Re: [PATCH v4 3/6] KVM: x86/pmu: Disable counters based on Host-Only/Guest-Only bits in SVM

[Yosry Ahmed]

On Mon, Apr 27, 2026 at 4:53 PM Sean Christopherson <seanjc@xxxxxxxxxx> wrote:
>
> On Mon, Apr 27, 2026, Yosry Ahmed wrote:
> > > We can have our cake and eat it too.  Add svm_pmu_handle_nested_transition(),
> > > but then also rename and rework reprogram_counters() to support both deferred and
> > > synchronous operation, e.g. something like so:
> > >
> > > ---
> > > static inline void __kvm_pmu_reprogram_counters(struct kvm_pmu *pmu, u64 diff,
> >
> > I don't like 'diff', I think just 'unsigned long *bitmap' and pass a
>
> Hard no.  I agree @diff is a weird name (I was literally just copy+pasting the
> existing code), but I _really_ don't like passing a pointer, especially not to
> an unsigned long.  The bitmap usage throughout the PMU code is mostly internal
> implementation details.  But for what is reprogram_counters(), and what will be
> __kvm_pmu_reprogram_counters(), the "counters to reprogram" is very tightly
> coupled to the architectural layout of PERF_GLOBAL_CTRL and PEBS_ENABLED.  And
> more broadly in the PMU, for the layout of GLOBAL_STATUS_BUFFER_OVF_BIT,
> MSR_CORE_PERF_GLOBAL_STATUS, and probably at least one other MSRs.  That all
> should be captured in the APIs.
>
> The other reason I don't want to pass a pointer is so that even when the source
> _is_ a PMU-internal bitmap, it's super duper obvious that the source bitmap isn't
> modified, and that it's operating on a snapshot in time.

Sounds good, I will keep u64 (and maybe name it 'counters'), or maybe
s64 as that's what kvm_pmu_handle_event() uses (and what the atomic
helpers take).

>
> > bitmap in here like most PMU code?
>
> FWIW, AFAICT, passing a bitmap as a function argument isn't common at all.  I
> only see kvm_for_each_pmc() and kvm_pmu_trigger_event() taking a bitmap.  There
> is a lot of bitmap _usage_, but rarely does KVM pass around a bitmap as a function
> argument.

We still need to pass a bitmap as an argument to the
reprogram_counters() callback that we'll add in
kvm_pmu_handle_event(), or would you rather we cast the bitmap to s64
here as well?