RE: [RFC,v3 1/2] vhost: support async dequeue for split ring

["Ding, Xuan" <xuan.ding@intel.com>]

Hi Maxime,

> -----Original Message-----
> From: Maxime Coquelin <maxime.coquelin@redhat.com>
> Sent: Thursday, March 31, 2022 5:15 PM
> To: Ding, Xuan <xuan.ding@intel.com>; Xia, Chenbo <chenbo.xia@intel.com>
> Cc: dev@dpdk.org; Hu, Jiayu <jiayu.hu@intel.com>; Jiang, Cheng1
> <cheng1.jiang@intel.com>; Pai G, Sunil <sunil.pai.g@intel.com>;
> liangma@liangbit.com; Wang, YuanX <yuanx.wang@intel.com>
> Subject: Re: [RFC,v3 1/2] vhost: support async dequeue for split ring
> 
> 
> 
> On 3/10/22 07:54, xuan.ding@intel.com wrote:
> > From: Xuan Ding <xuan.ding@intel.com>
> >
> > This patch implements asynchronous dequeue data path for vhost split
> > ring, with dmadev library integrated.
> >
> > Signed-off-by: Xuan Ding <xuan.ding@intel.com>
> > Signed-off-by: Yuan Wang <yuanx.wang@intel.com>
> > ---
> >   lib/vhost/rte_vhost_async.h |  37 ++-
> >   lib/vhost/version.map       |   1 +
> >   lib/vhost/vhost.h           |   1 +
> >   lib/vhost/virtio_net.c      | 504 ++++++++++++++++++++++++++++++++++++
> >   4 files changed, 541 insertions(+), 2 deletions(-)
> >
> > diff --git a/lib/vhost/rte_vhost_async.h b/lib/vhost/rte_vhost_async.h
> > index f1293c6a9d..b6ab0b06a2 100644
> > --- a/lib/vhost/rte_vhost_async.h
> > +++ b/lib/vhost/rte_vhost_async.h
> > @@ -155,9 +155,9 @@ int rte_vhost_async_get_inflight(int vid, uint16_t
> queue_id);
> >    * @param count
> >    *  Size of the packet array
> >    * @param dma_id
> > - *  the identifier of DMA device
> > + *  The identifier of DMA device
> >    * @param vchan_id
> > - *  the identifier of virtual DMA channel
> > + *  The identifier of virtual DMA channel
> 
> This is unrelated to the purpose of this patch, it can be moved in a dedicated
> trivial patch.

Okay, I will remove it from this patch set.

> 
> >    * @return
> >    *  Number of packets returned
> >    */
> > @@ -187,6 +187,39 @@ uint16_t
> rte_vhost_clear_queue_thread_unsafe(int vid, uint16_t queue_id,
> >   __rte_experimental
> >   int rte_vhost_async_dma_configure(int16_t dma_id, uint16_t
> > vchan_id);
> >
> > +/**
> > + * This function tries to receive packets from the guest with
> > +offloading
> > + * copies to the async channel. The packets that are transfer
> > +completed
> > + * are returned in "pkts". The other packets that their copies are
> > +submitted to
> > + * the async channel but not completed are called "in-flight packets".
> > + * This function will not return in-flight packets until their copies
> > +are
> > + * completed by the async channel.
> > + *
> > + * @param vid
> > + *  ID of vhost device to dequeue data
> > + * @param queue_id
> > + *  ID of virtqueue to dequeue data
> > + * @param mbuf_pool
> > + *  Mbuf_pool where host mbuf is allocated
> > + * @param pkts
> > + *  Blank array to keep successfully dequeued packets
> > + * @param count
> > + *  Size of the packet array
> > + * @param nr_inflight
> > + *  The amount of in-flight packets. If error occurred, its value is set to -1.
> > + * @param dma_id
> > + *  The identifier of DMA device
> > + * @param vchan_id
> > + *  The identifier of virtual DMA channel
> > + * @return
> > + *  Number of successfully dequeued packets  */ __rte_experimental
> > +uint16_t rte_vhost_async_try_dequeue_burst(int vid, uint16_t
> > +queue_id,
> > +	struct rte_mempool *mbuf_pool, struct rte_mbuf **pkts, uint16_t
> count,
> > +	int *nr_inflight, uint16_t dma_id, uint16_t vchan_id);
> > +
> >   #ifdef __cplusplus
> >   }
> >   #endif
> > diff --git a/lib/vhost/version.map b/lib/vhost/version.map index
> > 0a66c5840c..968d6d4290 100644
> > --- a/lib/vhost/version.map
> > +++ b/lib/vhost/version.map
> > @@ -87,6 +87,7 @@ EXPERIMENTAL {
> >
> >   	# added in 22.03
> >   	rte_vhost_async_dma_configure;
> 
> # added in 22.07

Sure, I will add in 22.07 in new version.

> 
> > +	rte_vhost_async_try_dequeue_burst;
> >   };
> >
> >   INTERNAL {
> > diff --git a/lib/vhost/vhost.h b/lib/vhost/vhost.h index
> > a9edc271aa..3799d41089 100644
> > --- a/lib/vhost/vhost.h
> > +++ b/lib/vhost/vhost.h
> > @@ -178,6 +178,7 @@ extern struct async_dma_info
> dma_copy_track[RTE_DMADEV_DEFAULT_MAX];
> >    */
> >   struct async_inflight_info {
> >   	struct rte_mbuf *mbuf;
> > +	struct virtio_net_hdr nethdr;
> >   	uint16_t descs; /* num of descs inflight */
> >   	uint16_t nr_buffers; /* num of buffers inflight for packed ring */
> >   };
> > diff --git a/lib/vhost/virtio_net.c b/lib/vhost/virtio_net.c index
> > 5f432b0d77..3816caca79 100644
> > --- a/lib/vhost/virtio_net.c
> > +++ b/lib/vhost/virtio_net.c
> > @@ -3141,3 +3141,507 @@ rte_vhost_dequeue_burst(int vid, uint16_t
> > queue_id,
> >
> >   	return count;
> >   }
> > +
> > +static __rte_always_inline int
> > +async_desc_to_mbuf_seg(struct virtio_net *dev, struct vhost_virtqueue
> *vq,
> > +			struct rte_mbuf *m, uint32_t mbuf_offset,
> > +			uint64_t buf_iova, uint32_t cpy_len) {
> > +	struct vhost_async *async = vq->async;
> > +	uint64_t mapped_len;
> > +	uint32_t buf_offset = 0;
> > +	void *host_iova;
> > +
> > +	while (cpy_len) {
> > +		host_iova = (void *)(uintptr_t)gpa_to_first_hpa(dev,
> > +					buf_iova + buf_offset, cpy_len,
> > +					&mapped_len);
> > +		if (unlikely(!host_iova)) {
> > +			VHOST_LOG_DATA(ERR, "(%s) %s: failed to get
> host_iova.\n",
> > +				dev->ifname, __func__);
> > +			return -1;
> > +		}
> > +
> > +		if (unlikely(async_iter_add_iovec(dev, async, host_iova,
> > +				(void *)(uintptr_t)rte_pktmbuf_iova_offset(m,
> mbuf_offset),
> > +				(size_t)mapped_len)))
> > +			return -1;
> > +
> > +		cpy_len -= (uint32_t)mapped_len;
> > +		mbuf_offset += (uint32_t)mapped_len;
> > +		buf_offset += (uint32_t)mapped_len;
> > +	}
> > +
> > +	return 0;
> > +}
> 
> It looks really similar to async_mbuf_to_desc_seg(), just the direction of the
> DMA copoy is changed.
> 
> Maybe we could refactor the two functions in a single one with adding a
> parameter for the direction. Something like this:
> 
> static __rte_always_inline int
> async_fill_seg(struct virtio_net *dev, struct vhost_virtqueue *vq,
> 		struct rte_mbuf *m, uint32_t mbuf_offset,
> 		uint64_t buf_iova, uint32_t cpy_len, bool to_desc) {
> 	struct vhost_async *async = vq->async;
> 	uint64_t mapped_len;
> 	uint32_t buf_offset = 0;
> 	void *src, *dst
> 	void *host_iova;
> 
> 	while (cpy_len) {
> 		host_iova = (void *)(uintptr_t)gpa_to_first_hpa(dev,
> 				buf_iova + buf_offset, cpy_len,
> &mapped_len);
> 		if (unlikely(!host_iova)) {
> 			VHOST_LOG_DATA(ERR, "(%s) %s: failed to get host
> iova.\n",
> 				       dev->ifname, __func__);
> 			return -1;
> 		}
> 
> 		if (to_desc) {
> 			src = (void *)(uintptr_t)rte_pktmbuf_iova_offset(m,
> mbuff_offset);
> 			dst = host_iova
> 		} else {
> 			dst = host_iova
> 			src = (void *)(uintptr_t)rte_pktmbuf_iova_offset(m,
> mbuff_offset);
> 		}
> 
> 		if (unlikely(async_iter_add_iovec(dev, async, src, dst,
> (size_t)mapped_len)))
> 			return -1;
> 
> 		cpy_len -= (uint32_t)mapped_len;
> 		mbuf_offset += (uint32_t)mapped_len;
> 		buf_offset += (uint32_t)mapped_len;
> 	}
> 
> 	return 0;
> }
> 
> 
> Then if you don't pass a variable but a static true/false value for to_desc, the
> compiler should generate the same code, so no performance degradation.

Thanks so much for the suggestion.
I noticed the async_mbuf_to_desc_seg() refactoring in enqueue path,
so I refactored the previous dequeue code to async_desc_to_mbuf_seg().
These two functions can indeed be combined into one, it is a good idea.

> 
> > +
> > +static __rte_always_inline int
> > +async_desc_to_mbuf(struct virtio_net *dev, struct vhost_virtqueue *vq,
> > +		  struct buf_vector *buf_vec, uint16_t nr_vec,
> > +		  struct rte_mbuf *m, struct rte_mempool *mbuf_pool,
> > +		  struct virtio_net_hdr *nethdr)
> > +{
> > +	uint64_t buf_addr, buf_iova;
> > +	uint32_t buf_avail, buf_offset, buf_len;
> > +	uint32_t mbuf_avail, mbuf_offset;
> > +	uint32_t cpy_len;
> > +	/* A counter to avoid desc dead loop chain */
> > +	uint16_t vec_idx = 0;
> > +	struct rte_mbuf *cur = m, *prev = m;
> > +	struct virtio_net_hdr tmp_hdr;
> > +	struct virtio_net_hdr *hdr = NULL;
> > +	struct vhost_async *async = vq->async;
> > +
> > +	buf_addr = buf_vec[vec_idx].buf_addr;
> > +	buf_len = buf_vec[vec_idx].buf_len;
> > +	buf_iova = buf_vec[vec_idx].buf_iova;
> > +
> > +	if (unlikely(buf_len < dev->vhost_hlen && nr_vec <= 1))
> > +		return -1;
> > +
> > +	if (virtio_net_with_host_offload(dev)) {
> > +		if (unlikely(buf_len < sizeof(struct virtio_net_hdr))) {
> > +			/*
> > +			 * No luck, the virtio-net header doesn't fit
> > +			 * in a contiguous virtual area.
> > +			 */
> > +			copy_vnet_hdr_from_desc(&tmp_hdr, buf_vec);
> > +			hdr = &tmp_hdr;
> > +		} else {
> > +			hdr = (struct virtio_net_hdr *)((uintptr_t)buf_addr);
> > +		}
> > +	}
> > +
> > +	/*
> > +	 * A virtio driver normally uses at least 2 desc buffers
> > +	 * for Tx: the first for storing the header, and others
> > +	 * for storing the data.
> > +	 */
> > +	if (unlikely(buf_len < dev->vhost_hlen)) {
> > +		buf_offset = dev->vhost_hlen - buf_len;
> > +		vec_idx++;
> > +		buf_addr = buf_vec[vec_idx].buf_addr;
> > +		buf_iova = buf_vec[vec_idx].buf_iova;
> > +		buf_len = buf_vec[vec_idx].buf_len;
> > +		buf_avail  = buf_len - buf_offset;
> > +	} else if (buf_len == dev->vhost_hlen) {
> > +		if (unlikely(++vec_idx >= nr_vec))
> > +			return -1;
> > +		buf_addr = buf_vec[vec_idx].buf_addr;
> > +		buf_iova = buf_vec[vec_idx].buf_iova;
> > +		buf_len = buf_vec[vec_idx].buf_len;
> > +
> > +		buf_offset = 0;
> > +		buf_avail = buf_len;
> > +	} else {
> > +		buf_offset = dev->vhost_hlen;
> > +		buf_avail = buf_vec[vec_idx].buf_len - dev->vhost_hlen;
> > +	}
> > +
> > +	PRINT_PACKET(dev, (uintptr_t)(buf_addr + buf_offset),
> > +(uint32_t)buf_avail, 0);
> > +
> > +	mbuf_offset = 0;
> > +	mbuf_avail  = m->buf_len - RTE_PKTMBUF_HEADROOM;
> > +
> > +	if (async_iter_initialize(dev, async))
> > +		return -1;
> > +
> > +	while (1) {
> > +		cpy_len = RTE_MIN(buf_avail, mbuf_avail);
> > +
> > +		if (async_desc_to_mbuf_seg(dev, vq, cur, mbuf_offset,
> buf_iova + buf_offset,
> > +					   cpy_len) < 0)
> > +			goto error;
> > +
> > +		mbuf_avail -= cpy_len;
> > +		buf_avail -= cpy_len;
> > +		mbuf_offset += cpy_len;
> > +		buf_offset += cpy_len;
> > +
> > +		/* This buf reaches to its end, get the next one */
> > +		if (buf_avail == 0) {
> > +			if (++vec_idx >= nr_vec)
> > +				break;
> > +
> > +			buf_addr = buf_vec[vec_idx].buf_addr;
> > +			buf_iova = buf_vec[vec_idx].buf_iova;
> > +			buf_len = buf_vec[vec_idx].buf_len;
> > +
> > +			buf_offset = 0;
> > +			buf_avail = buf_len;
> > +
> > +			PRINT_PACKET(dev, (uintptr_t)buf_addr,
> (uint32_t)buf_avail, 0);
> > +		}
> > +
> > +		/*
> > +		 * This mbuf reaches to its end, get a new one
> > +		 * to hold more data.
> > +		 */
> > +		if (mbuf_avail == 0) {
> > +			cur = rte_pktmbuf_alloc(mbuf_pool);
> > +			if (unlikely(cur == NULL)) {
> > +				VHOST_LOG_DATA(ERR,
> > +					"(%s) %s: failed to allocate memory
> for mbuf.\n",
> > +					dev->ifname, __func__);
> > +				goto error;
> > +			}
> > +
> > +			prev->next = cur;
> > +			prev->data_len = mbuf_offset;
> > +			m->nb_segs += 1;
> > +			m->pkt_len += mbuf_offset;
> > +			prev = cur;
> > +
> > +			mbuf_offset = 0;
> > +			mbuf_avail = cur->buf_len -
> RTE_PKTMBUF_HEADROOM;
> > +		}
> > +	}
> > +
> > +	prev->data_len = mbuf_offset;
> > +	m->pkt_len += mbuf_offset;
> > +
> > +	async_iter_finalize(async);
> > +	if (hdr)
> > +		*nethdr = *hdr;
> > +
> > +	return 0;
> > +
> > +error:
> > +	async_iter_cancel(async);
> > +	return -1;
> > +}
> 
> 
> You can do here the same refactoring I did for the enqueue path, i.e.
> merging copy_desc_to_mbuf and async_desc_to_mbuf in a single function.

Yes, I am preparing the v1 patch in 22.07, including the refactoring here.
Please see next version.

Thanks,
Xuan

> 
> > +static __rte_always_inline uint16_t
> > +async_poll_dequeue_completed_split(struct virtio_net *dev, uint16_t
> queue_id,
> > +		struct rte_mbuf **pkts, uint16_t count, uint16_t dma_id,
> > +		uint16_t vchan_id, bool legacy_ol_flags) {
> > +	uint16_t start_idx, from, i;
> > +	uint16_t nr_cpl_pkts = 0;
> > +	struct async_inflight_info *pkts_info;
> > +	struct vhost_virtqueue *vq = dev->virtqueue[queue_id];
> > +
> > +	pkts_info = vq->async->pkts_info;
> > +
> > +	vhost_async_dma_check_completed(dev, dma_id, vchan_id,
> > +VHOST_DMA_MAX_COPY_COMPLETE);
> > +
> > +	start_idx = async_get_first_inflight_pkt_idx(vq);
> > +
> > +	from = start_idx;
> > +	while (vq->async->pkts_cmpl_flag[from] && count--) {
> > +		vq->async->pkts_cmpl_flag[from] = false;
> > +		from = (from + 1) & (vq->size - 1);
> > +		nr_cpl_pkts++;
> > +	}
> > +
> > +	if (nr_cpl_pkts == 0)
> > +		return 0;
> > +
> > +	for (i = 0; i < nr_cpl_pkts; i++) {
> > +		from = (start_idx + i) & (vq->size - 1);
> > +		pkts[i] = pkts_info[from].mbuf;
> > +
> > +		if (virtio_net_with_host_offload(dev))
> > +			vhost_dequeue_offload(dev,
> &pkts_info[from].nethdr, pkts[i],
> > +					      legacy_ol_flags);
> > +	}
> > +
> > +	/* write back completed descs to used ring and update used idx */
> > +	write_back_completed_descs_split(vq, nr_cpl_pkts);
> > +	__atomic_add_fetch(&vq->used->idx, nr_cpl_pkts,
> __ATOMIC_RELEASE);
> > +	vhost_vring_call_split(dev, vq);
> > +
> > +	vq->async->pkts_inflight_n -= nr_cpl_pkts;
> > +
> > +	return nr_cpl_pkts;
> > +}
> > +
> > +static __rte_always_inline uint16_t
> > +virtio_dev_tx_async_split(struct virtio_net *dev, struct vhost_virtqueue
> *vq,
> > +		uint16_t queue_id, struct rte_mempool *mbuf_pool, struct
> rte_mbuf **pkts,
> > +		uint16_t count, uint16_t dma_id, uint16_t vchan_id, bool
> > +legacy_ol_flags) {
> > +	static bool allocerr_warned;
> > +	bool dropped = false;
> > +	uint16_t free_entries;
> > +	uint16_t pkt_idx, slot_idx = 0;
> > +	uint16_t nr_done_pkts = 0;
> > +	uint16_t pkt_err = 0;
> > +	uint16_t n_xfer;
> > +	struct vhost_async *async = vq->async;
> > +	struct async_inflight_info *pkts_info = async->pkts_info;
> > +	struct rte_mbuf *pkts_prealloc[MAX_PKT_BURST];
> > +	uint16_t pkts_size = count;
> > +
> > +	/**
> > +	 * The ordering between avail index and
> > +	 * desc reads needs to be enforced.
> > +	 */
> > +	free_entries = __atomic_load_n(&vq->avail->idx,
> __ATOMIC_ACQUIRE) - vq->last_avail_idx;
> > +	if (free_entries == 0)
> > +		goto out;
> > +
> > +	rte_prefetch0(&vq->avail->ring[vq->last_avail_idx & (vq->size -
> > +1)]);
> > +
> > +	async_iter_reset(async);
> > +
> > +	count = RTE_MIN(count, MAX_PKT_BURST);
> > +	count = RTE_MIN(count, free_entries);
> > +	VHOST_LOG_DATA(DEBUG, "(%s) about to dequeue %u buffers\n",
> > +dev->ifname, count);
> > +
> > +	if (rte_pktmbuf_alloc_bulk(mbuf_pool, pkts_prealloc, count))
> > +		goto out;
> > +
> > +	for (pkt_idx = 0; pkt_idx < count; pkt_idx++) {
> > +		uint16_t head_idx = 0;
> > +		uint16_t nr_vec = 0;
> > +		uint16_t to;
> > +		uint32_t buf_len;
> > +		int err;
> > +		struct buf_vector buf_vec[BUF_VECTOR_MAX];
> > +		struct rte_mbuf *pkt = pkts_prealloc[pkt_idx];
> > +
> > +		if (unlikely(fill_vec_buf_split(dev, vq, vq->last_avail_idx,
> > +						&nr_vec, buf_vec,
> > +						&head_idx, &buf_len,
> > +						VHOST_ACCESS_RO) < 0)) {
> > +			dropped = true;
> > +			break;
> > +		}
> > +
> > +		err = virtio_dev_pktmbuf_prep(dev, pkt, buf_len);
> > +		if (unlikely(err)) {
> > +			/**
> > +			 * mbuf allocation fails for jumbo packets when
> external
> > +			 * buffer allocation is not allowed and linear buffer
> > +			 * is required. Drop this packet.
> > +			 */
> > +			if (!allocerr_warned) {
> > +				VHOST_LOG_DATA(ERR,
> > +					"(%s) %s: Failed mbuf alloc of size %d
> from %s\n",
> > +					dev->ifname, __func__, buf_len,
> mbuf_pool->name);
> > +				allocerr_warned = true;
> > +			}
> > +			dropped = true;
> > +			break;
> > +		}
> > +
> > +		slot_idx = (async->pkts_idx + pkt_idx) & (vq->size - 1);
> > +		err = async_desc_to_mbuf(dev, vq, buf_vec, nr_vec, pkt,
> mbuf_pool,
> > +					&pkts_info[slot_idx].nethdr);
> > +		if (unlikely(err)) {
> > +			if (!allocerr_warned) {
> > +				VHOST_LOG_DATA(ERR,
> > +					"(%s) %s: Failed to offload copies to
> async channel.\n",
> > +					dev->ifname, __func__);
> > +				allocerr_warned = true;
> > +			}
> > +			dropped = true;
> > +			break;
> > +		}
> > +
> > +		pkts_info[slot_idx].mbuf = pkt;
> > +
> > +		/* store used descs */
> > +		to = async->desc_idx_split & (vq->size - 1);
> > +		async->descs_split[to].id = head_idx;
> > +		async->descs_split[to].len = 0;
> > +		async->desc_idx_split++;
> > +
> > +		vq->last_avail_idx++;
> > +	}
> > +
> > +	if (unlikely(dropped))
> > +		rte_pktmbuf_free_bulk(&pkts_prealloc[pkt_idx], count -
> pkt_idx);
> > +
> > +	n_xfer = vhost_async_dma_transfer(dev, vq, dma_id, vchan_id,
> async->pkts_idx,
> > +					  async->iov_iter, pkt_idx);
> > +
> > +	async->pkts_inflight_n += n_xfer;
> > +
> > +	pkt_err = pkt_idx - n_xfer;
> > +	if (unlikely(pkt_err)) {
> > +		VHOST_LOG_DATA(DEBUG,
> > +			"(%s) %s: failed to transfer data for queue id %d.\n",
> > +			dev->ifname, __func__, queue_id);
> > +
> > +		pkt_idx = n_xfer;
> > +		/* recover available ring */
> > +		vq->last_avail_idx -= pkt_err;
> > +
> > +		/**
> > +		 * recover async channel copy related structures and free
> pktmbufs
> > +		 * for error pkts.
> > +		 */
> > +		async->desc_idx_split -= pkt_err;
> > +		while (pkt_err-- > 0) {
> > +			rte_pktmbuf_free(pkts_info[slot_idx & (vq->size -
> 1)].mbuf);
> > +			slot_idx--;
> > +		}
> > +	}
> > +
> > +	async->pkts_idx += pkt_idx;
> > +	if (async->pkts_idx >= vq->size)
> > +		async->pkts_idx -= vq->size;
> > +
> > +out:
> > +	/* DMA device may serve other queues, unconditionally check
> completed. */
> > +	nr_done_pkts = async_poll_dequeue_completed_split(dev, queue_id,
> pkts, pkts_size,
> > +							  dma_id, vchan_id,
> legacy_ol_flags);
> > +
> > +	return nr_done_pkts;
> > +}
> > +
> > +__rte_noinline
> > +static uint16_t
> > +virtio_dev_tx_async_split_legacy(struct virtio_net *dev,
> > +		struct vhost_virtqueue *vq, uint16_t queue_id,
> > +		struct rte_mempool *mbuf_pool, struct rte_mbuf **pkts,
> > +		uint16_t count, uint16_t dma_id, uint16_t vchan_id) {
> > +	return virtio_dev_tx_async_split(dev, vq, queue_id, mbuf_pool,
> > +				pkts, count, dma_id, vchan_id, true); }
> > +
> > +__rte_noinline
> > +static uint16_t
> > +virtio_dev_tx_async_split_compliant(struct virtio_net *dev,
> > +		struct vhost_virtqueue *vq, uint16_t queue_id,
> > +		struct rte_mempool *mbuf_pool, struct rte_mbuf **pkts,
> > +		uint16_t count, uint16_t dma_id, uint16_t vchan_id) {
> > +	return virtio_dev_tx_async_split(dev, vq, queue_id, mbuf_pool,
> > +				pkts, count, dma_id, vchan_id, false); }
> > +
> > +uint16_t
> > +rte_vhost_async_try_dequeue_burst(int vid, uint16_t queue_id,
> > +	struct rte_mempool *mbuf_pool, struct rte_mbuf **pkts, uint16_t
> count,
> > +	int *nr_inflight, uint16_t dma_id, uint16_t vchan_id) {
> > +	struct virtio_net *dev;
> > +	struct rte_mbuf *rarp_mbuf = NULL;
> > +	struct vhost_virtqueue *vq;
> > +	int16_t success = 1;
> > +
> > +	*nr_inflight = -1;
> > +
> > +	dev = get_device(vid);
> > +	if (!dev)
> > +		return 0;
> > +
> > +	if (unlikely(!(dev->flags & VIRTIO_DEV_BUILTIN_VIRTIO_NET))) {
> > +		VHOST_LOG_DATA(ERR,
> > +			"(%s) %s: built-in vhost net backend is disabled.\n",
> > +			dev->ifname, __func__);
> > +		return 0;
> > +	}
> > +
> > +	if (unlikely(!is_valid_virt_queue_idx(queue_id, 1, dev->nr_vring))) {
> > +		VHOST_LOG_DATA(ERR,
> > +			"(%s) %s: invalid virtqueue idx %d.\n",
> > +			dev->ifname, __func__, queue_id);
> > +		return 0;
> > +	}
> > +
> > +	if (unlikely(!dma_copy_track[dma_id].vchans ||
> > +
> 	!dma_copy_track[dma_id].vchans[vchan_id].pkts_cmpl_flag_addr)) {
> > +		VHOST_LOG_DATA(ERR, "(%s) %s: invalid channel %d:%u.\n",
> dev->ifname, __func__,
> > +			       dma_id, vchan_id);
> > +		return 0;
> > +	}
> > +
> > +	vq = dev->virtqueue[queue_id];
> > +
> > +	if (unlikely(rte_spinlock_trylock(&vq->access_lock) == 0))
> > +		return 0;
> > +
> > +	if (unlikely(vq->enabled == 0)) {
> > +		count = 0;
> > +		goto out_access_unlock;
> > +	}
> > +
> > +	if (unlikely(!vq->async)) {
> > +		VHOST_LOG_DATA(ERR, "(%s) %s: async not registered for
> queue id %d.\n",
> > +			dev->ifname, __func__, queue_id);
> > +		count = 0;
> > +		goto out_access_unlock;
> > +	}
> > +
> > +	if (dev->features & (1ULL << VIRTIO_F_IOMMU_PLATFORM))
> > +		vhost_user_iotlb_rd_lock(vq);
> > +
> > +	if (unlikely(vq->access_ok == 0))
> > +		if (unlikely(vring_translate(dev, vq) < 0)) {
> > +			count = 0;
> > +			goto out;
> > +		}
> > +
> > +	/*
> > +	 * Construct a RARP broadcast packet, and inject it to the "pkts"
> > +	 * array, to looks like that guest actually send such packet.
> > +	 *
> > +	 * Check user_send_rarp() for more information.
> > +	 *
> > +	 * broadcast_rarp shares a cacheline in the virtio_net structure
> > +	 * with some fields that are accessed during enqueue and
> > +	 * __atomic_compare_exchange_n causes a write if performed
> compare
> > +	 * and exchange. This could result in false sharing between enqueue
> > +	 * and dequeue.
> > +	 *
> > +	 * Prevent unnecessary false sharing by reading broadcast_rarp first
> > +	 * and only performing compare and exchange if the read indicates it
> > +	 * is likely to be set.
> > +	 */
> > +	if (unlikely(__atomic_load_n(&dev->broadcast_rarp,
> __ATOMIC_ACQUIRE) &&
> > +			__atomic_compare_exchange_n(&dev-
> >broadcast_rarp,
> > +			&success, 0, 0, __ATOMIC_RELEASE,
> __ATOMIC_RELAXED))) {
> > +
> > +		rarp_mbuf = rte_net_make_rarp_packet(mbuf_pool, &dev-
> >mac);
> > +		if (rarp_mbuf == NULL) {
> > +			VHOST_LOG_DATA(ERR, "Failed to make RARP
> packet.\n");
> > +			count = 0;
> > +			goto out;
> > +		}
> > +		/*
> > +		 * Inject it to the head of "pkts" array, so that switch's mac
> > +		 * learning table will get updated first.
> > +		 */
> > +		pkts[0] = rarp_mbuf;
> > +		pkts++;
> > +		count -= 1;
> > +	}
> > +
> > +	if (unlikely(vq_is_packed(dev))) {
> > +		static bool not_support_pack_log;
> > +		if (!not_support_pack_log) {
> > +			VHOST_LOG_DATA(ERR,
> > +				"(%s) %s: async dequeue does not support
> packed ring.\n",
> > +				dev->ifname, __func__);
> > +			not_support_pack_log = true;
> > +		}
> > +		count = 0;
> > +		goto out;
> > +	}
> > +
> > +	if (dev->flags & VIRTIO_DEV_LEGACY_OL_FLAGS)
> > +		count = virtio_dev_tx_async_split_legacy(dev, vq, queue_id,
> > +				mbuf_pool, pkts, count, dma_id, vchan_id);
> > +	else
> > +		count = virtio_dev_tx_async_split_compliant(dev, vq,
> queue_id,
> > +				mbuf_pool, pkts, count, dma_id, vchan_id);
> > +
> > +	*nr_inflight = vq->async->pkts_inflight_n;
> > +
> > +out:
> > +	if (dev->features & (1ULL << VIRTIO_F_IOMMU_PLATFORM))
> > +		vhost_user_iotlb_rd_unlock(vq);
> > +
> > +out_access_unlock:
> > +	rte_spinlock_unlock(&vq->access_lock);
> > +
> > +	if (unlikely(rarp_mbuf != NULL))
> > +		count += 1;
> > +
> > +	return count;
> > +}