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

[Anthony Liguori]

On 05/17/2012 07:56 AM, Luiz Capitulino wrote:
> On Thu, 17 May 2012 08:49:44 +0100
> "Daniel P. Berrange"<address@hidden>  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.
>
> Having a way to detect the last balloon change would be perfect.

libvirt certainly would have to maintain a timeout and make a decision on what 
to do if the guest doesn't balloon to target.  Not sure how having events help 
at all here.

> > Should we perhaps just rate limit it to once per second ?
> >
> > BTW, if we're considering guest initiated events to be a potential
> > DOS in this way, then I should point out the RTC_CHANGE event
> > will already suffer this way, if a malicious guest continually
> > adjusts its hardware close. So we might want to apply rate limiting
> > to that event too ?
>
> I think several events can suffer from that. For example, a VNC
> client could repeatedly connect&  disconnect from QEMU. If we're going
> to fix this, then we'd need a general solution for it.

No, VNC clients are a whole different ballgame.  VNC connections will only 
happen from the management network, we don't worry about memory allocation from 
malicious VNC clients.

Regards,

Anthony Liguori

> But I think the balloon case is different, because we're not fighting
> malicious guests/clients, it's really the balloon operation that can
> cause the flood.