Re: [Qemu-devel] [PATCH] Add event notification for guest balloon changes

[Amit Shah]

On (Thu) 17 May 2012 [08:49:44], Daniel P. Berrange wrote:
> On Wed, May 16, 2012 at 01:58:34PM -0500, Anthony Liguori wrote:
> > On 05/16/2012 01:42 PM, Luiz Capitulino wrote:
> > >On Wed, 16 May 2012 11:10:47 +0100
> > >"Daniel P. Berrange"<address@hidden>  wrote:
> > >
> > >>From: "Daniel P. Berrange"<address@hidden>
> > >>
> > >>After setting a balloon target value, applications have to
> > >>continually poll 'query-balloon' to determine whether the
> > >>guest has reacted to this request. The virtio-balloon backend
> > >>knows exactly when the guest has reacted though, and thus it
> > >>is possible to emit a JSON event to tell the mgmt application
> > >>whenever the guest balloon changes.
> > >>
> > >>This introduces a new 'qemu_balloon_change()' API which is
> > >>to be called by balloon driver backends, whenever they have
> > >>a change in balloon value. This takes the 'actual' balloon
> > >>value, as would be found in the BalloonInfo struct.
> > >>
> > >>The qemu_balloon_change API emits a JSON monitor event which
> > >>looks like:
> > >>
> > >>   {"timestamp": {"seconds": 1337162462, "microseconds": 814521},
> > >>    "event": "BALLOON_CHANGE", "data": {"actual": 944766976}}
> > >
> > >It's missing an entry in QMP/qmp-events.txt and I have a comment below,
> > >but in general looks good.
> > >
> > >Amit, would be good to get your ack.
> > 
> > I think it would be safer to limit this event to (1) only firing
> > once target has been reached (2) firing if target is deviated from
> > without a corresponding change in target.
> > 
> > Otherwise, a guest could just flood libvirt with events.  This would
> > queue memory in QEMU indefinitely as the events got queued up to
> > potentially serving as a DoS against other guests.
> 
> Hmm, that's a good point, but my concern was that if we only emit
> the event when the target is reached, what happens if the guest
> gets very close to the target but never actually reaches it for
> some reason.
> 
> Should we perhaps just rate limit it to once per second ?

This also has a slight disadvantage of missing to send out a
notification if it was received within the 1-second window, but no
further events come from the guest.

Overall, the qmp emitter itself could gain a new API to rate-limit
events, if so configured.

                Amit