Re: [Qemu-devel] [PATCH] balloon: Fix failure of updating guest memory status

[Ladi Prosek]

On Thu, Jun 30, 2016 at 9:31 AM, Liang Li <address@hidden> wrote:
> After live migration, 'guest-stats' can't get the expected memory
> status in the guest. This issue is caused by commit 4eae2a657d.
> The value of 's->stats_vq_elem' will be NULL after live migration,
> and the check in the function 'balloon_stats_poll_cb()' will
> prevent the 'virtio_notify()' from executing. So guest will not
> update the memory status.
>
> Signed-off-by: Liang Li <address@hidden>
> Cc: Michael S. Tsirkin <address@hidden>
> Cc: Ladi Prosek <address@hidden>
> Cc: Paolo Bonzini <address@hidden>
> ---
>  hw/virtio/virtio-balloon.c | 8 +++++++-
>  1 file changed, 7 insertions(+), 1 deletion(-)
>
> diff --git a/hw/virtio/virtio-balloon.c b/hw/virtio/virtio-balloon.c
> index 557d3f9..cc6947f 100644
> --- a/hw/virtio/virtio-balloon.c
> +++ b/hw/virtio/virtio-balloon.c
> @@ -98,13 +98,19 @@ static void balloon_stats_poll_cb(void *opaque)
>  {
>      VirtIOBalloon *s = opaque;
>      VirtIODevice *vdev = VIRTIO_DEVICE(s);
> +    VirtQueueElement elem = {0};
>
> -    if (s->stats_vq_elem == NULL || !balloon_stats_supported(s)) {
> +    if (!balloon_stats_supported(s)) {
>          /* re-schedule */
>          balloon_stats_change_timer(s, s->stats_poll_interval);
>          return;
>      }
>
> +    if (s->stats_vq_elem == NULL) {
> +        virtqueue_push(s->svq, &elem, 0);
> +        virtio_notify(vdev, s->svq);
> +        return;
> +    }

What if we are not in the just-after-live-migration situation though?
If the guest simply didn't add a buffer to the queue for some reason,
wouldn't this newly added push/notify break the balloon protocol?

Thanks!
Ladi

>      virtqueue_push(s->svq, s->stats_vq_elem, s->stats_vq_offset);
>      virtio_notify(vdev, s->svq);
>      g_free(s->stats_vq_elem);
> --
> 1.9.1
>