Re: [dpdk-dev] [PATCH v2 2/2] vhost: introduce async enqueue for split ring

+#define VHOST_ASYNC_BATCH_THRESHOLD 8
+

Not very clear about why batch number is 8. It is better to save it in rte_vhost_async_features if the value come from hardware requirement. 

+
+static __rte_noinline uint32_t
+virtio_dev_rx_async_submit_split(struct virtio_net *dev,
+	struct vhost_virtqueue *vq, uint16_t queue_id,
+	struct rte_mbuf **pkts, uint32_t count)
+{
+	uint32_t pkt_idx = 0, pkt_burst_idx = 0;
+	uint16_t num_buffers;
+	struct buf_vector buf_vec[BUF_VECTOR_MAX];
+	uint16_t avail_head, last_idx, shadow_idx;
+
+	struct rte_vhost_iov_iter *it_pool = vq->it_pool;
+	struct iovec *vec_pool = vq->vec_pool;
+	struct rte_vhost_async_desc tdes[MAX_PKT_BURST];
+	struct iovec *src_iovec = vec_pool;
+	struct iovec *dst_iovec = vec_pool + (VHOST_MAX_ASYNC_VEC >> 1);
+	struct rte_vhost_iov_iter *src_it = it_pool;
+	struct rte_vhost_iov_iter *dst_it = it_pool + 1;
+	uint16_t n_free_slot, slot_idx;
+	int n_pkts = 0;
+
+	avail_head = *((volatile uint16_t *)&vq->avail->idx);
+	last_idx = vq->last_avail_idx;
+	shadow_idx = vq->shadow_used_idx;
+
+	/*
+	 * The ordering between avail index and
+	 * desc reads needs to be enforced.
+	 */
+	rte_smp_rmb();
+
+	rte_prefetch0(&vq->avail->ring[vq->last_avail_idx & (vq->size - 1)]);
+
+	for (pkt_idx = 0; pkt_idx < count; pkt_idx++) {
+		uint32_t pkt_len = pkts[pkt_idx]->pkt_len + dev->vhost_hlen;
+		uint16_t nr_vec = 0;
+
+		if (unlikely(reserve_avail_buf_split(dev, vq,
+						pkt_len, buf_vec,
&num_buffers,
+						avail_head, &nr_vec) < 0)) {
+			VHOST_LOG_DATA(DEBUG,
+				"(%d) failed to get enough desc from
vring\n",
+				dev->vid);
+			vq->shadow_used_idx -= num_buffers;
+			break;
+		}
+
+		VHOST_LOG_DATA(DEBUG, "(%d) current index %d | end
index %d\n",
+			dev->vid, vq->last_avail_idx,
+			vq->last_avail_idx + num_buffers);
+
+		if (async_mbuf_to_desc(dev, vq, pkts[pkt_idx],
+				buf_vec, nr_vec, num_buffers,
+				src_iovec, dst_iovec, src_it, dst_it) < 0) {
+			vq->shadow_used_idx -= num_buffers;
+			break;
+		}
+
+		slot_idx = (vq->async_pkts_idx + pkt_idx) & (vq->size - 1);
+		if (src_it->count) {
+			async_fill_des(&tdes[pkt_burst_idx], src_it, dst_it);
+			pkt_burst_idx++;
+			vq->async_pending_info[slot_idx] =
+				num_buffers | (src_it->nr_segs << 16);
+			src_iovec += src_it->nr_segs;
+			dst_iovec += dst_it->nr_segs;
+			src_it += 2;
+			dst_it += 2;

Patrick, 
In my understanding, nr_segs type definition can follow nr_vec type definition (uint16_t). By that can short the data saved in async_pkts_pending from 64bit to 32bit. 
Since those information will be used in datapath, the smaller size will get the better perf. 

It is better to replace integer 2 with macro. 

Thanks,
Marvin