Re: [Patch v2 2/2] perf/x86: Update cap_user_rdpmc base on rdpmc user disable state

["Chen, Zide" <zide.chen@intel.com>]

On 3/15/2026 10:08 PM, Dapeng Mi wrote:
> After introducing the RDPMC user disable feature, user-space RDPMC may
> return 0 instead of the actual event count. This creates an inconsistency
> with cap_user_rdpmc, where cap_user_rdpmc is set, but user-space RDPMC
> only returns 0.
> 
> To accurately represent the user-space RDPMC capability, update
> cap_user_rdpmc based on the RDPMC user disable state. If RDPMC user
> disable is enabled, cap_user_rdpmc is set to false, allowing user-space
> programs to fall back to the read() syscall to obtain the real event
> count.
> 
> Since arch_perf_update_userpage() could be called for software events,
> enhance x86_pmu_has_rdpmc_user_disable() to only check the x86 PMUs.
> 
> Fixes: 59af95e028d4 ("perf/x86/intel: Add support for rdpmc user disable feature")
> Signed-off-by: Dapeng Mi <dapeng1.mi@linux.intel.com>
> ---


Reviewed-by: Zide Chen <zide.chen@intel.com>


>  
> v2: Add is_x86_pmu() check before checking if rdpmc user disable feature
> is supported.
> 
> v1: https://lore.kernel.org/all/20260311075201.2951073-2-dapeng1.mi@linux.intel.com/
> 
>  arch/x86/events/core.c       | 3 +++
>  arch/x86/events/perf_event.h | 5 +++--
>  2 files changed, 6 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/x86/events/core.c b/arch/x86/events/core.c
> index 66b1a873c395..34eda8813716 100644
> --- a/arch/x86/events/core.c
> +++ b/arch/x86/events/core.c
> @@ -2794,6 +2794,9 @@ void arch_perf_update_userpage(struct perf_event *event,
>  	userpg->cap_user_time_zero = 0;
>  	userpg->cap_user_rdpmc =
>  		!!(event->hw.flags & PERF_EVENT_FLAG_USER_READ_CNT);
> +	if (x86_pmu_has_rdpmc_user_disable(event->pmu) &&
> +	    event->hw.config & ARCH_PERFMON_EVENTSEL_RDPMC_USER_DISABLE)
> +		userpg->cap_user_rdpmc = 0;
>  	userpg->pmc_width = x86_pmu.cntval_bits;
>  
>  	if (!using_native_sched_clock() || !sched_clock_stable())
> diff --git a/arch/x86/events/perf_event.h b/arch/x86/events/perf_event.h
> index 025f67726ecc..307361b106d2 100644
> --- a/arch/x86/events/perf_event.h
> +++ b/arch/x86/events/perf_event.h
> @@ -1351,8 +1351,9 @@ static inline u64 x86_pmu_get_event_config(struct perf_event *event)
>  
>  static inline bool x86_pmu_has_rdpmc_user_disable(struct pmu *pmu)
>  {
> -	return !!(hybrid(pmu, config_mask) &
> -		 ARCH_PERFMON_EVENTSEL_RDPMC_USER_DISABLE);
> +	return is_x86_pmu(pmu) &&
> +	       (hybrid(pmu, config_mask) &
> +		ARCH_PERFMON_EVENTSEL_RDPMC_USER_DISABLE);
>  }
>  
>  extern struct event_constraint emptyconstraint;