vhost: enforce desc flags and content read ordering

A read barrier is required to ensure that the ordering between
descriptor's flags and content reads is enforced.

1. read flags = desc->flags
if (flags & AVAIL_BIT)
2.   read desc->id

There is a control dependency between steps 1 and step 2.
2 could be speculatively executed before 1, which could result
in 'id' to not be updated yet.

Fixes: 2f3225a7d69b ("vhost: add vector filling support for packed ring")
Cc: stable@dpdk.org

Reported-by: Jason Wang <jasowang@redhat.com>
Signed-off-by: Maxime Coquelin <maxime.coquelin@redhat.com>
Acked-by: Ilya Maximets <i.maximets@samsung.com>
Acked-by: Michael S. Tsirkin <mst@redhat.com>
Acked-by: Tiwei Bie <tiwei.bie@intel.com>

lib/librte_vhost/virtio_net.c

@@ -481,6 +481,12 @@ fill_vec_buf_packed(struct virtio_net *dev, struct vhost_virtqueue *vq,
 	if (unlikely(!desc_is_avail(&descs[avail_idx], wrap_counter)))
 		return -1;
 
+	/*
+	 * The ordering between desc flags and desc
+	 * content reads need to be enforced.
+	 */
+	rte_smp_rmb();
+
 	*desc_count = 0;
 	*len = 0;