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

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

[Prasad Sodagudi]

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.

Signed-off-by: Prasad Sodagudi <psodagud@codeaurora.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,
-- 
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Re: [PATCH] perf: Change PMCR write to read-modify-write

[Julien Thierry]

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?

Thanks,

> Signed-off-by: Prasad Sodagudi <psodagud@codeaurora.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

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

[Sodagudi Prasad]

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.

-Thanks, Prasad

> 
> Thanks,
> 
>> Signed-off-by: Prasad Sodagudi <psodagud@codeaurora.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,
>> 

-- 
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Forum,
Linux Foundation Collaborative Project

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

[Julien Thierry]

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@codeaurora.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

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

[Mark Rutland]

On Thu, Mar 21, 2019 at 01:02:27PM -0700, 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.

Given the kernel reconfigures the PMU dynamically at runtime (e.g. to multiplex
events over counters), I don't follow how this is useful. Could you elaborate
on how specifically you intend to use this?

> I think, changing that register this register modifications to
> read-modify-write style make sense.

I disagree. 

In general, RESx bits may be allocated new meanings in future (and will reset
to UNKNOWN values), so it is not safe to simply preserve the reset value. Given
that, any kernel _must_ explicitly reset registers in order to ensure the
expected behaviour.

Thanks
Mark.

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

[Will Deacon]

[+MarkR]

On Wed, Mar 20, 2019 at 07:07:46PM -0700, 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.
> 
> Signed-off-by: Prasad Sodagudi <psodagud@codeaurora.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);

No, I don't think this is right at all. Many of these bits are UNKNOWN
out of reset, so you're throwing away our means of getting the PMU into
a deterministic state.

Who is setting which bits that you want to preserve and why?

Will

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