linux-kernel - Re: [PATCH v2 3/5] KVM: x86/pmu: Refresh Host-Only/Guest-Only eventsel at nested transitions

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Message-ID: <CALMp9eS7Za_vFdh8YBzycV2g87gZ9uj_S1MOYrgJ1+ShwVVWZw@mail.gmail.com>
Date: Fri, 30 Jan 2026 15:30:40 -0800
From: Jim Mattson <jmattson@...gle.com>
To: Yosry Ahmed <yosry.ahmed@...ux.dev>
Cc: Peter Zijlstra <peterz@...radead.org>, Ingo Molnar <mingo@...hat.com>, 
	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>, Thomas Gleixner <tglx@...nel.org>, Borislav Petkov <bp@...en8.de>, 
	Dave Hansen <dave.hansen@...ux.intel.com>, x86@...nel.org, 
	"H. Peter Anvin" <hpa@...or.com>, Sean Christopherson <seanjc@...gle.com>, Paolo Bonzini <pbonzini@...hat.com>, 
	Shuah Khan <shuah@...nel.org>, linux-perf-users@...r.kernel.org, 
	linux-kernel@...r.kernel.org, kvm@...r.kernel.org, 
	linux-kselftest@...r.kernel.org, mizhang@...gle.com, sandipan.das@....com
Subject: Re: [PATCH v2 3/5] KVM: x86/pmu: Refresh Host-Only/Guest-Only
 eventsel at nested transitions

On Fri, Jan 30, 2026 at 7:26 AM Yosry Ahmed <yosry.ahmed@...ux.dev> wrote:
>
> On Thu, Jan 29, 2026 at 03:28:08PM -0800, Jim Mattson wrote:
> > Add amd_pmu_refresh_host_guest_eventsel_hw() to recalculate eventsel_hw for
> > all PMCs based on the current vCPU state. This is needed because Host-Only
> > and Guest-Only counters must be enabled/disabled at:
> >
> >   - SVME changes: When EFER.SVME is modified, counters with Guest-Only bits
> >     need their hardware enable state updated.
> >
> >   - Nested transitions: When entering or leaving guest mode, Host-Only
> >     counters should be disabled/enabled and Guest-Only counters should be
> >     enabled/disabled accordingly.
> >
> > Introduce svm_enter_guest_mode() and svm_leave_guest_mode() wrappers that
> > call enter_guest_mode()/leave_guest_mode() followed by the PMU refresh,
> > ensuring the PMU state stays synchronized with guest mode transitions.
> >
> > Signed-off-by: Jim Mattson <jmattson@...gle.com>
> > ---
> >  arch/x86/kvm/svm/nested.c |  6 +++---
> >  arch/x86/kvm/svm/pmu.c    | 12 ++++++++++++
> >  arch/x86/kvm/svm/svm.c    |  2 ++
> >  arch/x86/kvm/svm/svm.h    | 17 +++++++++++++++++
> >  4 files changed, 34 insertions(+), 3 deletions(-)
> >
> > diff --git a/arch/x86/kvm/svm/nested.c b/arch/x86/kvm/svm/nested.c
> > index de90b104a0dd..a7d1901f256b 100644
> > --- a/arch/x86/kvm/svm/nested.c
> > +++ b/arch/x86/kvm/svm/nested.c
> > @@ -757,7 +757,7 @@ static void nested_vmcb02_prepare_control(struct vcpu_svm *svm,
> >       nested_svm_transition_tlb_flush(vcpu);
> >
> >       /* Enter Guest-Mode */
> > -     enter_guest_mode(vcpu);
> > +     svm_enter_guest_mode(vcpu);
>
> FWIW, I think this name is a bit confusing because we also have
> enter_svm_guest_mode(). So we end up with:
>
> enter_svm_guest_mode() -> nested_vmcb02_prepare_control() ->
> svm_enter_guest_mode() -> enter_guest_mode()
>
> I actually have another proposed change [1] that moves
> enter_guest_mode() directly into enter_svm_guest_mode(), so the sequence
> would end up being:
>
> enter_svm_guest_mode() -> svm_enter_guest_mode() -> enter_guest_mode()

Yes, that is confusing. What if I renamed the existing function to
something like svm_nested_switch_to_vmcb02()?

Alternatively, I could go back to introducing a new PMU_OP, call it
from {enter,leave}_guest_mode(), and drop the wrappers.

> [1] https://lore.kernel.org/kvm/20260115011312.3675857-9-yosry.ahmed@linux.dev/