Re: [PATCH v2 06/12] perf: arm_pmu: Remove event index to counter remapping

[Marc Zyngier <maz@kernel.org>]

On Wed, 26 Jun 2024 23:32:30 +0100,
"Rob Herring (Arm)" <robh@kernel.org> wrote:
> 
> Xscale and Armv6 PMUs defined the cycle counter at 0 and event counters
> starting at 1 and had 1:1 event index to counter numbering. On Armv7 and
> later, this changed the cycle counter to 31 and event counters start at
> 0. The drivers for Armv7 and PMUv3 kept the old event index numbering
> and introduced an event index to counter conversion. The conversion uses
> masking to convert from event index to a counter number. This operation
> relies on having at most 32 counters so that the cycle counter index 0
> can be transformed to counter number 31.
> 
> Armv9.4 adds support for an additional fixed function counter
> (instructions) which increases possible counters to more than 32, and
> the conversion won't work anymore as a simple subtract and mask. The
> primary reason for the translation (other than history) seems to be to
> have a contiguous mask of counters 0-N. Keeping that would result in
> more complicated index to counter conversions. Instead, store a mask of
> available counters rather than just number of events. That provides more
> information in addition to the number of events.
> 
> No (intended) functional changes.
> 
> Signed-off-by: Rob Herring (Arm) <robh@kernel.org>

[...]

> diff --git a/include/linux/perf/arm_pmu.h b/include/linux/perf/arm_pmu.h
> index b3b34f6670cf..e5d6d204beab 100644
> --- a/include/linux/perf/arm_pmu.h
> +++ b/include/linux/perf/arm_pmu.h
> @@ -96,7 +96,7 @@ struct arm_pmu {
>  	void		(*stop)(struct arm_pmu *);
>  	void		(*reset)(void *);
>  	int		(*map_event)(struct perf_event *event);
> -	int		num_events;
> +	DECLARE_BITMAP(cntr_mask, ARMPMU_MAX_HWEVENTS);

I'm slightly worried by this, as this size is never used, let alone
checked by the individual drivers. I can perfectly picture some new
(non-architectural) PMU driver having more counters than that, and
blindly setting bits outside of the allowed range.

One way to make it a bit safer would be to add a helper replacing the
various bitmap_set() calls, and enforcing that we never overflow this
bitmap.

Thanks,

	M.

-- 
Without deviation from the norm, progress is not possible.