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

[Peter Zijlstra <peterz@infradead.org>]

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)