[dpdk-dev] [PATCH v6 3/4] vhost: support async dequeue for split ring

[Wenwu Ma <wenwux.ma@intel.com>]

From: Yuan Wang <yuanx.wang@intel.com>

This patch implements asynchronous dequeue data path for split ring.
A new asynchronous dequeue function is introduced. With this function,
the application can try to receive packets from the guest with
offloading large copies to the async channel, thus saving precious CPU
cycles.

Signed-off-by: Yuan Wang <yuanx.wang@intel.com>
Signed-off-by: Jiayu Hu <jiayu.hu@intel.com>
Signed-off-by: Wenwu Ma <wenwux.ma@intel.com>
---
 doc/guides/prog_guide/vhost_lib.rst |   9 +
 lib/vhost/rte_vhost_async.h         |  39 +-
 lib/vhost/version.map               |   3 +
 lib/vhost/virtio_net.c              | 586 ++++++++++++++++++++++++++++
 4 files changed, 635 insertions(+), 2 deletions(-)

diff --git a/doc/guides/prog_guide/vhost_lib.rst b/doc/guides/prog_guide/vhost_lib.rst
index d18fb98910..bf90a2663b 100644
--- a/doc/guides/prog_guide/vhost_lib.rst
+++ b/doc/guides/prog_guide/vhost_lib.rst
@@ -281,6 +281,15 @@ The following is an overview of some key Vhost API functions:
   Poll enqueue completion status from async data path. Completed packets
   are returned to applications through ``pkts``.
 
+* ``rte_vhost_async_try_dequeue_burst(vid, queue_id, mbuf_pool, pkts, count, nr_inflight)``
+
+  This function tries to receive packets from the guest with offloading
+  large 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.
+
 Vhost-user Implementations
 --------------------------
 
diff --git a/lib/vhost/rte_vhost_async.h b/lib/vhost/rte_vhost_async.h
index 6faa31f5ad..04d7588217 100644
--- a/lib/vhost/rte_vhost_async.h
+++ b/lib/vhost/rte_vhost_async.h
@@ -83,12 +83,20 @@ struct rte_vhost_async_channel_ops {
 		uint16_t max_packets);
 };
 
+struct async_nethdr {
+	struct virtio_net_hdr hdr;
+	bool valid;
+};
+
 /**
- * inflight async packet information
+ * in-flight async packet information
  */
 struct async_inflight_info {
 	struct rte_mbuf *mbuf;
-	uint16_t descs; /* num of descs inflight */
+	union {
+		uint16_t descs; /* num of descs in-flight */
+		struct async_nethdr nethdr;
+	};
 	uint16_t nr_buffers; /* num of buffers inflight for packed ring */
 };
 
@@ -193,4 +201,31 @@ __rte_experimental
 uint16_t rte_vhost_poll_enqueue_completed(int vid, uint16_t queue_id,
 		struct rte_mbuf **pkts, uint16_t count);
 
+/**
+ * This function tries to receive packets from the guest with offloading
+ * large 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
+ *  queue id to dequeue data
+ * @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.
+ * @return
+ *  num 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);
+
 #endif /* _RTE_VHOST_ASYNC_H_ */
diff --git a/lib/vhost/version.map b/lib/vhost/version.map
index 9103a23cd4..a320f889cd 100644
--- a/lib/vhost/version.map
+++ b/lib/vhost/version.map
@@ -79,4 +79,7 @@ EXPERIMENTAL {
 
 	# added in 21.05
 	rte_vhost_get_negotiated_protocol_features;
+
+	# added in 21.08
+	rte_vhost_async_try_dequeue_burst;
 };
diff --git a/lib/vhost/virtio_net.c b/lib/vhost/virtio_net.c
index b93482587c..58317d7b75 100644
--- a/lib/vhost/virtio_net.c
+++ b/lib/vhost/virtio_net.c
@@ -3147,3 +3147,589 @@ rte_vhost_dequeue_burst(int vid, uint16_t queue_id,
 
 	return count;
 }
+
+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 iovec *src_iovec, struct iovec *dst_iovec,
+		  struct rte_vhost_iov_iter *src_it,
+		  struct rte_vhost_iov_iter *dst_it,
+		  struct async_nethdr *nethdr,
+		  bool legacy_ol_flags)
+{
+	uint64_t buf_addr, buf_iova;
+	uint64_t mapped_len;
+	uint32_t tlen = 0;
+	uint32_t buf_avail, buf_offset, buf_len;
+	uint32_t mbuf_avail, mbuf_offset;
+	uint32_t cpy_len, cpy_threshold;
+	/* A counter to avoid desc dead loop chain */
+	uint16_t vec_idx = 0;
+	int tvec_idx = 0;
+	struct rte_mbuf *cur = m, *prev = m;
+	struct virtio_net_hdr tmp_hdr;
+	struct virtio_net_hdr *hdr = NULL;
+	struct batch_copy_elem *batch_copy = vq->batch_copy_elems;
+
+	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;
+
+	cpy_threshold = vq->async_threshold;
+
+	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_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_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;
+	while (1) {
+		cpy_len = RTE_MIN(buf_avail, mbuf_avail);
+
+		while (cpy_len && cpy_len >= cpy_threshold) {
+			void *hpa = (void *)(uintptr_t)gpa_to_first_hpa(dev,
+						buf_iova + buf_offset, cpy_len,
+						&mapped_len);
+
+			if (unlikely(!hpa || mapped_len < cpy_threshold))
+				break;
+
+			async_fill_vec(src_iovec + tvec_idx, hpa,
+				(size_t)mapped_len);
+			async_fill_vec(dst_iovec + tvec_idx,
+				(void *)(uintptr_t)rte_pktmbuf_iova_offset(cur,
+							mbuf_offset),
+				(size_t)mapped_len);
+
+			tvec_idx++;
+			tlen += (uint32_t)mapped_len;
+			cpy_len -= (uint32_t)mapped_len;
+			mbuf_avail -= (uint32_t)mapped_len;
+			mbuf_offset += (uint32_t)mapped_len;
+			buf_avail -= (uint32_t)mapped_len;
+			buf_offset += (uint32_t)mapped_len;
+		}
+
+		if (cpy_len) {
+			if (vq->batch_copy_nb_elems >= vq->size ||
+				(hdr && cur == m)) {
+				rte_memcpy(
+					rte_pktmbuf_mtod_offset(cur, void *,
+							mbuf_offset),
+					(void *)((uintptr_t)(buf_addr +
+								buf_offset)),
+					cpy_len);
+			} else {
+				batch_copy[vq->batch_copy_nb_elems].dst =
+					rte_pktmbuf_mtod_offset(cur, void *,
+							mbuf_offset);
+				batch_copy[vq->batch_copy_nb_elems].src =
+					(void *)((uintptr_t)(buf_addr +
+								buf_offset));
+				batch_copy[vq->batch_copy_nb_elems].len =
+					cpy_len;
+				vq->batch_copy_nb_elems++;
+			}
+
+			mbuf_avail  -= cpy_len;
+			mbuf_offset += cpy_len;
+			buf_avail -= 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_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, "Failed to "
+					"allocate memory for mbuf.\n");
+				return -1;
+			}
+
+			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;
+
+	if (hdr && tlen) {
+		nethdr->valid = true;
+		nethdr->hdr = *hdr;
+	} else if (hdr)
+		vhost_dequeue_offload(hdr, m, legacy_ol_flags);
+
+	if (tlen) {
+		async_fill_iter(src_it, tlen, src_iovec, tvec_idx);
+		async_fill_iter(dst_it, tlen, dst_iovec, tvec_idx);
+	} else
+		src_it->count = 0;
+
+	return 0;
+}
+
+static __rte_always_inline uint16_t
+async_poll_dequeue_completed_split(struct virtio_net *dev,
+		struct vhost_virtqueue *vq, uint16_t queue_id,
+		struct rte_mbuf **pkts, uint16_t count, bool legacy_ol_flags)
+{
+	uint16_t n_pkts_cpl = 0, n_pkts_put = 0;
+	uint16_t start_idx, pkt_idx, from;
+	struct async_inflight_info *pkts_info;
+
+	pkt_idx = vq->async_pkts_idx & (vq->size - 1);
+	pkts_info = vq->async_pkts_info;
+	start_idx = virtio_dev_rx_async_get_info_idx(pkt_idx, vq->size,
+			vq->async_pkts_inflight_n);
+
+	if (count > vq->async_last_pkts_n) {
+		n_pkts_cpl = vq->async_ops.check_completed_copies(dev->vid,
+			queue_id, 0, count - vq->async_last_pkts_n);
+	}
+
+	n_pkts_cpl += vq->async_last_pkts_n;
+	if (unlikely(n_pkts_cpl == 0))
+		return 0;
+
+	n_pkts_put = RTE_MIN(count, n_pkts_cpl);
+
+	for (pkt_idx = 0; pkt_idx < n_pkts_put; pkt_idx++) {
+		from = (start_idx + pkt_idx) & (vq->size - 1);
+		pkts[pkt_idx] = pkts_info[from].mbuf;
+
+		if (pkts_info[from].nethdr.valid) {
+			vhost_dequeue_offload(&pkts_info[from].nethdr.hdr,
+					pkts[pkt_idx], legacy_ol_flags);
+		}
+	}
+	vq->async_last_pkts_n = n_pkts_cpl - n_pkts_put;
+
+	if (n_pkts_put) {
+		/* write back completed descs to used ring */
+		write_back_completed_descs_split(vq, n_pkts_put);
+		/* update used ring */
+		__atomic_add_fetch(&vq->used->idx,
+				n_pkts_put, __ATOMIC_RELEASE);
+
+		vq->async_pkts_inflight_n -= n_pkts_put;
+	}
+
+	return n_pkts_put;
+}
+
+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, bool legacy_ol_flags)
+{
+	static bool allocerr_warned;
+	bool dropped = false;
+	uint16_t pkt_idx;
+	uint16_t free_entries;
+	uint16_t slot_idx = 0;
+	uint16_t segs_await = 0;
+	uint16_t nr_done_pkts = 0, nr_async_pkts = 0, nr_async_cmpl_pkts = 0;
+	uint16_t nr_async_burst = 0;
+	uint16_t pkt_err = 0;
+	uint16_t iovec_idx = 0, it_idx = 0;
+	struct rte_vhost_iov_iter *it_pool = vq->it_pool;
+	struct iovec *vec_pool = vq->vec_pool;
+	struct iovec *src_iovec = vec_pool;
+	struct iovec *dst_iovec = vec_pool + (VHOST_MAX_ASYNC_VEC >> 1);
+	struct rte_vhost_async_desc tdes[MAX_PKT_BURST];
+	struct async_inflight_info *pkts_info = vq->async_pkts_info;
+	struct rte_mbuf *pkts_prealloc[MAX_PKT_BURST];
+
+	struct async_pkt_index {
+		uint16_t last_avail_idx;
+	} async_pkts_log[MAX_PKT_BURST];
+
+	/**
+	 * 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)]);
+
+	count = RTE_MIN(count, MAX_PKT_BURST);
+	count = RTE_MIN(count, free_entries);
+	VHOST_LOG_DATA(DEBUG, "(%d) about to dequeue %u buffers\n",
+			dev->vid, 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;
+		uint32_t buf_len;
+		int err;
+		struct buf_vector buf_vec[BUF_VECTOR_MAX];
+		struct rte_mbuf *pkt;
+
+		if (unlikely(fill_vec_buf_split(dev, vq, vq->last_avail_idx,
+						&nr_vec, buf_vec,
+						&head_idx, &buf_len,
+						VHOST_ACCESS_RO) < 0))
+			break;
+
+		err = virtio_dev_pktmbuf_prep(dev, pkts_prealloc[pkt_idx],
+						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,
+					"Failed mbuf alloc of size %d from %s on %s.\n",
+					buf_len, mbuf_pool->name, dev->ifname);
+				allocerr_warned = true;
+			}
+			dropped = true;
+			break;
+		}
+
+		pkt = pkts_prealloc[pkt_idx];
+
+		slot_idx = (vq->async_pkts_idx + nr_async_pkts) &
+				(vq->size - 1);
+		err = async_desc_to_mbuf(dev, vq, buf_vec, nr_vec, pkt,
+				mbuf_pool, &src_iovec[iovec_idx],
+				&dst_iovec[iovec_idx], &it_pool[it_idx],
+				&it_pool[it_idx + 1],
+				&pkts_info[slot_idx].nethdr, legacy_ol_flags);
+		if (unlikely(err)) {
+			if (!allocerr_warned) {
+				VHOST_LOG_DATA(ERR,
+					"Failed to copy desc to mbuf on %s.\n",
+					dev->ifname);
+				allocerr_warned = true;
+			}
+			dropped = true;
+			break;
+		}
+
+		if (it_pool[it_idx].count) {
+			uint16_t to = vq->async_desc_idx_split & (vq->size - 1);
+
+			async_fill_desc(&tdes[nr_async_burst], &it_pool[it_idx],
+				&it_pool[it_idx + 1]);
+			pkts_info[slot_idx].mbuf = pkt;
+			async_pkts_log[nr_async_pkts++].last_avail_idx =
+				vq->last_avail_idx;
+			nr_async_burst++;
+			iovec_idx += it_pool[it_idx].nr_segs;
+			it_idx += 2;
+			segs_await += it_pool[it_idx].nr_segs;
+
+			/* keep used desc */
+			vq->async_descs_split[to].id = head_idx;
+			vq->async_descs_split[to].len = 0;
+			vq->async_desc_idx_split++;
+		} else {
+			update_shadow_used_ring_split(vq, head_idx, 0);
+			pkts[nr_done_pkts++] = pkt;
+		}
+
+		vq->last_avail_idx++;
+
+		if (unlikely((nr_async_burst >= VHOST_ASYNC_BATCH_THRESHOLD) ||
+					((VHOST_MAX_ASYNC_VEC >> 1) -
+					 segs_await < BUF_VECTOR_MAX))) {
+			uint16_t nr_pkts;
+
+			nr_pkts = vq->async_ops.transfer_data(dev->vid,
+					queue_id, tdes, 0, nr_async_burst);
+			src_iovec = vec_pool;
+			dst_iovec = vec_pool + (VHOST_MAX_ASYNC_VEC >> 1);
+			it_idx = 0;
+			segs_await = 0;
+			vq->async_pkts_inflight_n += nr_pkts;
+
+			if (unlikely(nr_pkts < nr_async_burst)) {
+				pkt_err = nr_async_burst - nr_pkts;
+				nr_async_burst = 0;
+				break;
+			}
+			nr_async_burst = 0;
+		}
+	}
+
+	if (unlikely(dropped))
+		rte_pktmbuf_free_bulk(&pkts_prealloc[pkt_idx], count - pkt_idx);
+
+	if (nr_async_burst) {
+		uint32_t nr_pkts;
+
+		nr_pkts = vq->async_ops.transfer_data(dev->vid, queue_id,
+				tdes, 0, nr_async_burst);
+		vq->async_pkts_inflight_n += nr_pkts;
+
+		if (unlikely(nr_pkts < nr_async_burst))
+			pkt_err = nr_async_burst - nr_pkts;
+	}
+
+	do_data_copy_dequeue(vq);
+
+	if (unlikely(pkt_err)) {
+		uint16_t nr_err_dma = pkt_err;
+		uint16_t nr_err_sw;
+
+		nr_async_pkts -= nr_err_dma;
+
+		/**
+		 * revert shadow used ring and free pktmbufs for
+		 * CPU-copied pkts after the first DMA-error pkt.
+		 */
+		nr_err_sw = vq->last_avail_idx -
+			async_pkts_log[nr_async_pkts].last_avail_idx -
+			nr_err_dma;
+		vq->shadow_used_idx -= nr_err_sw;
+		while (nr_err_sw-- > 0)
+			rte_pktmbuf_free(pkts[--nr_done_pkts]);
+
+		/**
+		 * recover DMA-copy related structures and free pktmbufs
+		 * for DMA-error pkts.
+		 */
+		vq->async_desc_idx_split -= nr_err_dma;
+		while (nr_err_dma-- > 0) {
+			rte_pktmbuf_free(
+				pkts_info[slot_idx & (vq->size - 1)].mbuf);
+			slot_idx--;
+		}
+
+		/* recover available ring */
+		vq->last_avail_idx =
+			async_pkts_log[nr_async_pkts].last_avail_idx;
+	}
+
+	vq->async_pkts_idx += nr_async_pkts;
+
+	if (likely(vq->shadow_used_idx))
+		flush_shadow_used_ring_split(dev, vq);
+
+out:
+	if (nr_done_pkts < count && vq->async_pkts_inflight_n > 0) {
+		nr_async_cmpl_pkts = async_poll_dequeue_completed_split(dev, vq,
+					queue_id, &pkts[nr_done_pkts],
+					count - nr_done_pkts,
+					legacy_ol_flags);
+		nr_done_pkts += nr_async_cmpl_pkts;
+	}
+	if (likely(nr_done_pkts))
+		vhost_vring_call_split(dev, vq);
+
+	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)
+{
+	return virtio_dev_tx_async_split(dev, vq, queue_id, mbuf_pool,
+				pkts, count, 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)
+{
+	return virtio_dev_tx_async_split(dev, vq, queue_id, mbuf_pool,
+				pkts, count, 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)
+{
+	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,
+			"(%d) %s: built-in vhost net backend is disabled.\n",
+			dev->vid, __func__);
+		return 0;
+	}
+
+	if (unlikely(!is_valid_virt_queue_idx(queue_id, 1, dev->nr_vring))) {
+		VHOST_LOG_DATA(ERR,
+			"(%d) %s: invalid virtqueue idx %d.\n",
+			dev->vid, __func__, queue_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_registered)) {
+		VHOST_LOG_DATA(ERR, "(%d) %s: async not registered for queue id %d.\n",
+			dev->vid, __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_access_unlock;
+		}
+
+	/*
+	 * 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;
+		}
+		count -= 1;
+	}
+
+	if (unlikely(vq_is_packed(dev)))
+		return 0;
+
+	if (dev->flags & VIRTIO_DEV_LEGACY_OL_FLAGS)
+		count = virtio_dev_tx_async_split_legacy(dev, vq, queue_id,
+				mbuf_pool, pkts, count);
+	else
+		count = virtio_dev_tx_async_split_compliant(dev, vq, queue_id,
+				mbuf_pool, pkts, count);
+
+out:
+	*nr_inflight = vq->async_pkts_inflight_n;
+
+	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)) {
+		/*
+		 * Inject it to the head of "pkts" array, so that switch's mac
+		 * learning table will get updated first.
+		 */
+		memmove(&pkts[1], pkts, count * sizeof(struct rte_mbuf *));
+		pkts[0] = rarp_mbuf;
+		count += 1;
+	}
+
+	return count;
+}
-- 
2.25.1