[tip: perf/urgent] perf/x86/intel: Do not enable BTS for guests

["tip-bot2 for Fernand Sieber" <tip-bot2@linutronix.de>]

The following commit has been merged into the perf/urgent branch of tip:

Commit-ID:     91dcfae0ff2b9b9ab03c1ec95babaceefbffb9f4
Gitweb:        https://git.kernel.org/tip/91dcfae0ff2b9b9ab03c1ec95babaceefbffb9f4
Author:        Fernand Sieber <sieberf@amazon.com>
AuthorDate:    Thu, 11 Dec 2025 20:36:04 +02:00
Committer:     Peter Zijlstra <peterz@infradead.org>
CommitterDate: Wed, 21 Jan 2026 16:28:59 +01:00

perf/x86/intel: Do not enable BTS for guests

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 <jschoenh@amazon.de>
Suggested-by: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Fernand Sieber <sieberf@amazon.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: <stable@vger.kernel.org>
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 6296302..ad35c54 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)