virtio: order used ring after used index read
Michael S. Tsirkin [Sun, 25 Oct 2009 13:28:53 +0000 (15:28 +0200)]
On SMP guests, reads from the ring might bypass used index reads. This
causes guest crashes because host writes to used index to signal ring
data readiness.  Fix this by inserting rmb before used ring reads.

Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
Cc: stable@kernel.org

drivers/virtio/virtio_ring.c

index f536005..fbd2ecd 100644 (file)
@@ -285,6 +285,9 @@ static void *vring_get_buf(struct virtqueue *_vq, unsigned int *len)
                return NULL;
        }
 
+       /* Only get used array entries after they have been exposed by host. */
+       rmb();
+
        i = vq->vring.used->ring[vq->last_used_idx%vq->vring.num].id;
        *len = vq->vring.used->ring[vq->last_used_idx%vq->vring.num].len;