Message-ID: <CALMp9eRGj3rNeNN82H12f=XO5iXi5s2ri2K71CnjoVp9eKzz1w@mail.gmail.com>
Date: Thu, 22 Jan 2026 14:47:25 -0800
From: Jim Mattson <jmattson@...gle.com>
To: Sean Christopherson <seanjc@...gle.com>
Cc: Paolo Bonzini <pbonzini@...hat.com>, Thomas Gleixner <tglx@...utronix.de>, 
	Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>, 
	Dave Hansen <dave.hansen@...ux.intel.com>, x86@...nel.org, 
	"H. Peter Anvin" <hpa@...or.com>, Peter Zijlstra <peterz@...radead.org>, 
	Arnaldo Carvalho de Melo <acme@...nel.org>, Namhyung Kim <namhyung@...nel.org>, 
	Mark Rutland <mark.rutland@....com>, 
	Alexander Shishkin <alexander.shishkin@...ux.intel.com>, Jiri Olsa <jolsa@...nel.org>, 
	Ian Rogers <irogers@...gle.com>, Adrian Hunter <adrian.hunter@...el.com>, 
	James Clark <james.clark@...aro.org>, Shuah Khan <shuah@...nel.org>, kvm@...r.kernel.org, 
	linux-kernel@...r.kernel.org, linux-perf-users@...r.kernel.org, 
	linux-kselftest@...r.kernel.org
Subject: Re: [PATCH 2/6] KVM: x86/pmu: Disable HG_ONLY events as appropriate
 for current vCPU state

On Thu, Jan 22, 2026 at 8:33 AM Sean Christopherson <seanjc@...gle.com> wrote:
>
> On Wed, Jan 21, 2026, Jim Mattson wrote:
> > Introduce amd_pmu_dormant_hg_event(), which determines whether an AMD PMC
> > should be dormant (i.e. not count) based on the guest's Host-Only and
> > Guest-Only event selector bits and the current vCPU state.
> >
> > Update amd_pmu_set_eventsel_hw() to clear the event selector's enable bit
> > when the event is dormant.
> >
> > Signed-off-by: Jim Mattson <jmattson@...gle.com>
> > ---
> >  arch/x86/include/asm/perf_event.h |  2 ++
> >  arch/x86/kvm/svm/pmu.c            | 23 +++++++++++++++++++++++
> >  2 files changed, 25 insertions(+)
> >
> > diff --git a/arch/x86/include/asm/perf_event.h b/arch/x86/include/asm/perf_event.h
> > index 0d9af4135e0a..7649d79d91a6 100644
> > --- a/arch/x86/include/asm/perf_event.h
> > +++ b/arch/x86/include/asm/perf_event.h
> > @@ -58,6 +58,8 @@
> >  #define AMD64_EVENTSEL_INT_CORE_ENABLE                       (1ULL << 36)
> >  #define AMD64_EVENTSEL_GUESTONLY                     (1ULL << 40)
> >  #define AMD64_EVENTSEL_HOSTONLY                              (1ULL << 41)
> > +#define AMD64_EVENTSEL_HG_ONLY                               \
>
> I would strongly prefer to avoid the HG acronym, as it's not immediately obvious
> that it's HOST_GUEST, and avoiding long lines even with the full HOST_GUEST is
> pretty easy.

In this instance, I'm happy to make the suggested change, but I think
your overall distaste for HG_ONLY is unwarranted.
These bits are documented in the APM as:

> HG_ONLY (Host/Guest Only)—Bits 41:40, read/write

> The name should also have "MASK" at the end to make it more obvious this is a
> multi-flag macro, i.e. not a single-flag value.  Otherwise the intent and thus
> correctness of code like this isn't obvious:
>
>         if (eventsel & AMD64_EVENTSEL_HG_ONLY)
>
> How about AMD64_EVENTSEL_HOST_GUEST_MASK?

Sure.

> > +     (AMD64_EVENTSEL_HOSTONLY | AMD64_EVENTSEL_GUESTONLY)
> >
> >  #define AMD64_EVENTSEL_INT_CORE_SEL_SHIFT            37
> >  #define AMD64_EVENTSEL_INT_CORE_SEL_MASK             \
> > diff --git a/arch/x86/kvm/svm/pmu.c b/arch/x86/kvm/svm/pmu.c
> > index 33c139b23a9e..f619417557f9 100644
> > --- a/arch/x86/kvm/svm/pmu.c
> > +++ b/arch/x86/kvm/svm/pmu.c
> > @@ -147,10 +147,33 @@ static int amd_pmu_get_msr(struct kvm_vcpu *vcpu, struct msr_data *msr_info)
> >       return 1;
> >  }
> >
> > +static bool amd_pmu_dormant_hg_event(struct kvm_pmc *pmc)
>
> I think I would prefer to flip the polarity, even though the only caller would
> then need to invert the return value.  Partly because I think we can come up with
> a more intuitive name, partly because it'll make the last check in particular
> more intutive, i.e. IMO, checking "guest == guest"
>
>         return !!(hg_only & AMD64_EVENTSEL_GUESTONLY) == is_guest_mode(vcpu);
>
> is more obvious than checking "host == guest":
>
>         return !!(hg_only & AMD64_EVENTSEL_GUESTONLY) == is_guest_mode(vcpu);
>
> Maybe amd_pmc_is_active() or amd_pmc_counts_in_current_mode()?

I think amd_pmc_is_active() is a much stronger statement, implying
that both enable bits are also set.

Similarly, amd_pmc_counts_in_current_mode() sounds like it looks at
OS/USR bits as well.

I'll see if I can think of a better name that isn't misleading. I
actually went with this polarity because of the naming problem. But, I
agree that the reverse polarity is marginally better.

> > +{
> > +     u64 hg_only = pmc->eventsel & AMD64_EVENTSEL_HG_ONLY;
> > +     struct kvm_vcpu *vcpu = pmc->vcpu;
> > +
> > +     if (hg_only == 0)
>
> !hg_only

Now, you're just being petty. But, okay.

> In the spirit of avoiding the "hg" acronym, what if we do something like this?
>
>         const u64 HOST_GUEST_MASK = AMD64_EVENTSEL_HOST_GUEST_MASK;

Ugh. No. You can't both prefer the longer name and yet avoid it like
the plague. If you need to introduce a shorter alias, the longer name
is a bad choice.

>         struct kvm_vcpu *vcpu = pmc->vcpu;
>         u64 eventsel = pmc->eventsel;
>
>         /*
>          * PMCs count in both host and guest if neither {HOST,GUEST}_ONLY flags
>          * are set, or if both flags are set.
>          */
>         if (!(eventsel & HOST_GUEST_MASK) ||
>             ((eventsel & HOST_GUEST_MASK) == HOST_GUEST_MASK))
>                 return true;
>
>         /* {HOST,GUEST}_ONLY bits are ignored when SVME is clear. */
>         if (!(vcpu->arch.efer & EFER_SVME))
>                 return true;
>
>         return !!(eventsel & AMD64_EVENTSEL_GUESTONLY) == is_guest_mode(vcpu);
>
> > +             /* Not an HG_ONLY event */
>
> Please don't put comments inside single-line if-statements.  99% of the time
> it's easy to put the comment outside of the if-statement, and doing so encourages
> a more verbose comment and avoids a "does this if-statement need curly-braces"
> debate.

There is no debate. A comment is not a statement. But, okay.

> > +             return false;
> > +
> > +     if (!(vcpu->arch.efer & EFER_SVME))
> > +             /* HG_ONLY bits are ignored when SVME is clear */
> > +             return false;
> > +
> > +     /* Always active if both HG_ONLY bits are set */
> > +     if (hg_only == AMD64_EVENTSEL_HG_ONLY)
>
> I vote to check this condition at the same time !hg_only is checked.  From a
> *very* pedantic perspective, one could argue it's "wrong" to check the bits when
> SVME=0, but the purpose of the helper is to detect if the PMC is active or not.
> Precisely following the architectural behavior is unnecessary.

Even I am not that pedantic.

> > +             return false;
> > +
> > +     return !!(hg_only & AMD64_EVENTSEL_HOSTONLY) == is_guest_mode(vcpu);
> > +}
> > +
> >  static void amd_pmu_set_eventsel_hw(struct kvm_pmc *pmc)
> >  {
> >       pmc->eventsel_hw = (pmc->eventsel & ~AMD64_EVENTSEL_HOSTONLY) |
> >               AMD64_EVENTSEL_GUESTONLY;
> > +
> > +     if (amd_pmu_dormant_hg_event(pmc))
> > +             pmc->eventsel_hw &= ~ARCH_PERFMON_EVENTSEL_ENABLE;
> >  }
> >
> >  static int amd_pmu_set_msr(struct kvm_vcpu *vcpu, struct msr_data *msr_info)
> > --
> > 2.52.0.457.g6b5491de43-goog
> >

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