>   drivers/virtio/virtio_ring.c | 491 ++++++++++++++++++++++++++++++++++-
>   1 file changed, 481 insertions(+), 10 deletions(-)
>
> diff --git a/drivers/virtio/virtio_ring.c b/drivers/virtio/virtio_ring.c
> index 62d7c407841a..c6c5deb0e3ae 100644
> --- a/drivers/virtio/virtio_ring.c
> +++ b/drivers/virtio/virtio_ring.c
> @@ -58,7 +58,8 @@
>   
>   struct vring_desc_state {
>   	void *data;			/* Data for callback. */
> -	struct vring_desc *indir_desc;	/* Indirect descriptor, if any. */
> +	void *indir_desc;		/* Indirect descriptor, if any. */
> +	int num;			/* Descriptor list length. */
>   };
>   
>   struct vring_virtqueue {
> @@ -116,6 +117,9 @@ struct vring_virtqueue {
>   			/* Last written value to driver->flags in
>   			 * guest byte order. */
>   			u16 event_flags_shadow;
> +
> +			/* ID allocation. */
> +			struct idr buffer_id;

I'm not sure idr is fit for the performance critical case here. Need to 
measure its performance impact, especially if we have few unused slots.

>   		};
>   	};
>   
> @@ -142,6 +146,16 @@ struct vring_virtqueue {
>   
>   #define to_vvq(_vq) container_of(_vq, struct vring_virtqueue, vq)
>   
> +static inline bool virtqueue_use_indirect(struct virtqueue *_vq,
> +					  unsigned int total_sg)
> +{
> +	struct vring_virtqueue *vq = to_vvq(_vq);
> +
> +	/* If the host supports indirect descriptor tables, and we have multiple
> +	 * buffers, then go indirect. FIXME: tune this threshold */
> +	return (vq->indirect && total_sg > 1 && vq->vq.num_free);
> +}
> +
>   /*
>    * Modern virtio devices have feature bits to specify whether they need a
>    * quirk and bypass the IOMMU. If not there, just use the DMA API.
> @@ -327,9 +341,7 @@ static inline int virtqueue_add_split(struct virtqueue *_vq,
>   
>   	head = vq->free_head;
>   
> -	/* If the host supports indirect descriptor tables, and we have multiple
> -	 * buffers, then go indirect. FIXME: tune this threshold */
> -	if (vq->indirect && total_sg > 1 && vq->vq.num_free)
> +	if (virtqueue_use_indirect(_vq, total_sg))
>   		desc = alloc_indirect_split(_vq, total_sg, gfp);
>   	else {
>   		desc = NULL;
> @@ -741,6 +753,63 @@ 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_one_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);
> +
> +	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 u16 alloc_id_packed(struct vring_virtqueue *vq)
> +{
> +	u16 id;
> +
> +	id = idr_alloc(&vq->buffer_id, NULL, 0, vq->vring_packed.num,
> +		       GFP_KERNEL);
> +	return id;
> +}
> +
> +static void free_id_packed(struct vring_virtqueue *vq, u16 id)
> +{
> +	idr_remove(&vq->buffer_id, id);
> +}
> +
>   static inline int virtqueue_add_packed(struct virtqueue *_vq,
>   				       struct scatterlist *sgs[],
>   				       unsigned int total_sg,
> @@ -750,47 +819,446 @@ 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, wrap_counter, id;
> +	bool indirect;
> +
> +	START_USE(vq);
> +
> +	BUG_ON(data == NULL);
> +	BUG_ON(ctx && vq->indirect);
> +
> +	if (unlikely(vq->broken)) {
> +		END_USE(vq);
> +		return -EIO;
> +	}
> +
> +#ifdef DEBUG
> +	{
> +		ktime_t now = ktime_get();
> +
> +		/* No kick or get, with .1 second between?  Warn. */
> +		if (vq->last_add_time_valid)
> +			WARN_ON(ktime_to_ms(ktime_sub(now, vq->last_add_time))
> +					    > 100);
> +		vq->last_add_time = now;
> +		vq->last_add_time_valid = true;
> +	}
> +#endif
> +
> +	BUG_ON(total_sg == 0);
> +
> +	head = vq->next_avail_idx;
> +	wrap_counter = vq->wrap_counter;
> +
> +	if (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. */
> +		if (out_sgs)
> +			vq->notify(&vq->vq);
> +		if (indirect)
> +			kfree(desc);
> +		END_USE(vq);
> +		return -ENOSPC;
> +	}
> +
> +	id = alloc_id_packed(vq);
> +
> +	for (n = 0; n < out_sgs + in_sgs; n++) {
> +		for (sg = sgs[n]; sg; sg = sg_next(sg)) {
> +			dma_addr_t addr = vring_map_one_sg(vq, sg, n < out_sgs ?
> +					       DMA_TO_DEVICE : DMA_FROM_DEVICE);
> +			if (vring_mapping_error(vq, addr))
> +				goto unmap_release;
> +
> +			flags = cpu_to_virtio16(_vq->vdev, VRING_DESC_F_NEXT |
> +					(n < out_sgs ? 0 : VRING_DESC_F_WRITE) |
> +					VRING_DESC_F_AVAIL(vq->wrap_counter) |
> +					VRING_DESC_F_USED(!vq->wrap_counter));
> +			if (!indirect && i == head)
> +				head_flags = flags;
> +			else
> +				desc[i].flags = flags;
> +
> +			desc[i].addr = cpu_to_virtio64(_vq->vdev, addr);
> +			desc[i].len = cpu_to_virtio32(_vq->vdev, sg->length);
> +			i++;
> +			if (!indirect && i >= vq->vring_packed.num) {
> +				i = 0;
> +				vq->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(wrap_counter) |
> +					     VRING_DESC_F_USED(!wrap_counter));
> +		vq->vring_packed.desc[head].addr = cpu_to_virtio64(_vq->vdev,
> +								   addr);
> +		vq->vring_packed.desc[head].len = cpu_to_virtio32(_vq->vdev,
> +				total_sg * sizeof(struct vring_packed_desc));
> +		vq->vring_packed.desc[head].id = cpu_to_virtio16(_vq->vdev, id);
> +	}
> +
> +	/* We're using some buffers from the free list. */
> +	vq->vq.num_free -= descs_used;
> +
> +	/* Update free pointer */
> +	if (indirect) {
> +		n = head + 1;
> +		if (n >= vq->vring_packed.num) {
> +			n = 0;
> +			vq->wrap_counter ^= 1;
> +		}
> +		vq->next_avail_idx = n;
> +	} else
> +		vq->next_avail_idx = i;
> +
> +	/* Store token and indirect buffer state. */
> +	vq->desc_state[id].num = descs_used;
> +	vq->desc_state[id].data = data;
> +	if (indirect)
> +		vq->desc_state[id].indir_desc = desc;
> +	else
> +		vq->desc_state[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_one_packed(vq, &desc[i]);
> +		i++;
> +		if (!indirect && i >= vq->vring_packed.num)
> +			i = 0;
> +	}
> +
> +	vq->wrap_counter = wrap_counter;
> +
> +	if (indirect)
> +		kfree(desc);
> +
> +	free_id_packed(vq, id);
> +
> +	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 = *(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 head,
> +			      unsigned int id, void **ctx)
> +{
> +	struct vring_packed_desc *desc;
> +	unsigned int i, j;
> +
> +	/* Clear data ptr. */
> +	vq->desc_state[id].data = NULL;
> +
> +	i = head;
> +
> +	for (j = 0; j < vq->desc_state[id].num; j++) {
> +		desc = &vq->vring_packed.desc[i];
> +		vring_unmap_one_packed(vq, desc);

As mentioned in previous discussion, this probably won't work for the 
case of out of order completion since it depends on the information in 
the descriptor ring. We probably need to extend ctx to record such 
information.

Thanks

> +		i++;
> +		if (i >= vq->vring_packed.num)
> +			i = 0;
> +	}
> +
> +	vq->vq.num_free += vq->desc_state[id].num;
> +
> +	if (vq->indirect) {
> +		u32 len;
> +
> +		/* Free the indirect table, if any, now that it's unmapped. */
> +		desc = vq->desc_state[id].indir_desc;
> +		if (!desc)
> +			goto out;
> +
> +		len = virtio32_to_cpu(vq->vq.vdev,
> +				      vq->vring_packed.desc[head].len);
> +
> +		for (j = 0; j < len / sizeof(struct vring_packed_desc); j++)
> +			vring_unmap_one_packed(vq, &desc[j]);
> +
> +		kfree(desc);
> +		vq->desc_state[id].indir_desc = NULL;
> +	} else if (ctx) {
> +		*ctx = vq->desc_state[id].indir_desc;
> +	}
> +
> +out:
> +	free_id_packed(vq, id);
>   }
>   
>   static inline bool more_used_packed(const struct vring_virtqueue *vq)
>   {
> -	return false;
> +	u16 last_used, flags;
> +	bool avail, used;
> +
> +	if (vq->vq.num_free == vq->vring_packed.num)
> +		return false;
> +
> +	last_used = vq->last_used_idx;
> +	flags = virtio16_to_cpu(vq->vq.vdev,
> +				vq->vring_packed.desc[last_used].flags);
> +	avail = flags & VRING_DESC_F_AVAIL(1);
> +	used = flags & VRING_DESC_F_USED(1);
> +
> +	return avail == used;
>   }
>   
>   static 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[id].data)) {
> +		BAD_RING(vq, "id %u is not a head!\n", id);
> +		return NULL;
> +	}
> +
> +	vq->last_used_idx += vq->desc_state[id].num;
> +	if (vq->last_used_idx >= vq->vring_packed.num)
> +		vq->last_used_idx -= vq->vring_packed.num;
> +
> +	/* detach_buf_packed clears data, so grab it now. */
> +	ret = vq->desc_state[id].data;
> +	detach_buf_packed(vq, last_used, 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) {
> +		virtio_wmb(vq->weak_barriers);
> +		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;
>   }
>   
>   static bool virtqueue_poll_packed(struct virtqueue *_vq, unsigned last_used_idx)
>   {
> -	return false;
> +	struct vring_virtqueue *vq = to_vvq(_vq);
> +	bool avail, used;
> +	u16 flags;
> +
> +	virtio_mb(vq->weak_barriers);
> +	flags = virtio16_to_cpu(vq->vq.vdev,
> +			vq->vring_packed.desc[last_used_idx].flags);
> +	avail = flags & VRING_DESC_F_AVAIL(1);
> +	used = flags & VRING_DESC_F_USED(1);
> +	return avail == used;
>   }
>   
>   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) {
> +		virtio_wmb(vq->weak_barriers);
> +		vq->event_flags_shadow = VRING_EVENT_F_ENABLE;
> +		vq->vring_packed.driver->flags = cpu_to_virtio16(_vq->vdev,
> +							vq->event_flags_shadow);
> +	}
> +
> +	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);
> +	u16 flags, head, id, i;
> +	unsigned int len;
> +	void *buf;
> +
> +	START_USE(vq);
> +
> +	/* Detach the used descriptors. */
> +	if (more_used_packed(vq)) {
> +		buf = virtqueue_get_buf_ctx_packed(_vq, &len, NULL);
> +		END_USE(vq);
> +		return buf;
> +	}
> +
> +	/* Detach the available descriptors. */
> +	for (i = vq->last_used_idx; i != vq->next_avail_idx;
> +			i = (i + 1) % vq->vring_packed.num) {
> +		flags = virtio16_to_cpu(vq->vq.vdev,
> +				vq->vring_packed.desc[i].flags);
> +		while (flags & VRING_DESC_F_NEXT) {
> +			i = (i + 1) % vq->vring_packed.num;
> +			flags = virtio16_to_cpu(vq->vq.vdev,
> +					vq->vring_packed.desc[i].flags);
> +		}
> +		id = virtio16_to_cpu(_vq->vdev, vq->vring_packed.desc[i].id);
> +		if (!vq->desc_state[id].data)
> +			continue;
> +
> +		len = vq->desc_state[id].num - 1;
> +		head = (i < len ? i + vq->vring_packed.num : i) - len;
> +
> +		/* detach_buf clears data, so grab it now. */
> +		buf = vq->desc_state[id].data;
> +		detach_buf_packed(vq, head, id, 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;
>   }
>   
> @@ -1198,6 +1666,7 @@ struct virtqueue *__vring_new_virtqueue(unsigned int index,
>   		vq->next_avail_idx = 0;
>   		vq->wrap_counter = 1;
>   		vq->event_flags_shadow = 0;
> +		idr_init(&vq->buffer_id);
>   	} else {
>   		vq->vring = vring.vring_split;
>   		vq->avail_flags_shadow = 0;
> @@ -1384,6 +1853,8 @@ void vring_del_virtqueue(struct virtqueue *_vq)
>   					      (void *)vq->vring.desc,
>   				 vq->queue_dma_addr);
>   	}
> +	if (vq->packed)
> +		idr_destroy(&vq->buffer_id);
>   	list_del(&_vq->list);
>   	kfree(vq);
>   }

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