[dpdk-dev] [PATCH RFC 3/4] vhost: avoid reordering of used->idx and last_used_idx updating.

[Ilya Maximets <i.maximets@samsung.com>]

Calling rte_vhost_enqueue_burst() simultaneously from different threads
for the same queue_id requires additional SMP memory barrier to avoid
reordering of used->idx and last_used_idx updates.

In case of virtio_dev_rx() memory barrier rte_mb() simply moved one
instruction higher.

Signed-off-by: Ilya Maximets <i.maximets@samsung.com>
---
 lib/librte_vhost/vhost_rxtx.c | 16 ++++++++++++----
 1 file changed, 12 insertions(+), 4 deletions(-)

diff --git a/lib/librte_vhost/vhost_rxtx.c b/lib/librte_vhost/vhost_rxtx.c
index 9095fb1..a03f687 100644
--- a/lib/librte_vhost/vhost_rxtx.c
+++ b/lib/librte_vhost/vhost_rxtx.c
@@ -281,10 +281,13 @@ virtio_dev_rx(struct virtio_net *dev, uint16_t queue_id,
 		rte_pause();
 
 	*(volatile uint16_t *)&vq->used->idx += count;
-	vq->last_used_idx = res_end_idx;
 
-	/* flush used->idx update before we read avail->flags. */
+	/*
+	 * Flush used->idx update to make it visible to virtio and all other
+	 * threads before allowing to modify it.
+	 */
 	rte_mb();
+	vq->last_used_idx = res_end_idx;
 
 	/* Kick the guest if necessary. */
 	if (!(vq->avail->flags & VRING_AVAIL_F_NO_INTERRUPT))
@@ -586,19 +589,24 @@ virtio_dev_merge_rx(struct virtio_net *dev, uint16_t queue_id,
 			rte_pause();
 
 		*(volatile uint16_t *)&vq->used->idx += entry_success;
+		/*
+		 * Flush used->idx update to make it visible to all
+		 * other threads before allowing to modify it.
+		 */
+		rte_smp_wmb();
+
 		vq->last_used_idx = res_cur_idx;
 	}
 
 merge_rx_exit:
 	if (likely(pkt_idx)) {
-		/* flush used->idx update before we read avail->flags. */
+		/* Flush used->idx update to make it visible to virtio. */
 		rte_mb();
 
 		/* Kick the guest if necessary. */
 		if (!(vq->avail->flags & VRING_AVAIL_F_NO_INTERRUPT))
 			eventfd_write(vq->callfd, (eventfd_t)1);
 	}
-
 	return pkt_idx;
 }
 
-- 
2.5.0