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

[Eduardo Habkost]

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.

Right, so it means invtsc migration without TSC scaling will be
possible in practice only if we tolerate a small variance on TSC
frequency on migration. The question is: should we? Can we
tolerate a 0.5% variance on TSC frequency and still expose invtsc
to the guest?

-- 
Eduardo