From: Paolo Bonzini <pbonzini@redhat.com>

If the vring has not been set up, it is not necessary for vring_used_idx
to do anything (as is already the case when the caller is virtio_load).
This is harmless for now, but it will be a problem when the
MemoryRegionCache has not been set up.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
---
 hw/virtio/virtio.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/hw/virtio/virtio.c b/hw/virtio/virtio.c
index d62509d..cdafcec 100644
--- a/hw/virtio/virtio.c
+++ b/hw/virtio/virtio.c
@@ -2156,7 +2156,9 @@ void virtio_queue_set_last_avail_idx(VirtIODevice *vdev, int n, uint16_t idx)
 
 void virtio_queue_update_used_idx(VirtIODevice *vdev, int n)
 {
-    vdev->vq[n].used_idx = vring_used_idx(&vdev->vq[n]);
+    if (vdev->vq[n].vring.desc) {
+        vdev->vq[n].used_idx = vring_used_idx(&vdev->vq[n]);
+    }
 }
 
 void virtio_queue_invalidate_signalled_used(VirtIODevice *vdev, int n)
-- 
MST