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

[Eduardo Habkost <ehabkost@redhat.com>]

On Wed, Jan 18, 2017 at 09:55:43AM -0200, Marcelo Tosatti wrote:
> On Tue, Jan 10, 2017 at 05:36:48PM +0100, Paolo Bonzini wrote:
> > 
> > 
> > On 05/01/2017 11:48, Marcelo Tosatti wrote:
> > >> 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.
> > 
> > I don't think this is a scenario that can work reliably.  The computed
> > TSC frequency may vary by 0.5% or so on every boot (e.g. you may get
> > 2497000 kHz or 2511000 kHz for a 2.5 GHz TSC).  You can start the VM on
> > host A, reboot host B, and then you'll be unable to migrate.
> > 
> > Paolo
> 
> Including an acceptable error in the comparison seems to be
> acceptable to work around that case.

How much error is acceptable when exposing the invtsc flag to the
guest?

-- 
Eduardo