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

[Rob Herring]

On Mon, Jul 1, 2024 at 7:52 AM Will Deacon <will at kernel.org> 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 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.

I think the list of places we're initializing cntr_mask is short
enough to check and additions to arm_pmu users are rare enough I would
not be too worried about it.

If anything, I think the issue is with the bitmap API in that it has
no bounds checking. I'm sure it will get on someone's radar to fix at
some point.

But if we want to have something check, this is what I have:

static inline void armpmu_set_counter_mask(struct arm_pmu *pmu,
                                          unsigned int start, unsigned int nr)
{
       if (WARN_ON(start + nr > ARMPMU_MAX_HWEVENTS))
               return;
       bitmap_set(pmu->cntr_mask, start, nr);
}

Rob