Re: [PATCH] KVM: x86/pmu: Do not accidentally create BTS events

Peter Zijlstra
2025-12-02T12:44:32Z

On Tue, Dec 02, 2025 at 11:03:11AM +0100, Peter Zijlstra wrote:
> On Mon, Dec 01, 2025 at 04:23:57PM +0200, Fernand Sieber wrote:
> > arch/x86/kvm/pmu.c | 13 +++++++++++++
> > 1 file changed, 13 insertions(+)
> >
> > diff --git a/arch/x86/kvm/pmu.c b/arch/x86/kvm/pmu.c
> > index 487ad19a236e..547512028e24 100644
> > --- a/arch/x86/kvm/pmu.c
> > +++ b/arch/x86/kvm/pmu.c
> > @@ -225,6 +225,19 @@ static u64 get_sample_period(struct kvm_pmc *pmc, u64 counter_value)
> > {
> > u64 sample_period = (-counter_value) & pmc_bitmask(pmc);
> >
> > + /*
> > + * A sample_period of 1 might get mistaken by perf for a BTS event, see
> > + * intel_pmu_has_bts_period(). This would prevent re-arming the counter
> > + * via pmc_resume_counter(), followed by the accidental creation of an
> > + * actual BTS event, which we do not want.
> > + *
> > + * Avoid this by bumping the sampling period. Note, that we do not lose
> > + * any precision, because the same quirk happens later anyway (for
> > + * different reasons) in x86_perf_event_set_period().
> > + */
> > + if (sample_period == 1)
> > + sample_period = 2;
> > +
> > if (!sample_period)
> > sample_period = pmc_bitmask(pmc) + 1;
> > return sample_period;
>
> Oh gawd, I so hate this kvm code. It is so ludicrously bad. The way it
> keeps recreating counters is just stupid. And then they complain it
> sucks, it does :-(
>
> Anyway, yes this is terrible. Let me try and untangle all this, see if
> there's a saner solution.

Does something like so work? It is still terrible, but perhaps slightly
less so.

diff --git a/arch/x86/events/perf_event.h b/arch/x86/events/perf_event.h
index 2b969386dcdd..493e6ba51e06 100644
--- a/arch/x86/events/perf_event.h
+++ b/arch/x86/events/perf_event.h
@@ -1558,13 +1558,22 @@ static inline bool intel_pmu_has_bts_period(struct perf_event *event, u64 period
struct hw_perf_event *hwc = &event->hw;
unsigned int hw_event, bts_event;

- if (event->attr.freq)
+ /*
+ * Only use BTS for fixed rate period==1 events.
+ */
+ if (event->attr.freq || period != 1)
+ return false;
+
+ /*
+ * BTS doesn't virtualize.
+ */
+ if (event->attr.exclude_host)
return false;

hw_event = hwc->config & INTEL_ARCH_EVENT_MASK;
bts_event = x86_pmu.event_map(PERF_COUNT_HW_BRANCH_INSTRUCTIONS);

- return hw_event == bts_event && period == 1;
+ return hw_event == bts_event;
}

static inline bool intel_pmu_has_bts(struct perf_event *event)