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

[Daniel P. Berrange]

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 ?

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 ?


Daniel
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