Re: KVM PMU virtualization

Joerg Roedel
2010-02-26T11:06:44Z

On Fri, Feb 26, 2010 at 11:46:59AM +0100, Ingo Molnar wrote:
>
> * Joerg Roedel <joro@8bytes.org> wrote:
>
> > On Fri, Feb 26, 2010 at 11:46:34AM +0200, Avi Kivity wrote:
> > > On 02/26/2010 10:42 AM, Ingo Molnar wrote:
> > >> Note that the 'soft PMU' still sucks from a design POV as there's no generic
> > >> hw interface to the PMU. So there would have to be a 'soft AMD' and a 'soft
> > >> Intel' PMU driver at minimum.
> > >>
> > >
> > > Right, this will severely limit migration domains to hosts of the same
> > > vendor and processor generation. There is a middle ground, though,
> > > Intel has recently moved to define an "architectural pmu" which is not
> > > model specific. I don't know if AMD adopted it. We could offer both
> > > options - native host capabilities, with a loss of compatibility, and
> > > the architectural pmu, with loss of model specific counters.
> >
> > I only had a quick look yet on the architectural pmu from intel but it looks
> > like it can be emulated for a guest on amd using existing features.
>
> AMD CPUs dont have enough events for that, they cannot do the 3 fixed events
> in addition to the 2 generic ones.

Good point. Maybe we can emulate that with some counter round-robin
usage if the guest really uses all 5 counters.

> Nor do you really want to standardize on KVM guests on returning
> 'GenuineIntel' in CPUID, so that the various guest side OSs use the Intel PMU
> drivers, right?

Isn't there a cpuid bit indicating the availability of architectural
perfmon?

Joerg