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: <626dbe6541266f61e8b505202cf49c94c4fee12e@linux.dev>
Date: Fri, 30 Jan 2026 23:40:57 +0000
From: "Yosry Ahmed" <yosry.ahmed@...ux.dev>
To: "Jim Mattson" <jmattson@...gle.com>
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

January 30, 2026 at 3:30 PM, "Jim Mattson" <jmattson@...gle.com> wrote:

> 
> 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.

We could just call amd_pmu_refresh_host_guest_eventsel_hw() every time we call enter_guest_mode() and leave_guest_mode(), which is more error-prone but there's already other things in that category.

We could also call it from svm_switch_vmcb(), which will add some calls to extra places but I assume that would be fine?

I personally prefer the former tbh, as it's otherwise easy to miss.

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