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