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

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Date:   Wed, 29 May 2019 11:46:50 +0100
From:   Raphael Gault <raphael.gault@....com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        mingo@...hat.com, catalin.marinas@....com, will.deacon@....com,
        acme@...nel.org, mark.rutland@....com
Subject: Re: [RFC 4/7] arm64: pmu: Add function implementation to update event
 index in userpage.

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.

>> +}

Thanks,

-- 
Raphael Gault