[PATCH 5.10 106/161] perf/x86/intel: Do not enable BTS for guests

Greg Kroah-Hartman
2026-02-04T14:50:14Z

From: Fernand Sieber <sieberf@amazon.com>

commit 91dcfae0ff2b9b9ab03c1ec95babaceefbffb9f4 upstream.

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
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
arch/x86/events/perf_event.h | 13 +++++++++++--
1 file changed, 11 insertions(+), 2 deletions(-)

--- a/arch/x86/events/perf_event.h
+++ b/arch/x86/events/perf_event.h
@@ -1088,13 +1088,22 @@ static inline bool intel_pmu_has_bts_per
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)