Re: [PATCH] KVM: x86: vPMU: truncate counter value to allowed width

[Roman Kagan <rkagan@amazon.de>]

On Fri, Jun 30, 2023 at 08:45:04AM -0700, Sean Christopherson wrote:
> On Fri, Jun 30, 2023, Roman Kagan wrote:
> > On Fri, Jun 30, 2023 at 07:28:29AM -0700, Sean Christopherson wrote:
> > > On Fri, Jun 30, 2023, Roman Kagan wrote:
> > > > On Thu, Jun 29, 2023 at 05:11:06PM -0700, Sean Christopherson wrote:
> > > > > @@ -74,6 +74,14 @@ static inline u64 pmc_read_counter(struct kvm_pmc *pmc)
> > > > >         return counter & pmc_bitmask(pmc);
> > > > >  }
> > > > >
> > > > > +static inline void pmc_write_counter(struct kvm_pmc *pmc, u64 val)
> > > > > +{
> > > > > +       if (pmc->perf_event && !pmc->is_paused)
> > > > > +               perf_event_set_count(pmc->perf_event, val);
> > > > > +
> > > > > +       pmc->counter = val;
> > > >
> > > > Doesn't this still have the original problem of storing wider value than
> > > > allowed?
> > >
> > > Yes, this was just to fix the counter offset weirdness.  My plan is to apply your
> > > patch on top.  Sorry for not making that clear.
> >
> > Ah, got it, thanks!
> >
> > Also I'm now chasing a problem that we occasionally see
> >
> > [3939579.462832] Uhhuh. NMI received for unknown reason 30 on CPU 43.
> > [3939579.462836] Do you have a strange power saving mode enabled?
> > [3939579.462836] Dazed and confused, but trying to continue
> >
> > in the guests when perf is used.  These messages disappear when
> > 9cd803d496e7 ("KVM: x86: Update vPMCs when retiring instructions") is
> > reverted.  I haven't yet figured out where exactly the culprit is.
> 
> Can you reverting de0f619564f4 ("KVM: x86/pmu: Defer counter emulated overflow
> via pmc->prev_counter")?  I suspect the problem is the prev_counter mess.

We observe the problem on a branch that predates this commit

Thanks,
Roman.



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