[dpdk-dev] [PATCH 3/6] vhost: introduce last avail idx for Tx

[Yuanhan Liu]

So far, we retrieve both the used ring avail ring idx by last_used_idx
var; it won't be a problem because we used ring is updated immediately
after those avail entries are consumed.

But that's not true when Tx zero copy is enabled, that used ring is updated
only when the mbuf is consumed. Thus, we need use another var to note
the last avail ring idx we have consumed.

Therefore, last_avail_idx is introduced.

Signed-off-by: Yuanhan Liu <yuanhan.liu at linux.intel.com>
---
 lib/librte_vhost/vhost.h      |  2 +-
 lib/librte_vhost/virtio_net.c | 19 +++++++++++--------
 2 files changed, 12 insertions(+), 9 deletions(-)

diff --git a/lib/librte_vhost/vhost.h b/lib/librte_vhost/vhost.h
index 2d52987..8565fa1 100644
--- a/lib/librte_vhost/vhost.h
+++ b/lib/librte_vhost/vhost.h
@@ -70,7 +70,7 @@ struct vhost_virtqueue {
 	struct vring_used	*used;
 	uint32_t		size;
 
-	/* Last index used on the available ring */
+	uint16_t		last_avail_idx;
 	volatile uint16_t	last_used_idx;
 #define VIRTIO_INVALID_EVENTFD		(-1)
 #define VIRTIO_UNINITIALIZED_EVENTFD	(-2)
diff --git a/lib/librte_vhost/virtio_net.c b/lib/librte_vhost/virtio_net.c
index 8a151af..1c2ee47 100644
--- a/lib/librte_vhost/virtio_net.c
+++ b/lib/librte_vhost/virtio_net.c
@@ -846,16 +846,17 @@ rte_vhost_dequeue_burst(int vid, uint16_t queue_id,
 		}
 	}
 
-	avail_idx =  *((volatile uint16_t *)&vq->avail->idx);
-	free_entries = avail_idx - vq->last_used_idx;
+	free_entries = *((volatile uint16_t *)&vq->avail->idx) -
+			vq->last_avail_idx;
 	if (free_entries == 0)
 		goto out;
 
 	LOG_DEBUG(VHOST_DATA, "(%d) %s\n", dev->vid, __func__);
 
-	/* Prefetch available ring to retrieve head indexes. */
-	used_idx = vq->last_used_idx & (vq->size - 1);
-	rte_prefetch0(&vq->avail->ring[used_idx]);
+	/* Prefetch available and used ring */
+	avail_idx = vq->last_avail_idx & (vq->size - 1);
+	used_idx  = vq->last_used_idx  & (vq->size - 1);
+	rte_prefetch0(&vq->avail->ring[avail_idx]);
 	rte_prefetch0(&vq->used->ring[used_idx]);
 
 	count = RTE_MIN(count, MAX_PKT_BURST);
@@ -865,8 +866,9 @@ rte_vhost_dequeue_burst(int vid, uint16_t queue_id,
 
 	/* Retrieve all of the head indexes first to avoid caching issues. */
 	for (i = 0; i < count; i++) {
-		used_idx = (vq->last_used_idx + i) & (vq->size - 1);
-		desc_indexes[i] = vq->avail->ring[used_idx];
+		avail_idx = (vq->last_avail_idx + i) & (vq->size - 1);
+		used_idx  = (vq->last_used_idx  + i) & (vq->size - 1);
+		desc_indexes[i] = vq->avail->ring[avail_idx];
 
 		vq->used->ring[used_idx].id  = desc_indexes[i];
 		vq->used->ring[used_idx].len = 0;
@@ -900,7 +902,8 @@ rte_vhost_dequeue_burst(int vid, uint16_t queue_id,
 	rte_smp_wmb();
 	rte_smp_rmb();
 	vq->used->idx += i;
-	vq->last_used_idx += i;
+	vq->last_avail_idx += i;
+	vq->last_used_idx  += i;
 	vhost_log_used_vring(dev, vq, offsetof(struct vring_used, idx),
 			sizeof(vq->used->idx));
 
-- 
1.9.0