Re: [Patch v8 22/23] perf/x86: Activate back-to-back NMI detection for arch-PEBS induced NMIs

[sashiko-bot@kernel.org]

Thank you for your contribution! Sashiko AI review found 1 potential issue(s) to consider:
- [Medium] Using the `BIT()` macro for shifts >= 32 on 32-bit architectures causes undefined behavior, defeating the back-to-back NMI detection for fixed counters.
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commit 67718335973298488df3db65888b9d78109347ad
Author: Dapeng Mi <dapeng1.mi@linux.intel.com>

perf/x86: Activate back-to-back NMI detection for arch-PEBS induced NMIs

This commit modifies the PEBS handlers to return the count of actually processed
events. This activates back-to-back NMI detection in the PMI handler, avoiding
the "suspicious NMI" warning that occurs when multiple back-to-back NMIs are
triggered by PEBS events.

> diff --git a/arch/x86/events/intel/ds.c b/arch/x86/events/intel/ds.c
> index 8a653edce392a..e0d3076277026 100644
> --- a/arch/x86/events/intel/ds.c
> +++ b/arch/x86/events/intel/ds.c
[ ... ]
> @@ -3198,6 +3201,7 @@ static void intel_pmu_drain_pebs_nhm(struct pt_regs *iregs, struct perf_sample_d
>  		if ((counts[bit] == 0) && (error[bit] == 0))
>  			continue;
>  
> +		events_bitmap |= BIT(bit);
>  		event = cpuc->events[bit];

[Severity: Medium]
Will this shift cause undefined behavior on 32-bit architectures?

Since bit can be greater than or equal to 32 for fixed counters (where
INTEL_PMC_IDX_FIXED is 32), using the BIT() macro on 32-bit x86 kernels
results in a shift by 32 or more bits on an unsigned long, which is undefined
behavior in C.

In practice, this typically results in a modulo shift that aliases fixed
counters to general-purpose counters, causing hweight64(events_bitmap) to
under-report the number of unique events handled. 

This could defeat the back-to-back NMI detection and fail to suppress the
"suspicious NMI" warning for fixed counters on 32-bit systems.

Should this use BIT_ULL(bit) instead?

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Sashiko AI review · https://sashiko.dev/#/patchset/20260529075645.580362-1-dapeng1.mi@linux.intel.com?part=22