Re: [libvirt] TSC frequency configuration & invtsc migration (was Re: [PATCH 4/4] kvm: Allow migration with invtsc)

[Eduardo Habkost <ehabkost@redhat.com>]

On Thu, Jan 05, 2017 at 12:33:51PM +0000, Daniel P. Berrange wrote:
> On Thu, Jan 05, 2017 at 10:19:50AM -0200, Eduardo Habkost wrote:
> > On Thu, Jan 05, 2017 at 08:48:32AM -0200, Marcelo Tosatti wrote:
> > > On Wed, Jan 04, 2017 at 11:36:31PM -0200, Eduardo Habkost wrote:
[...]
> > > > Scenario 2:
> > > > 
> > > > Host A has TSC scaling, host B doesn't have TSC scaling. We want
> > > > to be able to start the VM on host A, and migrate to B. In this
> > > > case, the only possible solution is to use B's frequency when
> > > > starting the VM. The QEMU process doesn't have enough information
> > > > to make that decision.
> > > 
> > > That is a good point. But again, its a special case and 
> > > should be supported by -cpu xxx,tsc-frequency=zzzz.
> > > 
> > > However, for the vast majority of 99.999% cases, the issue
> > > can be handled entirely in QEMU, without libvirt involvement,
> > > and without adding extra steps to the management software.
> > 
> > I agree it should cover most cases. The only problem here is that
> > it can break migration in unexpected ways.
> > 
> > Then my point is: assuming that libvirt will prefer to require
> > explicit TSC frequency configuration to enable invtsc migration
> > (instead of getting unpredictable migration compatibility), is
> > the added complexity to migration code worth the effort, if
> > choosing an explicit frequency is safer and more predictable? I
> > believe this is where we disagree.
> 
> I believe that if libvirt detects that QEMU supports the 'tsc-frequency'
> option, then libvirt should set it by default in the XML, if not already
> set by the mgmt app. That way, libvirt can validate TSC freq comapt
> for migration before it even launches QEMU in the target host.

If you do this unconditionally, you have another problem: if
tsc-frequency is set explicitly, migration is only possible if
TSC frequency of the destination matches[1], or if TSC scaling is
supported by the destination. It's a good idea to set a TSC
frequency only if invtsc is enabled explicitly in the config.

[1] Currently the frequency needs to match exactly. That's a
separate issue: we should probably add a knob to allow a slight
variation in TSC frequency (e.g. <1% difference).

-- 
Eduardo