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 13:39:51 +0100
From:   Raphael Gault <raphael.gault@....com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Robin Murphy <robin.murphy@....com>, mark.rutland@....com,
        catalin.marinas@....com, will.deacon@....com,
        linux-kernel@...r.kernel.org, acme@...nel.org, mingo@...hat.com,
        linux-arm-kernel@...ts.infradead.org
Subject: Re: [RFC 4/7] arm64: pmu: Add function implementation to update event
 index in userpage.

Hi Peter,

On 5/29/19 1:32 PM, Peter Zijlstra wrote:
> 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.
> 

Indeed that's true. As for the pseudo-code in perf_event_mmap_page. It 
is compatible with what I do here. The two approach are only different 
in form but it is in both case necessary to subtract 1 on the returned 
value in order to access the correct hardware counter.

Thank you,

-- 
Raphael Gault