Re: Performance monitoring units and KVM

Andi Kleen
2008-01-31T03:12:32Z

On Thu, Jan 31, 2008 at 12:44:10AM +0530, Balaji Rao wrote:
> On Wednesday 30 January 2008 11:56:25 pm Andi Kleen wrote:
> > There is no really an architectural PMU if you consider
> > boxes beyond relatively new Intel CPUs (which got one)
> >
> But since kvm runs only on such CPUs, it should not really be a problem in
> migrating between various Intel models at least.

I'm not 100% sure, but I think there are P4 based (Family 15) CPUs which have
VT but not ArchPerfMon. AFAIK ArchPerfMon is only in Family 6 CPUs.
Family 15 has a completely different PerfMon interface.

> > But on the other hand in my experience most PMU users use
> > relatively simple few counters (e.g. 90+% likely the local
> > variant of CPU_CYCLES_NONHALTED) so it would be in theory
> > possible to translate those by traps from a different CPU's
> > format in the monitor into the local MSR.
> >
> > The only trouble is that is no architectural way to tell
> > the guest "i support only counter X Y Z" and also no
> > nice way to reject a particular counter except for just
> > not ticking.
> Can't this be exported through CPUID ?

Sure it could, but that would be a new interface. If you were
free to define a new interface you could also just go completely
hypercall based.

-Andi