Re: Questing regarding KVM Guest PMU

[Avi Kivity <avi@redhat.com>]

On 04/05/2012 03:37 PM, Gleb Natapov wrote:
> 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.
>

First, there is the standard Linus rant to never break userspace (and
for us a guest kernel is userspace).  Second, the Intel rule may have
been added later (as preparation for a change in behaviour), so it may
actually work correctly on older hardware.

-- 
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