[dpdk-dev] [PATCH v4] vhost: batch used descs chains write-back with packed ring

[Maxime Coquelin]

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

Also, move the write barrier in logging cache sync, so that it
is done only when logging is enabled. It means there is now
one more barrier for split ring when logging is enabled.

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

Signed-off-by: Maxime Coquelin <maxime.coquelin at redhat.com>
---
 lib/librte_vhost/vhost.h      |  2 ++
 lib/librte_vhost/virtio_net.c | 19 ++++++++++++++++---
 2 files changed, 18 insertions(+), 3 deletions(-)

diff --git a/lib/librte_vhost/vhost.h b/lib/librte_vhost/vhost.h
index 3b3265c4b..7d1d8a308 100644
--- a/lib/librte_vhost/vhost.h
+++ b/lib/librte_vhost/vhost.h
@@ -457,6 +457,8 @@ vhost_log_cache_sync(struct virtio_net *dev, struct vhost_virtqueue *vq)
 		   !dev->log_base))
 		return;
 
+	rte_smp_wmb();
+
 	log_base = (unsigned long *)(uintptr_t)dev->log_base;
 
 	/*
diff --git a/lib/librte_vhost/virtio_net.c b/lib/librte_vhost/virtio_net.c
index 8c657a101..02c1fd3a4 100644
--- a/lib/librte_vhost/virtio_net.c
+++ b/lib/librte_vhost/virtio_net.c
@@ -97,6 +97,8 @@ flush_shadow_used_ring_packed(struct virtio_net *dev,
 {
 	int i;
 	uint16_t used_idx = vq->last_used_idx;
+	uint16_t head_idx = vq->last_used_idx;
+	uint16_t head_flags = 0;
 
 	/* Split loop in two to save memory barriers */
 	for (i = 0; i < vq->shadow_used_idx; i++) {
@@ -126,12 +128,17 @@ flush_shadow_used_ring_packed(struct virtio_net *dev,
 			flags &= ~VRING_DESC_F_AVAIL;
 		}
 
-		vq->desc_packed[vq->last_used_idx].flags = flags;
+		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) {
@@ -140,7 +147,13 @@ flush_shadow_used_ring_packed(struct virtio_net *dev,
 		}
 	}
 
-	rte_smp_wmb();
+	vq->desc_packed[head_idx].flags = head_flags;
+
+	vhost_log_cache_used_vring(dev, vq,
+				head_idx *
+				sizeof(struct vring_packed_desc),
+				sizeof(struct vring_packed_desc));
+
 	vq->shadow_used_idx = 0;
 	vhost_log_cache_sync(dev, vq);
 }
-- 
2.17.2