Re: [Qemu-devel] [PATCH 2/2] virtio-balloon: resume collecting stats on vmload

[Michael S. Tsirkin]

On Thu, Sep 01, 2016 at 09:14:00PM +0300, Roman Kagan wrote:
> Upon save/restore virtio-balloon stats acquisition stops.  The reason is
> that the in-use virtqueue element is not saved, and upon restore it's
> not released to the guest, making further progress impossible.
> 
> Saving the information about the used element would introduce
> unjustified vmstate incompatibility.
> 
> However, the number of virtqueue in-use elements can be deduced from the
> data available on restore (see bccdef6b1a204db0f41ffb6e24ce373e4d7890d4
> "virtio: recalculate vq->inuse after migration").
> 
> So make the stats virtqueue forget that an element was popped from it,
> and start over.  As this tackles the problem on the "load" side, it
> is compatible with the state saved by earlier QEMU versions.
> 
> Signed-off-by: Roman Kagan <address@hidden>
> Cc: "Michael S. Tsirkin" <address@hidden>
> Cc: Ladi Prosek <address@hidden>
> Cc: Stefan Hajnoczi <address@hidden>
> ---
>  hw/virtio/virtio-balloon.c | 21 ++++++++++++++++++++-
>  1 file changed, 20 insertions(+), 1 deletion(-)
> 
> diff --git a/hw/virtio/virtio-balloon.c b/hw/virtio/virtio-balloon.c
> index 5af429a..8660052 100644
> --- a/hw/virtio/virtio-balloon.c
> +++ b/hw/virtio/virtio-balloon.c
> @@ -406,7 +406,13 @@ static void virtio_balloon_save_device(VirtIODevice 
> *vdev, QEMUFile *f)
>  
>  static int virtio_balloon_load(QEMUFile *f, void *opaque, size_t size)
>  {
> -    return virtio_load(VIRTIO_DEVICE(opaque), f, 1);
> +    VirtIODevice *vdev = VIRTIO_DEVICE(opaque);
> +    VirtIOBalloon *s = VIRTIO_BALLOON(vdev);
> +    int ret = virtio_load(vdev, f, 1);
> +    /* rewind needs vq->inuse populated which happens in virtio_load() after
> +     * vdev->load */
> +    virtqueue_rewind(s->svq);
> +    return ret;
>  }
>  
>  static int virtio_balloon_load_device(VirtIODevice *vdev, QEMUFile *f,
> @@ -468,6 +474,18 @@ static void virtio_balloon_device_reset(VirtIODevice 
> *vdev)
>      }
>  }
>  
> +static void virtio_balloon_set_status(VirtIODevice *vdev, uint8_t status)
> +{
> +    VirtIOBalloon *s = VIRTIO_BALLOON(vdev);
> +
> +    if (vdev->vm_running && balloon_stats_supported(s) &&
> +        (status & VIRTIO_CONFIG_S_DRIVER_OK) && !s->stats_vq_elem) {
> +        /* poll stats queue for the element we may have discarded when the VM
> +         * was stopped */
> +        virtio_balloon_receive_stats(vdev, s->svq);
> +    }
> +}
> +
>  static void virtio_balloon_instance_init(Object *obj)
>  {
>      VirtIOBalloon *s = VIRTIO_BALLOON(obj);
> @@ -505,6 +523,7 @@ static void virtio_balloon_class_init(ObjectClass *klass, 
> void *data)
>      vdc->get_features = virtio_balloon_get_features;
>      vdc->save = virtio_balloon_save_device;
>      vdc->load = virtio_balloon_load_device;
> +    vdc->set_status = virtio_balloon_set_status;
>  }
>  
>  static const TypeInfo virtio_balloon_info = {


I'm sorry - I don't like this patch. This means that
virtio_balloon_receive_stats will be called and will poke
at the ring even if the ring was never kicked.

This is why in my opinion it is cleaner to have
virtqueue_rewind return a true/false status,
and only process the ring if ring was rewinded.

I think Stefan's patches did something similar.

> -- 
> 2.7.4