Re: [RFC 4/7] arm64: pmu: Add function implementation to update event index in userpage.

[Robin Murphy]

On 29/05/2019 11:46, Raphael Gault wrote:
> Hi Peter,
>
> On 5/29/19 10:46 AM, Peter Zijlstra wrote:
> > On Tue, May 28, 2019 at 04:03:17PM +0100, Raphael Gault wrote:
> > > +static int armv8pmu_access_event_idx(struct perf_event *event)
> > > +{
> > > +ÂÂÂ if (!(event->hw.flags & ARMPMU_EL0_RD_CNTR))
> > > +ÂÂÂÂÂÂÂ return 0;
> > > +
> > > +ÂÂÂ /*
> > > +ÂÂÂÂ * We remap the cycle counter index to 32 to
> > > +ÂÂÂÂ * match the offset applied to the rest of
> > > +ÂÂÂÂ * the counter indeces.
> > > +ÂÂÂÂ */
> > > +ÂÂÂ if (event->hw.idx == ARMV8_IDX_CYCLE_COUNTER)
> > > +ÂÂÂÂÂÂÂ return 32;
> > > +
> > > +ÂÂÂ return event->hw.idx;
> >
> > Is there a guarantee event->hw.idx is never 0? Or should you, just like
> > x86, use +1 here?
>
> You are right, I should use +1 here. Thanks for pointing that out.

Isn't that already the case though, since we reserve index 0 for the 
cycle counter? I'm looking at ARMV8_IDX_TO_COUNTER() here...

Robin.