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

[Mingwei Zhang]

On Fri, Jun 30, 2023 at 10:16 AM Jim Mattson <jmattson@xxxxxxxxxx> wrote:
>
> On Fri, Jun 30, 2023 at 10:08 AM Mingwei Zhang <mizhang@xxxxxxxxxx> wrote:
> >
> > On Fri, Jun 30, 2023 at 8:45 AM Sean Christopherson <seanjc@xxxxxxxxxx> 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.
> >
> > For sure it is prev_counter issue, I have done some instrumentation as follows:
> >
> > diff --git a/arch/x86/kvm/pmu.c b/arch/x86/kvm/pmu.c
> > index 48a0528080ab..946663a42326 100644
> > --- a/arch/x86/kvm/pmu.c
> > +++ b/arch/x86/kvm/pmu.c
> > @@ -322,8 +322,11 @@ static void reprogram_counter(struct kvm_pmc *pmc)
> >         if (!pmc_event_is_allowed(pmc))
> >                 goto reprogram_complete;
> >
> > -       if (pmc->counter < pmc->prev_counter)
> > +       if (pmc->counter < pmc->prev_counter) {
> > +               pr_info("pmc->counter: %llx\tpmc->prev_counter: %llx\n",
> > +                       pmc->counter, pmc->prev_counter);
> >                 __kvm_perf_overflow(pmc, false);
> > +       }
> >
> >         if (eventsel & ARCH_PERFMON_EVENTSEL_PIN_CONTROL)
> >                 printk_once("kvm pmu: pin control bit is ignored\n");
> >
> > I find some interesting changes on prev_counter:
> >
> > [  +7.295348] pmc->counter: 12 pmc->prev_counter: fffffffffb3d
> > [  +0.622991] pmc->counter: 3 pmc->prev_counter: fffffffffb1a
> > [  +6.943282] pmc->counter: 1 pmc->prev_counter: fffffffff746
> > [  +4.483523] pmc->counter: 0 pmc->prev_counter: ffffffffffff
> > [ +12.817772] pmc->counter: 0 pmc->prev_counter: ffffffffffff
> > [ +21.721233] pmc->counter: 0 pmc->prev_counter: ffffffffffff
> >
> > The first 3 logs will generate this:
> >
> > [ +11.811925] Uhhuh. NMI received for unknown reason 20 on CPU 2.
> > [  +0.000003] Dazed and confused, but trying to continue
> >
> > While the last 3 logs won't. This is quite reasonable as looking into
> > de0f619564f4 ("KVM: x86/pmu: Defer counter emulated overflow via
> > pmc->prev_counter"), counter and prev_counter should only have 1 diff
> > in value.
>
> prev_counter isn't actually sync'ed at this point, is it? This comes
> back to that "setting a running counter" nonsense. We want to add 1 to
> the current counter, but we don't actually know what the current
> counter is.
>
> My interpretation of the above is that, in the first three cases, PMU
> hardware has already detected an overflow. In the last three cases,
> software counting has detected an overflow.
>
> If the last three occur while executing the guest's PMI handler (i.e.
> NMIs are blocked), then this could corroborate my conjecture about
> IA32_DEBUGCTL.Freeze_PerfMon_On_PMI.
>

I see. I wonder if we can just do this:

diff --git a/arch/x86/kvm/pmu.c b/arch/x86/kvm/pmu.c
index 48a0528080ab..8d28158e58f2 100644
--- a/arch/x86/kvm/pmu.c
+++ b/arch/x86/kvm/pmu.c
@@ -322,7 +322,7 @@ static void reprogram_counter(struct kvm_pmc *pmc)
        if (!pmc_event_is_allowed(pmc))
                goto reprogram_complete;

-       if (pmc->counter < pmc->prev_counter)
+       if (pmc->counter == 0)
                __kvm_perf_overflow(pmc, false);

        if (eventsel & ARCH_PERFMON_EVENTSEL_PIN_CONTROL)

Since this is software emulation, we (KVM) should only handle overflow
by plusing one?