[dpdk-dev] [PATCH] vhost: fix invalid use of stored last used index

[Balazs Nemeth <bnemeth@redhat.com>]

The optimization introduced by commit d18db8049c7c ("vhost: read last
used index once") didn't account for the fact that
vhost_flush_enqueue_shadow_packed increments the last_used_idx. For this
reason, store last_used_idx after the potential call to
vhost_flush_enqueue_shadow_packed.

Fixes: d18db8049c7c ("vhost: read last used index once")
Signed-off-by: Balazs Nemeth <bnemeth@redhat.com>
---
 lib/vhost/virtio_net.c | 7 +++++--
 1 file changed, 5 insertions(+), 2 deletions(-)

diff --git a/lib/vhost/virtio_net.c b/lib/vhost/virtio_net.c
index abfd67d38c..8da8a86a10 100644
--- a/lib/vhost/virtio_net.c
+++ b/lib/vhost/virtio_net.c
@@ -218,14 +218,17 @@ vhost_flush_enqueue_batch_packed(struct virtio_net *dev,
 {
 	uint16_t i;
 	uint16_t flags;
-	uint16_t last_used_idx = vq->last_used_idx;
-	struct vring_packed_desc *desc_base = &vq->desc_packed[last_used_idx];
+	uint16_t last_used_idx;
+	struct vring_packed_desc *desc_base;
 
 	if (vq->shadow_used_idx) {
 		do_data_copy_enqueue(dev, vq);
 		vhost_flush_enqueue_shadow_packed(dev, vq);
 	}
 
+	last_used_idx = vq->last_used_idx;
+	desc_base = &vq->desc_packed[last_used_idx];
+
 	flags = PACKED_DESC_ENQUEUE_USED_FLAG(vq->used_wrap_counter);
 
 	vhost_for_each_try_unroll(i, 0, PACKED_BATCH_SIZE) {
-- 
2.30.2