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

[Will Deacon]

On Thu, Jun 27, 2024 at 12:05:23PM +0100, Marc Zyngier wrote:
> On Wed, 26 Jun 2024 23:32:30 +0100,
> "Rob Herring (Arm)" <robh at 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 at 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.

I tend to agree.

> 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.

Or perhaps wd could leave the 'num_events' field intact and allocate the
new bitmap dynamically?

Rob -- what do you prefer? I think the rest of the series is ready to go.

Will