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

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Message-ID: <0100f2bd-7940-0b81-4c03-205b295a048f@arm.com>
Date:   Wed, 29 May 2019 13:25:46 +0100
From:   Raphael Gault <raphael.gault@....com>
To:     Robin Murphy <robin.murphy@....com>,
        Peter Zijlstra <peterz@...radead.org>
Cc:     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 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.

> Robin.

-- 
Raphael Gault