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

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commit 91dcfae0ff 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>
This commit is contained in:
Fernand Sieber
2026-01-30 10:27:40 +01:00
committed by Greg Kroah-Hartman
parent cad7003d95
commit dd40d94ffd
1 changed files with 11 additions and 2 deletions
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arch/x86/events/perf_event.h
+11 -2
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@@ -1423,13 +1423,22 @@ 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)