Re: [Qemu-devel] [PATCH] virtio-balloon: stats vq migration spec compliance

[Stefan Hajnoczi]

On Tue, Aug 16, 2016 at 12:32:55AM +0300, Michael S. Tsirkin wrote:
> virtio spec says that devices must not touch VQs
> before DRIVER_OK is set.
> 
> Additionally, balloon must not touch stats VQ
> unless the stats feature bit has been negotiated.
> 
> Cc: Ladi Prosek <address@hidden>
> Signed-off-by: Michael S. Tsirkin <address@hidden>
> ---
>  hw/virtio/virtio-balloon.c | 4 +++-
>  1 file changed, 3 insertions(+), 1 deletion(-)
> 
> diff --git a/hw/virtio/virtio-balloon.c b/hw/virtio/virtio-balloon.c
> index 65457e9..7189260 100644
> --- a/hw/virtio/virtio-balloon.c
> +++ b/hw/virtio/virtio-balloon.c
> @@ -427,6 +427,7 @@ static void virtio_balloon_vmstate_cb(void *opaque, int 
> running,
>                                        RunState state)
>  {
>      VirtIOBalloon *s = opaque;
> +    VirtIODevice *vdev = VIRTIO_DEVICE(s);
>  
>      if (!running) {
>          /* put the stats element back if the VM is not running */
> @@ -436,7 +437,8 @@ static void virtio_balloon_vmstate_cb(void *opaque, int 
> running,
>              s->stats_vq_elem = NULL;
>          }
>  
> -    } else {
> +    } else if (balloon_stats_supported(s) &&
> +               (vdev->status & VIRTIO_CONFIG_S_DRIVER_OK)) {
>          /* poll stats queue for the element we may have discarded
>           * when the VM was stopped */
>          virtio_balloon_receive_stats(VIRTIO_DEVICE(s), s->svq);

Another suspect case is when the feature bit was negotiated,
VIRTIO_CONFIG_S_DRIVER_OK is set, but the virtqueue PFN wasn't set up.

In that case virtqueue_pop() will access junk values.

I'm not sure if anything terrible can happen but a check that
vq->vring.desc != 0 would be nice - just like we do in
virtio_queue_notify_vq().

Can you add a virtio_queue_get_addr() call to see if the PFN has been
set?

Stefan

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