[dpdk-dev] [PATCH v2 4/5] vhost: simplify descriptor's buffer prefetching

[Maxime Coquelin <maxime.coquelin@redhat.com>]

Now that we have a single function to map the descriptors
buffers, let's prefetch them there as it is the earliest
place we can do it.

Signed-off-by: Maxime Coquelin <maxime.coquelin@redhat.com>
---
 lib/librte_vhost/virtio_net.c | 32 ++------------------------------
 1 file changed, 2 insertions(+), 30 deletions(-)

diff --git a/lib/librte_vhost/virtio_net.c b/lib/librte_vhost/virtio_net.c
index 8ba526c070..364c468be8 100644
--- a/lib/librte_vhost/virtio_net.c
+++ b/lib/librte_vhost/virtio_net.c
@@ -286,6 +286,8 @@ map_one_desc(struct virtio_net *dev, struct vhost_virtqueue *vq,
 		if (unlikely(!desc_addr))
 			return -1;
 
+		rte_prefetch0((void *)(uintptr_t)desc_addr);
+
 		buf_vec[vec_id].buf_iova = desc_iova;
 		buf_vec[vec_id].buf_addr = desc_addr;
 		buf_vec[vec_id].buf_len  = desc_chunck_len;
@@ -665,9 +667,6 @@ copy_mbuf_to_desc(struct virtio_net *dev, struct vhost_virtqueue *vq,
 	buf_iova = buf_vec[vec_idx].buf_iova;
 	buf_len = buf_vec[vec_idx].buf_len;
 
-	if (nr_vec > 1)
-		rte_prefetch0((void *)(uintptr_t)buf_vec[1].buf_addr);
-
 	if (unlikely(buf_len < dev->vhost_hlen && nr_vec <= 1)) {
 		error = -1;
 		goto out;
@@ -710,10 +709,6 @@ copy_mbuf_to_desc(struct virtio_net *dev, struct vhost_virtqueue *vq,
 			buf_iova = buf_vec[vec_idx].buf_iova;
 			buf_len = buf_vec[vec_idx].buf_len;
 
-			/* Prefetch next buffer address. */
-			if (vec_idx + 1 < nr_vec)
-				rte_prefetch0((void *)(uintptr_t)
-						buf_vec[vec_idx + 1].buf_addr);
 			buf_offset = 0;
 			buf_avail  = buf_len;
 		}
@@ -811,8 +806,6 @@ virtio_dev_rx_split(struct virtio_net *dev, struct vhost_virtqueue *vq,
 			break;
 		}
 
-		rte_prefetch0((void *)(uintptr_t)buf_vec[0].buf_addr);
-
 		VHOST_LOG_DEBUG(VHOST_DATA, "(%d) current index %d | end index %d\n",
 			dev->vid, vq->last_avail_idx,
 			vq->last_avail_idx + num_buffers);
@@ -860,8 +853,6 @@ virtio_dev_rx_packed(struct virtio_net *dev, struct vhost_virtqueue *vq,
 			break;
 		}
 
-		rte_prefetch0((void *)(uintptr_t)buf_vec[0].buf_addr);
-
 		VHOST_LOG_DEBUG(VHOST_DATA, "(%d) current index %d | end index %d\n",
 			dev->vid, vq->last_avail_idx,
 			vq->last_avail_idx + num_buffers);
@@ -1121,16 +1112,12 @@ copy_desc_to_mbuf(struct virtio_net *dev, struct vhost_virtqueue *vq,
 		goto out;
 	}
 
-	if (likely(nr_vec > 1))
-		rte_prefetch0((void *)(uintptr_t)buf_vec[1].buf_addr);
-
 	if (virtio_net_with_host_offload(dev)) {
 		if (unlikely(buf_len < sizeof(struct virtio_net_hdr))) {
 			copy_vnet_hdr_from_desc(&tmp_hdr, buf_vec);
 			hdr = &tmp_hdr;
 		} else {
 			hdr = (struct virtio_net_hdr *)((uintptr_t)buf_addr);
-			rte_prefetch0(hdr);
 		}
 	}
 
@@ -1160,9 +1147,6 @@ copy_desc_to_mbuf(struct virtio_net *dev, struct vhost_virtqueue *vq,
 		buf_avail = buf_vec[vec_idx].buf_len - dev->vhost_hlen;
 	}
 
-	rte_prefetch0((void *)(uintptr_t)
-			(buf_addr + buf_offset));
-
 	PRINT_PACKET(dev,
 			(uintptr_t)(buf_addr + buf_offset),
 			(uint32_t)buf_avail, 0);
@@ -1228,14 +1212,6 @@ copy_desc_to_mbuf(struct virtio_net *dev, struct vhost_virtqueue *vq,
 			buf_iova = buf_vec[vec_idx].buf_iova;
 			buf_len = buf_vec[vec_idx].buf_len;
 
-			/*
-			 * Prefecth desc n + 1 buffer while
-			 * desc n buffer is processed.
-			 */
-			if (vec_idx + 1 < nr_vec)
-				rte_prefetch0((void *)(uintptr_t)
-						buf_vec[vec_idx + 1].buf_addr);
-
 			buf_offset = 0;
 			buf_avail  = buf_len;
 
@@ -1379,8 +1355,6 @@ virtio_dev_tx_split(struct virtio_net *dev, struct vhost_virtqueue *vq,
 		if (likely(dev->dequeue_zero_copy == 0))
 			update_shadow_used_ring_split(vq, head_idx, 0);
 
-		rte_prefetch0((void *)(uintptr_t)buf_vec[0].buf_addr);
-
 		pkts[i] = rte_pktmbuf_alloc(mbuf_pool);
 		if (unlikely(pkts[i] == NULL)) {
 			RTE_LOG(ERR, VHOST_DATA,
@@ -1490,8 +1464,6 @@ virtio_dev_tx_packed(struct virtio_net *dev, struct vhost_virtqueue *vq,
 			update_shadow_used_ring_packed(vq, buf_id, 0,
 					desc_count);
 
-		rte_prefetch0((void *)(uintptr_t)buf_vec[0].buf_addr);
-
 		pkts[i] = rte_pktmbuf_alloc(mbuf_pool);
 		if (unlikely(pkts[i] == NULL)) {
 			RTE_LOG(ERR, VHOST_DATA,
-- 
2.21.0