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

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Date:   Fri, 30 Jun 2023 09:40:37 -0700
From:   Jim Mattson <jmattson@...gle.com>
To:     Roman Kagan <rkagan@...zon.de>,
        Sean Christopherson <seanjc@...gle.com>,
        Jim Mattson <jmattson@...gle.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Eric Hankland <ehankland@...gle.com>, kvm@...r.kernel.org,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Like Xu <likexu@...cent.com>, x86@...nel.org,
        Thomas Gleixner <tglx@...utronix.de>,
        linux-kernel@...r.kernel.org, "H. Peter Anvin" <hpa@...or.com>,
        Borislav Petkov <bp@...en8.de>, Ingo Molnar <mingo@...hat.com>,
        Mingwei Zhang <mizhang@...gle.com>
Subject: Re: [PATCH] KVM: x86: vPMU: truncate counter value to allowed width

On Fri, Jun 30, 2023 at 8:21 AM Roman Kagan <rkagan@...zon.de> 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.

Maybe this is because KVM doesn't virtualize
IA32_DEBUGCTL.Freeze_PerfMon_On_PMI?

Consider:

1. PMC0 overflows, GLOBAL_STATUS[0] is set, and an NMI is delivered.
2. Before the guest's PMI handler clears GLOBAL_CTRL, PMC1 overflows,
GLOBAL_STATUS[1] is set, and an NMI is queued for delivery after the
next IRET.
3. The guest's PMI handler clears GLOBAL_CTRL, reads 3 from
GLOBAL_STATUS, writes 3 to GLOBAL_OVF_CTRL, re-enables GLOBAL_CTRL,
and IRETs.
4. The queued NMI is delivered, but GLOBAL_STATUS is now 0. No one
claims the NMI, so we get the spurious NMI message.

I don't know why this would require counting the retirement of
emulated instructions. It seems that hardware PMC overflow in the
early part of the guest's PMI handler would also be a problem.

> Thanks,
> Roman.
>
>
>
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