Re: [Qemu-devel] [PATCH] virtio-balloon: return empty data when no stats are available

[Luiz Capitulino <lcapitulino@redhat.com>]

On Mon, 19 May 2014 08:41:21 +0200
Ján Tomko <jtomko@redhat.com> wrote:

> If the guest hasn't updated the stats yet, instead of returning
> an error, return '-1' for the stats and '0' as 'last-update'.
> 
> This lets applications ignore this without parsing the error message.
> 
> Related libvirt patch and discussion:
> https://www.redhat.com/archives/libvir-list/2014-May/msg00460.html
> 
> Signed-off-by: Ján Tomko <jtomko@redhat.com>
> ---
>  hw/virtio/virtio-balloon.c | 7 ++-----
>  1 file changed, 2 insertions(+), 5 deletions(-)
> 
> diff --git a/hw/virtio/virtio-balloon.c b/hw/virtio/virtio-balloon.c
> index 971a921..6661c53 100644
> --- a/hw/virtio/virtio-balloon.c
> +++ b/hw/virtio/virtio-balloon.c
> @@ -111,11 +111,6 @@ static void balloon_stats_get_all(Object *obj, struct Visitor *v,
>      VirtIOBalloon *s = opaque;
>      int i;
>  
> -    if (!s->stats_last_update) {
> -        error_setg(errp, "guest hasn't updated any stats yet");
> -        return;
> -    }
> -
>      visit_start_struct(v, NULL, "guest-stats", name, 0, errp);
>      visit_type_int(v, &s->stats_last_update, "last-update", errp);
>  
> @@ -361,6 +356,8 @@ static void virtio_balloon_device_realize(DeviceState *dev, Error **errp)
>      s->dvq = virtio_add_queue(vdev, 128, virtio_balloon_handle_output);
>      s->svq = virtio_add_queue(vdev, 128, virtio_balloon_receive_stats);
>  
> +    reset_stats(s);
> +
>      register_savevm(dev, "virtio-balloon", -1, 1,
>                      virtio_balloon_save, virtio_balloon_load, s);
>  

Looks good to me, but I have some comments:

- I'm assuming you tested this against libivrt, right? It's good to add
  a comment in the changelog mentioning that

- You have to update the documentation in docs/virtio-balloon-stats.txt
  as well. There are two sections to update, the one describing the last-update
  key (where you should mention what it means when last-update=0) and the
  section saying that polling can be enabled even when the guest doesn't have
  stats support (I'd mention that last-update=0 for that case)

- Please, rebase on top of latest master