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

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