It is not possible to instantiate a virtio-blk device with 0 virtqueues.
The following check is located in ->realize():

  if (!conf->num_queues) {
      error_setg(errp, "num-queues property must be larger than 0");
      return;
  }

Later on we access s->vq_aio_context[0] under the assumption that there
is as least one virtqueue. Hanna Czenczek <hreitz@redhat.com> noted that
it would help to show that the array index is already valid.

Add an assertion to document that s->vq_aio_context[0] is always
safe...and catch future code changes that break this assumption.

Suggested-by: Hanna Czenczek <hreitz@redhat.com>
Reviewed-by: Manos Pitsidianakis <manos.pitsidianakis@linaro.org>
Reviewed-by: Hanna Czenczek <hreitz@redhat.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
---
 hw/block/virtio-blk.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/hw/block/virtio-blk.c b/hw/block/virtio-blk.c
index 6e3e3a23ee..e430ba583c 100644
--- a/hw/block/virtio-blk.c
+++ b/hw/block/virtio-blk.c
@@ -1824,6 +1824,7 @@ static int virtio_blk_start_ioeventfd(VirtIODevice *vdev)
      * Try to change the AioContext so that block jobs and other operations can
      * co-locate their activity in the same AioContext. If it fails, nevermind.
      */
+    assert(nvqs > 0); /* enforced during ->realize() */
     r = blk_set_aio_context(s->conf.conf.blk, s->vq_aio_context[0],
                             &local_err);
     if (r < 0) {
-- 
2.43.0