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

[Peter Zijlstra <peterz@infradead.org>]

On Wed, May 29, 2019 at 01:25:46PM +0100, Raphael Gault wrote:
> Hi Robin, Hi Peter,
> 
> On 5/29/19 11:50 AM, Robin Murphy wrote:
> > 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)
> > > > > +{
> > > > > +    if (!(event->hw.flags & ARMPMU_EL0_RD_CNTR))
> > > > > +        return 0;
> > > > > +
> > > > > +    /*
> > > > > +     * We remap the cycle counter index to 32 to
> > > > > +     * match the offset applied to the rest of
> > > > > +     * the counter indeces.
> > > > > +     */
> > > > > +    if (event->hw.idx == ARMV8_IDX_CYCLE_COUNTER)
> > > > > +        return 32;
> > > > > +
> > > > > +    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...
> > 
> 
> Well the current behaviour is correct and takes care of the zero case with
> the ARMV8_IDX_CYCLE_COUNTER check. But using ARMV8_IDX_TO_COUNTER() and add
> 1 would also work. However this seems indeed redundant with the current
> value held in event->hw.idx.

Note that whatever you pick now will become ABI. Also note that the
comment/pseudo-code in perf_event_mmap_page suggests to use idx-1 for
the actual hardware access.