[dpdk-dev] [PATCH 1/7] vhost: use shadow used ring in dequeue path

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

Relax used ring contention by reusing the shadow used
ring feature used by enqueue path.

Signed-off-by: Maxime Coquelin <maxime.coquelin@redhat.com>
---
 lib/librte_vhost/virtio_net.c | 45 ++++++++++---------------------------------
 1 file changed, 10 insertions(+), 35 deletions(-)

diff --git a/lib/librte_vhost/virtio_net.c b/lib/librte_vhost/virtio_net.c
index 98ad8e936..7e70a927f 100644
--- a/lib/librte_vhost/virtio_net.c
+++ b/lib/librte_vhost/virtio_net.c
@@ -1019,35 +1019,6 @@ copy_desc_to_mbuf(struct virtio_net *dev, struct vhost_virtqueue *vq,
 	return error;
 }
 
-static __rte_always_inline void
-update_used_ring(struct virtio_net *dev, struct vhost_virtqueue *vq,
-		 uint32_t used_idx, uint32_t desc_idx)
-{
-	vq->used->ring[used_idx].id  = desc_idx;
-	vq->used->ring[used_idx].len = 0;
-	vhost_log_cache_used_vring(dev, vq,
-			offsetof(struct vring_used, ring[used_idx]),
-			sizeof(vq->used->ring[used_idx]));
-}
-
-static __rte_always_inline void
-update_used_idx(struct virtio_net *dev, struct vhost_virtqueue *vq,
-		uint32_t count)
-{
-	if (unlikely(count == 0))
-		return;
-
-	rte_smp_wmb();
-	rte_smp_rmb();
-
-	vhost_log_cache_sync(dev, vq);
-
-	vq->used->idx += count;
-	vhost_log_used_vring(dev, vq, offsetof(struct vring_used, idx),
-			sizeof(vq->used->idx));
-	vhost_vring_call(dev, vq);
-}
-
 static __rte_always_inline struct zcopy_mbuf *
 get_zmbuf(struct vhost_virtqueue *vq)
 {
@@ -1146,6 +1117,7 @@ rte_vhost_dequeue_burst(int vid, uint16_t queue_id,
 		goto out_access_unlock;
 
 	vq->batch_copy_nb_elems = 0;
+	vq->shadow_used_idx = 0;
 
 	if (dev->features & (1ULL << VIRTIO_F_IOMMU_PLATFORM))
 		vhost_user_iotlb_rd_lock(vq);
@@ -1164,8 +1136,7 @@ rte_vhost_dequeue_burst(int vid, uint16_t queue_id,
 
 			if (mbuf_is_consumed(zmbuf->mbuf)) {
 				used_idx = vq->last_used_idx++ & (vq->size - 1);
-				update_used_ring(dev, vq, used_idx,
-						 zmbuf->desc_idx);
+				update_shadow_used_ring(vq, zmbuf->desc_idx, 0);
 				nr_updated += 1;
 
 				TAILQ_REMOVE(&vq->zmbuf_list, zmbuf, next);
@@ -1176,7 +1147,9 @@ rte_vhost_dequeue_burst(int vid, uint16_t queue_id,
 			}
 		}
 
-		update_used_idx(dev, vq, nr_updated);
+		flush_shadow_used_ring(dev, vq);
+		vhost_vring_call(dev, vq);
+		vq->shadow_used_idx = 0;
 	}
 
 	/*
@@ -1233,7 +1206,7 @@ rte_vhost_dequeue_burst(int vid, uint16_t queue_id,
 		desc_indexes[i] = vq->avail->ring[avail_idx];
 
 		if (likely(dev->dequeue_zero_copy == 0))
-			update_used_ring(dev, vq, used_idx, desc_indexes[i]);
+			update_shadow_used_ring(vq, desc_indexes[i], 0);
 	}
 
 	/* Prefetch descriptor index. */
@@ -1326,8 +1299,10 @@ rte_vhost_dequeue_burst(int vid, uint16_t queue_id,
 
 	if (likely(dev->dequeue_zero_copy == 0)) {
 		do_data_copy_dequeue(vq);
-		vq->last_used_idx += i;
-		update_used_idx(dev, vq, i);
+		if (unlikely(i < count))
+			vq->shadow_used_idx = i;
+		flush_shadow_used_ring(dev, vq);
+		vhost_vring_call(dev, vq);
 	}
 
 out:
-- 
2.14.3