Re: [Qemu-devel] [PATCH 0/3] re-enable balloon stats

[Luiz Capitulino]

On Tue, 11 Dec 2012 11:38:08 -0600
mdroth <address@hidden> wrote:

> On Tue, Dec 11, 2012 at 03:14:02PM +0000, Dietmar Maurer wrote:
> > > > > I'm not sure I like this for two reasons. First, there will be cases
> > > > > where the user doesn't want this to be enabled. Second, we'll be
> > > > > forcing an interval on users.
> > > >
> > > > So when should we set the stats-polling-interval? I first thought I
> > > > simply set that at VM start, but that triggers an error:
> > > >
> > > > "guest doesn't support balloon stats"
> > > >
> > > > because the balloon driver is not loaded.
> > > 
> > > Yes, it's required to have the balloon driver loaded. The stats are 
> > > reported by
> > > it.
> > 
> > That does not really answers my question. So you think the management
> > framework should start the VM, and then wait until the balloon driver is
> > loaded? That sound very clumsy to me.
> > 
> > I mean, I just want to set the polling interval. Why does that need the 
> > balloon drive loaded 
> > inside the guest (polling is done by the host)?
> 
> I agree. Should lack of a balloon module disable the timer
> completely, or just silently fail? Management can always reference
> stats-last-update to check that stats are being reported properly. -1 would
> mean driver was never loaded, longer update intervals might mean guest
> was rebooted/suspended/etc, but in both cases it makes sense that the
> timer still try to fetch stats, and the determination of what stats are
> valid/invalid be left to management.

I could move the check for the stats feature bit from the function that
enables polling to the timer callback. This would solve Dietmar's use-case,
but it would silently fail if the feature is never negotiated (as you said
above).

> Alternatively, we can hook into the feature negotiation path and
> defer the actual timer start until the required feature is negotiated,

That's also possible, and helps not wasting resources.

> and disable it (temporarilly or permanently based on guest behavior)
> somewhere in the reset path). I'm not sure this will keep it from
> running during hibernate/sleep...maybe hook into runstate changes?

Then it starts to get somewhat complicated.