Re: [Qemu-devel] [PATCH 4/4] virtio-balloon: keep collecting stats on save/restore

[Roman Kagan]

On Fri, Aug 19, 2016 at 11:57:44AM +0200, Ladi Prosek wrote:
> On Thu, Aug 18, 2016 at 8:27 PM, Roman Kagan <address@hidden> wrote:
> > Upon save/restore virtio-balloon stats acquisition stops.  The reason is
> > that the fact that the (only) virtqueue element is being used by QEMU is
> > not recorded anywhere on save, so 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.
> >
> > So instead just make sure the element is pushed before save, leaving the
> > ball on the guest side.  For that, add vm state change handler to
> > virtio-ballon which would take care of pushing the element if there is
> > one.
> 
> Please take a look at:
> https://lists.gnu.org/archive/html/qemu-devel/2016-08/msg01038.html

Thanks for the pointer, I wish I did a better maillist search before
submitting.

> virtqueue_discard looks like a better choice than virtqueue_push.
> There's no need to cause churn on the guest side and receive an extra
> set of stats in between the regular timer callbacks.

I forgot about virtqueue_discard; however, I still tend to slightly
prefer the push version as it IMO makes things easier to follow: the
pumping of stats is always initiated by the guest (both initially on
guest driver load and upon restore), and ->stats_vq_elem being non-NULL
is enough to know that it needs pushing (both in the timer callback and
in vm state change handler).

> Also in that thread see the note about stats_vq_offset which is
> misused and can be killed.

Indeed.

Thanks,
Roman.

> > Signed-off-by: Roman Kagan <address@hidden>
> > Cc: "Michael S. Tsirkin" <address@hidden>
> > ---
> >  hw/virtio/virtio-balloon.c         | 27 ++++++++++++++++++++++-----
> >  include/hw/virtio/virtio-balloon.h |  1 +
> >  2 files changed, 23 insertions(+), 5 deletions(-)
> >
> > diff --git a/hw/virtio/virtio-balloon.c b/hw/virtio/virtio-balloon.c
> > index b56fecd..66a926a 100644
> > --- a/hw/virtio/virtio-balloon.c
> > +++ b/hw/virtio/virtio-balloon.c
> > @@ -88,10 +88,19 @@ static void balloon_stats_change_timer(VirtIOBalloon 
> > *s, int64_t secs)
> >      timer_mod(s->stats_timer, qemu_clock_get_ms(QEMU_CLOCK_VIRTUAL) + secs 
> > * 1000);
> >  }
> >
> > +static void balloon_stats_push_elem(VirtIOBalloon *s)
> > +{
> > +    VirtIODevice *vdev = VIRTIO_DEVICE(s);
> > +
> > +    virtqueue_push(s->svq, s->stats_vq_elem, s->stats_vq_offset);
> > +    virtio_notify(vdev, s->svq);
> > +    g_free(s->stats_vq_elem);
> > +    s->stats_vq_elem = NULL;
> > +}
> > +
> >  static void balloon_stats_poll_cb(void *opaque)
> >  {
> >      VirtIOBalloon *s = opaque;
> > -    VirtIODevice *vdev = VIRTIO_DEVICE(s);
> >
> >      if (!s->stats_vq_elem) {
> >          /* The guest hasn't sent the stats yet (either not enabled or we 
> > came
> > @@ -100,10 +109,7 @@ static void balloon_stats_poll_cb(void *opaque)
> >          return;
> >      }
> >
> > -    virtqueue_push(s->svq, s->stats_vq_elem, s->stats_vq_offset);
> > -    virtio_notify(vdev, s->svq);
> > -    g_free(s->stats_vq_elem);
> > -    s->stats_vq_elem = NULL;
> > +    balloon_stats_push_elem(s);
> >  }
> >
> >  static void balloon_stats_get_all(Object *obj, Visitor *v, const char 
> > *name,
> > @@ -411,6 +417,15 @@ static int virtio_balloon_load_device(VirtIODevice 
> > *vdev, QEMUFile *f,
> >      return 0;
> >  }
> >
> > +static void balloon_vm_state_change(void *opaque, int running, RunState 
> > state)
> > +{
> > +    VirtIOBalloon *s = opaque;
> > +
> > +    if (!running && s->stats_vq_elem) {
> > +        balloon_stats_push_elem(s);
> > +    }
> > +}
> > +
> >  static void virtio_balloon_device_realize(DeviceState *dev, Error **errp)
> >  {
> >      VirtIODevice *vdev = VIRTIO_DEVICE(dev);
> > @@ -433,6 +448,7 @@ 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, 1, virtio_balloon_receive_stats);
> >
> > +    s->change = qemu_add_vm_change_state_handler(balloon_vm_state_change, 
> > s);
> >      reset_stats(s);
> >  }
> >
> > @@ -441,6 +457,7 @@ static void virtio_balloon_device_unrealize(DeviceState 
> > *dev, Error **errp)
> >      VirtIODevice *vdev = VIRTIO_DEVICE(dev);
> >      VirtIOBalloon *s = VIRTIO_BALLOON(dev);
> >
> > +    qemu_del_vm_change_state_handler(s->change);
> >      balloon_stats_destroy_timer(s);
> >      qemu_remove_balloon_handler(s);
> >      virtio_cleanup(vdev);
> > diff --git a/include/hw/virtio/virtio-balloon.h 
> > b/include/hw/virtio/virtio-balloon.h
> > index 1ea13bd..d72ff7f 100644
> > --- a/include/hw/virtio/virtio-balloon.h
> > +++ b/include/hw/virtio/virtio-balloon.h
> > @@ -43,6 +43,7 @@ typedef struct VirtIOBalloon {
> >      int64_t stats_last_update;
> >      int64_t stats_poll_interval;
> >      uint32_t host_features;
> > +    VMChangeStateEntry *change;
> >  } VirtIOBalloon;
> >
> >  #endif
> > --
> > 2.7.4
> >
> >