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