Re: [PATCH v2] perf/x86/intel: Do not enable BTS for guests

[David Woodhouse]

On Thu, 2025-12-11 at 20:36 +0200, Fernand Sieber wrote:
> 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@xxxxxxxxx>
> Suggested-by: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
> Signed-off-by: Fernand Sieber <sieberf@xxxxxxxxxx>

Did this get applied?

I think you want Cc: stable@xxxxxxxxxxxxxxx here in the body of the
commit itself, not just on the actual email.

> ---
>  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 3161ec0a3416..f2e2d9b03367 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)

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