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

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.