From 91dcfae0ff2b9b9ab03c1ec95babaceefbffb9f4 Mon Sep 17 00:00:00 2001 From: Fernand Sieber Date: Thu, 11 Dec 2025 20:36:04 +0200 Subject: [PATCH] perf/x86/intel: Do not enable BTS for guests MIME-Version: 1.0 Content-Type: text/plain; charset=utf8 Content-Transfer-Encoding: 8bit By default when users program perf to sample branch instructions (PERF_COUNT_HW_BRANCH_INSTRUCTIONS) with a sample period of 1, perf interprets this as a special case and enables BTS (Branch Trace Store) as an optimization to avoid taking an interrupt on every branch. Since BTS doesn't virtualize, this optimization doesn't make sense when the request originates from a guest. Add an additional check that prevents this optimization for virtualized events (exclude_host). Reported-by: Jan H. Schönherr Suggested-by: Peter Zijlstra Signed-off-by: Fernand Sieber Signed-off-by: Peter Zijlstra (Intel) Cc: Link: https://patch.msgid.link/20251211183604.868641-1-sieberf@amazon.com --- arch/x86/events/perf_event.h | 13 +++++++++++-- 1 file changed, 11 insertions(+), 2 deletions(-) diff --git a/arch/x86/events/perf_event.h b/arch/x86/events/perf_event.h index 62963022b517..ad35c546243e 100644 --- a/arch/x86/events/perf_event.h +++ b/arch/x86/events/perf_event.h @@ -1574,13 +1574,22 @@ 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) -- 2.47.3