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

Peter Zijlstra
2025-12-02T10:03:18Z

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.