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

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).