linux-kernel - Re: [PATCH] perf: Change PMCR write to read-modify-write

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Date:   Fri, 22 Mar 2019 08:31:46 +0000
From:   Julien Thierry <julien.thierry@....com>
To:     Sodagudi Prasad <psodagud@...eaurora.org>
Cc:     will.deacon@....com, mingo@...hat.com, catalin.marinas@....com,
        linux-arm-kernel@...ts.infradead.org, peterz@...radead.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] perf: Change PMCR write to read-modify-write



On 21/03/2019 20:02, Sodagudi Prasad wrote:
> On 2019-03-21 06:34, Julien Thierry wrote:
>> Hi Prasad,
>>
>> On 21/03/2019 02:07, Prasad Sodagudi wrote:
>>> Preserves the bitfields of PMCR_EL0(AArch64) during PMU reset.
>>> Reset routine should write a 1 to PMCR.C and PMCR.P fields only
>>> to reset the counters. Other fields should not be changed
>>> as they could be set before PMU initialization and their
>>> value must be preserved even after reset.
>>>
>>
>> Are there any particular bit you are concerned about? Apart from the RO
>> ones and the Res0 ones (to which we are already writing 0), I see:
>>
>> DP -> irrelevant for non-secure
>> X -> This one is the only potentially interesting, however it resets to
>> an architecturally unknown value, so unless we know for a fact it was
>> set before hand, we probably want to clear it
>> D -> ignored when we have LC set (and we do)
>> E -> Since this is the function we use to reset the PMU on the current
>> CPU, we probably want to set this bit to 0 regardless of its previous
>> value
>>
>> So, is there any issue this patch is solving?
> 
> Hi Julien,
> 
> Thanks for taking a look into this patch. Yes. On our Qualcomm
> platforms, observed that X bit is getting cleared by kernel.
> This bit is getting set by firmware for Qualcomm use cases and
> non-secure world is resetting without this patch.
> I think, changing that register this register modifications to
> read-modify-write style make sense.
> 

Maybe for the X bit, but for the E bit this seems like the wrong thing
to do. We want to set the E bit to 0 here.

And for platforms that don't have a firmware touching the X bit (or
rather the pmcr as a whole), I'd like to understand whether it would be
valid to leave this bit set to an architecturally unknown value and
preserving that value.

Thanks,

>>> Signed-off-by: Prasad Sodagudi <psodagud@...eaurora.org>
>>> ---
>>>  arch/arm64/kernel/perf_event.c | 4 ++--
>>>  1 file changed, 2 insertions(+), 2 deletions(-)
>>>
>>> diff --git a/arch/arm64/kernel/perf_event.c
>>> b/arch/arm64/kernel/perf_event.c
>>> index 4addb38..0c1afdd 100644
>>> --- a/arch/arm64/kernel/perf_event.c
>>> +++ b/arch/arm64/kernel/perf_event.c
>>> @@ -868,8 +868,8 @@ static void armv8pmu_reset(void *info)
>>>       * Initialize & Reset PMNC. Request overflow interrupt for
>>>       * 64 bit cycle counter but cheat in armv8pmu_write_counter().
>>>       */
>>> -    armv8pmu_pmcr_write(ARMV8_PMU_PMCR_P | ARMV8_PMU_PMCR_C |
>>> -                ARMV8_PMU_PMCR_LC);
>>> +    armv8pmu_pmcr_write(armv8pmu_pmcr_read() | ARMV8_PMU_PMCR_P |
>>> +            ARMV8_PMU_PMCR_C | ARMV8_PMU_PMCR_LC);
>>>  }
>>>
>>>  static int __armv8_pmuv3_map_event(struct perf_event *event,
>>>
> 

-- 
Julien Thierry