Re: [PATCH v6 02/10] arm64: perf: Enable PMU counter direct access for perf event

[Will Deacon]

On Tue, Mar 30, 2021 at 04:08:11PM -0500, Rob Herring wrote:
> On Tue, Mar 30, 2021 at 12:09 PM Rob Herring <robh@xxxxxxxxxx> wrote:
> > On Tue, Mar 30, 2021 at 10:31 AM Will Deacon <will@xxxxxxxxxx> wrote:
> > > The logic here feels like it
> > > could with a bit of untangling.
> >
> > Yes, I don't love it, but couldn't come up with anything better. It is
> > complicated by the fact that flags have to be set before we assign the
> > counter and can't set/change them when we assign the counter. It would
> > take a lot of refactoring with armpmu code to fix that.
> 
> How's this instead?:
> 
> if (armv8pmu_event_want_user_access(event) || !armv8pmu_event_is_64bit(event))
>         event->hw.flags |= ARMPMU_EL0_RD_CNTR;
> 
> /*
> * At this point, the counter is not assigned. If a 64-bit counter is
> * requested, we must make sure the h/w has 64-bit counters if we set
> * the event size to 64-bit because chaining is not supported with
> * userspace access. This may still fail later on if the CPU cycle
> * counter is in use.
> */
> if (armv8pmu_event_is_64bit(event) &&
>     (!armv8pmu_event_want_user_access(event) ||
>      armv8pmu_has_long_event(cpu_pmu) || (hw_event_id ==
> ARMV8_PMUV3_PERFCTR_CPU_CYCLES)))
>         event->hw.flags |= ARMPMU_EVT_64BIT;

I thought there were some cases where we could assign cycles event to an
event counter; does that not happen anymore?

Will