Re: [Qemu-devel] [PATCH 2/4] virtio-balloon: make stats virtqueue length 1

[Stefan Hajnoczi <stefanha@gmail.com>]

[-- Attachment #1: Type: text/plain, Size: 1974 bytes --]

On Thu, Aug 18, 2016 at 09:27:52PM +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>
> ---
>  hw/virtio/virtio-balloon.c | 9 +--------
>  1 file changed, 1 insertion(+), 8 deletions(-)
> 
> diff --git a/hw/virtio/virtio-balloon.c b/hw/virtio/virtio-balloon.c
> index 5af429a..0baf4b3 100644
> --- a/hw/virtio/virtio-balloon.c
> +++ b/hw/virtio/virtio-balloon.c
> @@ -262,13 +262,6 @@ 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);
> -    }
> -
>      s->stats_vq_elem = elem;
>  
>      /* Initialize the stats to get rid of any stale values.  This is only
> @@ -443,7 +436,7 @@ 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);

This change breaks live migration compatibility.  The device is not
allowed to change in existing machine types.

I think Ladi's fix for this bug is the way to go.  It preserves
migration compatibility.

[-- Attachment #2: signature.asc --]
[-- Type: application/pgp-signature, Size: 473 bytes --]