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

[Li, Liang Z]

> 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?
> 
Could you elaborate how it happens?  
The added code only works for the situation just after live migration. right?

Thanks!
Liang