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 -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
