Re: [PATCH 2/2] perf/x86: Disable precise sampling for PERF_SAMPLE_STACK_USER

[Mi, Dapeng]

The patch is only validated on Intel platforms. @Ravi may help to validate
on AMD platforms. Thanks.

On 9/8/2026 3:51 PM, Dapeng Mi wrote:
> PERF_SAMPLE_STACK_USER needs to return the user stack and user registers
> to user space when the PMI exits. Since the skid from the PEBS/IBS sample
> and PMI delivery, the PEBS/IBS register snapshot (especially IP/SP/BP)
> can diverge from the user stack at PMI return. That mismatch breaks DWARF
> unwinding.
>
> Precise sampling provides no benefit in this case, so disable PEBS/IBS
> precise sampling and allow only PMI-based sampling when
> PERF_SAMPLE_STACK_USER is requested.
>
> Reported-by: Gennady Kupava <gennady.kupava@xxxxxxxxx>
> Closes: https://lore.kernel.org/all/CAPu-DQqF0aF6=GS8Z6KKWeeX_V5LiXeKU_rJQZC+uGg8zuTPNw@xxxxxxxxxxxxxx/
> Cc: Ravi Bangoria <ravi.bangoria@xxxxxxx>
> Fixes: c5ebcedb566e ("perf: Add ability to attach user stack dump to sample")
> Signed-off-by: Dapeng Mi <dapeng1.mi@xxxxxxxxxxxxxxx>
> ---
>  arch/x86/events/amd/ibs.c    |  3 +++
>  arch/x86/events/core.c       |  3 +++
>  arch/x86/events/perf_event.h | 19 +++++++++++++++++++
>  3 files changed, 25 insertions(+)
>
> diff --git a/arch/x86/events/amd/ibs.c b/arch/x86/events/amd/ibs.c
> index 3531f9c23b8c..c67b659d64e2 100644
> --- a/arch/x86/events/amd/ibs.c
> +++ b/arch/x86/events/amd/ibs.c
> @@ -327,6 +327,9 @@ static int perf_ibs_init(struct perf_event *event)
>  	if (has_branch_stack(event))
>  		return -EOPNOTSUPP;
>  
> +	if (!x86_pmu_allow_sample_user_stack(event, true))
> +		return -EINVAL;
> +
>  	/* handle exclude_{user,kernel} in the IRQ handler */
>  	if (event->attr.exclude_host || event->attr.exclude_guest ||
>  	    event->attr.exclude_idle)
> diff --git a/arch/x86/events/core.c b/arch/x86/events/core.c
> index 8b3ea0adb965..ceb5038d7565 100644
> --- a/arch/x86/events/core.c
> +++ b/arch/x86/events/core.c
> @@ -594,6 +594,9 @@ int x86_pmu_hw_config(struct perf_event *event)
>  		/* There's no sense in having PEBS for non sampling events: */
>  		if (!is_sampling_event(event))
>  			return -EINVAL;
> +
> +		if (!x86_pmu_allow_sample_user_stack(event, false))
> +			return -EINVAL;
>  	}
>  	/*
>  	 * check that PEBS LBR correction does not conflict with
> diff --git a/arch/x86/events/perf_event.h b/arch/x86/events/perf_event.h
> index 71ed5b2acea2..9fbcf2bcef51 100644
> --- a/arch/x86/events/perf_event.h
> +++ b/arch/x86/events/perf_event.h
> @@ -1274,6 +1274,25 @@ static inline bool is_counter_pair(struct hw_perf_event *hwc)
>  	return hwc->flags & PERF_X86_EVENT_PAIR;
>  }
>  
> +static inline bool
> +x86_pmu_allow_sample_user_stack(struct perf_event *event, bool ibs)
> +{
> +	if (!(event->attr.sample_type & PERF_SAMPLE_STACK_USER))
> +		return true;
> +
> +	/*
> +	 * PERF_SAMPLE_STACK_USER needs to return the user stack and
> +	 * user registers to user space when the PMI exits. Since the skid
> +	 * from the PEBS/IBS sample and PMI delivery, the PEBS/IBS register
> +	 * snapshot (especially IP/SP/BP) can diverge from the user stack
> +	 * at PMI return. That mismatch breaks DWARF unwinding.
> +	 *
> +	 * Therefore, disable precise sampling for PERF_SAMPLE_STACK_USER
> +	 * and permit only PMI-based sampling for this case.
> +	 */
> +	return !(event->attr.precise_ip || ibs);
> +}
> +
>  static inline void __x86_pmu_enable_event(struct hw_perf_event *hwc,
>  					  u64 enable_mask)
>  {