Re: Exporting kvm_max_guest_tsc_khz to userspace (was Re: [PATCH 4/4] kvm: Allow migration with invtsc)

Eduardo Habkost
2017-01-11T13:26:55Z

On Mon, Jan 09, 2017 at 03:58:11PM +0100, Paolo Bonzini wrote:
>
>
> On 08/01/2017 21:28, Eduardo Habkost wrote:
> >> Well just export KVM_GET_TSC_KHZ in a QMP command right? Its pretty
> >> easy.
> >>
> >> Let me know if you need any help coding or testing.
> > I just found out that KVM doesn't provide something that QEMU and
> > libvirt need: the value of kvm_max_guest_tsc_khz. Without it, we
> > have no way to know if a given VM is really migratable to a host.
> >
> > Could we add a KVM_CAP_MAX_TSC_KHZ capability for that?
>
> The ratio is really quite high, 256x the host frequency for AMD and
> 65536x for Intel. Anything below 2^32 Hz (above that, there would
> probably be other failures) is safe.

2^32 Hz (~4.3 GHz) sounds like a limit likely to be hit by future
CPUs. Which kind of failures do you think we could see?

--
Eduardo