linux-kernel - Re: [PATCH v3 02/10] perf: arm_spe: Support FEAT

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Message-ID: <3578865d-a2c2-4cb6-9271-abf880403097@linaro.org>
Date: Tue, 15 Jul 2025 12:23:08 +0100
From: James Clark <james.clark@...aro.org>
To: Will Deacon <will@...nel.org>
Cc: Catalin Marinas <catalin.marinas@....com>,
 Mark Rutland <mark.rutland@....com>, Jonathan Corbet <corbet@....net>,
 Marc Zyngier <maz@...nel.org>, Oliver Upton <oliver.upton@...ux.dev>,
 Joey Gouly <joey.gouly@....com>, Suzuki K Poulose <suzuki.poulose@....com>,
 Zenghui Yu <yuzenghui@...wei.com>, Peter Zijlstra <peterz@...radead.org>,
 Ingo Molnar <mingo@...hat.com>, Arnaldo Carvalho de Melo <acme@...nel.org>,
 Namhyung Kim <namhyung@...nel.org>,
 Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
 Jiri Olsa <jolsa@...nel.org>, Ian Rogers <irogers@...gle.com>,
 Adrian Hunter <adrian.hunter@...el.com>, leo.yan@....com,
 linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
 linux-perf-users@...r.kernel.org, linux-doc@...r.kernel.org,
 kvmarm@...ts.linux.dev
Subject: Re: [PATCH v3 02/10] perf: arm_spe: Support FEAT_SPEv1p4 filters



On 14/07/2025 2:26 pm, Will Deacon wrote:
> On Thu, Jun 05, 2025 at 11:49:00AM +0100, James Clark wrote:
>> FEAT_SPEv1p4 (optional from Armv8.8) adds some new filter bits, so
>> remove them from the previous version's RES0 bits using
>> PMSEVFR_EL1_RES0_V1P4_EXCL. It also makes some previously available bits
>> unavailable again, so add those back using PMSEVFR_EL1_RES0_V1P4_INCL.
>> E.g:
>>
>>    E[30], bit [30]
>>    When FEAT_SPEv1p4 is _not_ implemented ...
>>
>> FEAT_SPE_V1P3 has the same filters as V1P2 so explicitly add it to the
>> switch.
>>
>> Reviewed-by: Leo Yan <leo.yan@....com>
>> Tested-by: Leo Yan <leo.yan@....com>
>> Signed-off-by: James Clark <james.clark@...aro.org>
>> ---
>>   arch/arm64/include/asm/sysreg.h | 7 +++++++
>>   drivers/perf/arm_spe_pmu.c      | 5 ++++-
>>   2 files changed, 11 insertions(+), 1 deletion(-)
>>
>> diff --git a/arch/arm64/include/asm/sysreg.h b/arch/arm64/include/asm/sysreg.h
>> index f1bb0d10c39a..880090df3efc 100644
>> --- a/arch/arm64/include/asm/sysreg.h
>> +++ b/arch/arm64/include/asm/sysreg.h
>> @@ -358,6 +358,13 @@
>>   	(PMSEVFR_EL1_RES0_IMP & ~(BIT_ULL(18) | BIT_ULL(17) | BIT_ULL(11)))
>>   #define PMSEVFR_EL1_RES0_V1P2	\
>>   	(PMSEVFR_EL1_RES0_V1P1 & ~BIT_ULL(6))
>> +#define PMSEVFR_EL1_RES0_V1P4_EXCL \
>> +	(BIT_ULL(2) | BIT_ULL(4) | GENMASK_ULL(10, 8) | GENMASK_ULL(23, 19))
>> +#define PMSEVFR_EL1_RES0_V1P4_INCL \
>> +	(GENMASK_ULL(31, 26))
>> +#define PMSEVFR_EL1_RES0_V1P4	\
>> +	(PMSEVFR_EL1_RES0_V1P4_INCL | \
>> +	(PMSEVFR_EL1_RES0_V1P2 & ~PMSEVFR_EL1_RES0_V1P4_EXCL))
>>   
>>   /* Buffer error reporting */
>>   #define PMBSR_EL1_FAULT_FSC_SHIFT	PMBSR_EL1_MSS_SHIFT
>> diff --git a/drivers/perf/arm_spe_pmu.c b/drivers/perf/arm_spe_pmu.c
>> index 3efed8839a4e..d9f6d229dce8 100644
>> --- a/drivers/perf/arm_spe_pmu.c
>> +++ b/drivers/perf/arm_spe_pmu.c
>> @@ -701,9 +701,12 @@ static u64 arm_spe_pmsevfr_res0(u16 pmsver)
>>   	case ID_AA64DFR0_EL1_PMSVer_V1P1:
>>   		return PMSEVFR_EL1_RES0_V1P1;
>>   	case ID_AA64DFR0_EL1_PMSVer_V1P2:
>> +	case ID_AA64DFR0_EL1_PMSVer_V1P3:
>> +		return PMSEVFR_EL1_RES0_V1P2;
>> +	case ID_AA64DFR0_EL1_PMSVer_V1P4:
>>   	/* Return the highest version we support in default */
>>   	default:
>> -		return PMSEVFR_EL1_RES0_V1P2;
>> +		return PMSEVFR_EL1_RES0_V1P4;
> 
> See my reply [1] to Leo about this function, but I think we should just
> remove it.
> 
> Will
> 
> [1] https://lore.kernel.org/all/20250707-arm_spe_support_hitm_overhead_v1_public-v3-0-33ea82da3280@arm.com/

We're only refusing filters that we know for sure are RES0. Unless 
there's a mistake, the ones that are maybes are still up to userspace to 
decide whether it wants to use them or not.

I think it could be quite useful for some automated tool to fall back to 
another behavior if it needs an event that isn't implemented.

If they were _all_ defined as maybes like "When FEAT_SPEv1p4 is 
implemented or filtering on event 2 is optionally supported" then I 
would agree it's not definite enough to bother restricting them. But a 
lot of them are known for sure like "When FEAT_SPEv1p4 is not 
implemented ...", so I don't see the harm in preventing use of those.

Or as I mentioned in the other thread if we think we can probe the valid 
filters that would be even better.

Thanks
James