Re: Questing regarding KVM Guest PMU

[Gleb Natapov <gleb@redhat.com>]

On Thu, Apr 05, 2012 at 03:27:18PM +0300, Avi Kivity wrote:
> On 04/04/2012 01:29 PM, Gleb Natapov wrote:
> > > >
> > > ok. seems to be. will move over to perf as its working fine inside guest.
> > > 
> > Good riddance IMO. I managed to run it on a guest (but not on my
> > host!). The thing is buggy. It does not use global ctrl MSR to enable
> > counters and kvm has all of them disabled by default. I didn't find what
> > value this MSR should have after reset, so this may be either kvm bug or
> > real BIOSes enable all counters in global ctrl MSR for PMUv1
> > compatibility. Doing "wrmsr 0x38f 0x70000000f" solves this problem. The
> > second problem is that oprofile reprogram PMU counters without
> > disabling them first and this is explicitly prohibited by Intel SDM.
> > The patch below solve that, but oprofile is the one who should be fixed.
> 
> Both should be fixed, there may be other profilers affected.
> 
Global ctrl msr issue I need to investigate further, but second one is
bug that should be fixed in oprofile. Intel spec clearly says:

 EN (Enable Counters) Flag (bit 22) — When set, performance counting
 is enabled in the corresponding performance-monitoring counter; when
 clear, the corresponding counter is disabled. The event logic unit
 for a UMASK must be disabled by setting IA32_PERFEVTSELx[bit 22] = 0,
 before writing to IA32_PMCx.

I suspect that on real HW they got wrong result too. It simply subtly
wrong, so it is not as noticeable as with kvm.

> >
> > diff --git a/arch/x86/kvm/pmu.c b/arch/x86/kvm/pmu.c
> > index a73f0c1..be05028 100644
> > --- a/arch/x86/kvm/pmu.c
> > +++ b/arch/x86/kvm/pmu.c
> > @@ -396,6 +396,7 @@ int kvm_pmu_set_msr(struct kvm_vcpu *vcpu, u32 index, u64 data)
> >  				(pmc = get_fixed_pmc(pmu, index))) {
> >  			data = (s64)(s32)data;
> >  			pmc->counter += data - read_pmc(pmc);
> > +			reprogram_gp_counter(pmc, pmc->eventsel);
> >  			return 0;
> >  		} else if ((pmc = get_gp_pmc(pmu, index, MSR_P6_EVNTSEL0))) {
> >  			if (data == pmc->eventsel)
> >
> 
> -- 
> error compiling committee.c: too many arguments to function

--
			Gleb.