virtio: order index/descriptor reads

virtio has the equivalent of:

	if (vq->last_avail_index != vring_avail_idx(vq)) {
		read descriptor head at vq->last_avail_index;
	}

In theory, processor can reorder descriptor head
read to happen speculatively before the index read.
this would trigger the following race:

	host descriptor head read <- reads invalid head from ring
		guest writes valid descriptor head
		guest writes avail index
	host avail index read <- observes valid index

as a result host will use an invalid head value.
This was not observed in the field by me but after
the experience with the previous two races
I think it is prudent to address this theoretical race condition.

Signed-off-by: Michael S. Tsirkin <mst@redhat.com>

hw/virtio.c

@@ -287,6 +287,11 @@ static int virtqueue_num_heads(VirtQueue *vq, unsigned int idx)
                      idx, vring_avail_idx(vq));
         exit(1);
     }
+    /* On success, callers read a descriptor at vq->last_avail_idx.
+     * Make sure descriptor read does not bypass avail index read. */
+    if (num_heads) {
+        smp_rmb();
+    }
 
     return num_heads;
 }