[dpdk-dev] [PATCH] vhost: batch used descriptors chains write-back with packed ring

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

Instead of writing back descriptors chains in order, let's
write the first chain flags last in order to improve batching.

With Kernel's pktgen benchmark, ~3% performance gain is measured.

Signed-off-by: Maxime Coquelin <maxime.coquelin@redhat.com>
---
 lib/librte_vhost/virtio_net.c | 37 ++++++++++++++++++++++-------------
 1 file changed, 23 insertions(+), 14 deletions(-)

diff --git a/lib/librte_vhost/virtio_net.c b/lib/librte_vhost/virtio_net.c
index 5e1a1a727..f54642c2d 100644
--- a/lib/librte_vhost/virtio_net.c
+++ b/lib/librte_vhost/virtio_net.c
@@ -135,19 +135,10 @@ flush_shadow_used_ring_packed(struct virtio_net *dev,
 			struct vhost_virtqueue *vq)
 {
 	int i;
-	uint16_t used_idx = vq->last_used_idx;
+	uint16_t head_flags, head_idx = vq->last_used_idx;
 
-	/* Split loop in two to save memory barriers */
-	for (i = 0; i < vq->shadow_used_idx; i++) {
-		vq->desc_packed[used_idx].id = vq->shadow_used_packed[i].id;
-		vq->desc_packed[used_idx].len = vq->shadow_used_packed[i].len;
-
-		used_idx += vq->shadow_used_packed[i].count;
-		if (used_idx >= vq->size)
-			used_idx -= vq->size;
-	}
-
-	rte_smp_wmb();
+	if (unlikely(vq->shadow_used_idx == 0))
+		return;
 
 	for (i = 0; i < vq->shadow_used_idx; i++) {
 		uint16_t flags;
@@ -165,12 +156,22 @@ flush_shadow_used_ring_packed(struct virtio_net *dev,
 			flags &= ~VRING_DESC_F_AVAIL;
 		}
 
-		vq->desc_packed[vq->last_used_idx].flags = flags;
+		vq->desc_packed[vq->last_used_idx].id =
+			vq->shadow_used_packed[i].id;
+		vq->desc_packed[vq->last_used_idx].len =
+			vq->shadow_used_packed[i].len;
+
+		if (i > 0) {
+			vq->desc_packed[vq->last_used_idx].flags = flags;
 
-		vhost_log_cache_used_vring(dev, vq,
+			vhost_log_cache_used_vring(dev, vq,
 					vq->last_used_idx *
 					sizeof(struct vring_packed_desc),
 					sizeof(struct vring_packed_desc));
+		} else {
+			head_idx = vq->last_used_idx;
+			head_flags = flags;
+		}
 
 		vq->last_used_idx += vq->shadow_used_packed[i].count;
 		if (vq->last_used_idx >= vq->size) {
@@ -180,7 +181,15 @@ flush_shadow_used_ring_packed(struct virtio_net *dev,
 	}
 
 	rte_smp_wmb();
+
+	vq->desc_packed[head_idx].flags = head_flags;
 	vq->shadow_used_idx = 0;
+
+	vhost_log_cache_used_vring(dev, vq,
+				head_idx *
+				sizeof(struct vring_packed_desc),
+				sizeof(struct vring_packed_desc));
+
 	vhost_log_cache_sync(dev, vq);
 }
 
-- 
2.17.2