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

[Mark Rutland <mark.rutland@arm.com>]

On Mon, Jul 01, 2024 at 02:52:16PM +0100, Will Deacon wrote:
> 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@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.
> 
> I tend to agree.

It's the same size as other bitmaps and arrays in struct arm_pmu, e.g.
the first two fields:

| struct pmu_hw_events {
|         /*  
|          * The events that are active on the PMU for the given index.
|          */
|         struct perf_event       *events[ARMPMU_MAX_HWEVENTS];
| 
|         /*  
|          * A 1 bit for an index indicates that the counter is being used for
|          * an event. A 0 means that the counter can be used.
|          */
|         DECLARE_BITMAP(used_mask, ARMPMU_MAX_HWEVENTS);

... so IMO it's fine as-is, since anything not bound by
ARMPMU_MAX_HWEVENTS would already be wrong today.

> > 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?

I don't think we should allocate the bitmap dynamically, since then we'd
have to do likewise for all the other fields sized by
ARMPMU_MAX_HWEVENTS.

I'm not averse to a check when setting bits in the new cntr_mask (which
I guess would WARN() and not set the bit), but as above I think it's
fine as-is.

Mark.