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

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.