Re: [Qemu-devel] [PATCH v2 2/6] virtio-balloon: make stats virtqueue length 1

["Michael S. Tsirkin" <mst@redhat.com>]

On Fri, Aug 19, 2016 at 04:39:21PM +0300, Roman Kagan wrote:
> The protocol for virtio-balloon stats virtqueue doesn't allow more than
> one element in the virtqueue.
> 
> So, instead of trying to compensate for guest misbehavior if it sends
> new data before the slot has been released by the host, just define the
> stats virtqueue length to 1 initially and rely on the generic virtio
> code to handle overflows.
> 
> Signed-off-by: Roman Kagan <rkagan@virtuozzo.com>
> Cc: "Michael S. Tsirkin" <mst@redhat.com>
> Cc: Ladi Prosek <lprosek@redhat.com>
> Cc: Stefan Hajnoczi <stefanha@redhat.com>


This would need a bunch of compat handlers to
avoid changes for old machine types.

> ---
> v1 > v2:
>  - added assert in virtio_balloon_receive_stats
> 
>  hw/virtio/virtio-balloon.c | 10 +++-------
>  1 file changed, 3 insertions(+), 7 deletions(-)
> 
> diff --git a/hw/virtio/virtio-balloon.c b/hw/virtio/virtio-balloon.c
> index 5af429a..fb8784e 100644
> --- a/hw/virtio/virtio-balloon.c
> +++ b/hw/virtio/virtio-balloon.c
> @@ -262,12 +262,8 @@ static void virtio_balloon_receive_stats(VirtIODevice *vdev, VirtQueue *vq)
>          goto out;
>      }
>  
> -    if (s->stats_vq_elem != NULL) {
> -        /* This should never happen if the driver follows the spec. */
> -        virtqueue_push(vq, s->stats_vq_elem, 0);
> -        virtio_notify(vdev, vq);
> -        g_free(s->stats_vq_elem);
> -    }
> +    /* enforced by stats virtqueue depth being 1 */
> +    assert(!s->stats_vq_elem);
>  
>      s->stats_vq_elem = elem;
>  
> @@ -443,7 +439,7 @@ static void virtio_balloon_device_realize(DeviceState *dev, Error **errp)
>  
>      s->ivq = virtio_add_queue(vdev, 128, virtio_balloon_handle_output);
>      s->dvq = virtio_add_queue(vdev, 128, virtio_balloon_handle_output);
> -    s->svq = virtio_add_queue(vdev, 128, virtio_balloon_receive_stats);
> +    s->svq = virtio_add_queue(vdev, 1, virtio_balloon_receive_stats);
>  
>      reset_stats(s);
>  }
> -- 
> 2.7.4