linux-kernel - Re: [PATCH v3 01/22] arm64: cpufeature: Add cpucap for HPMN0

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Message-ID: <gsnt34b61fd0.fsf@coltonlewis-kvm.c.googlers.com>
Date: Tue, 08 Jul 2025 22:34:35 +0000
From: Colton Lewis <coltonlewis@...gle.com>
To: Mark Rutland <mark.rutland@....com>
Cc: kvm@...r.kernel.org, pbonzini@...hat.com, corbet@....net, 
	linux@...linux.org.uk, catalin.marinas@....com, will@...nel.org, 
	maz@...nel.org, oliver.upton@...ux.dev, mizhang@...gle.com, 
	joey.gouly@....com, suzuki.poulose@....com, yuzenghui@...wei.com, 
	shuah@...nel.org, linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org, 
	linux-arm-kernel@...ts.infradead.org, kvmarm@...ts.linux.dev, 
	linux-perf-users@...r.kernel.org, linux-kselftest@...r.kernel.org
Subject: Re: [PATCH v3 01/22] arm64: cpufeature: Add cpucap for HPMN0

Mark Rutland <mark.rutland@....com> writes:

> On Thu, Jun 26, 2025 at 08:04:37PM +0000, Colton Lewis wrote:
>> Add a capability for FEAT_HPMN0, whether MDCR_EL2.HPMN can specify 0
>> counters reserved for the guest.

>> This required changing HPMN0 to an UnsignedEnum in tools/sysreg
>> because otherwise not all the appropriate macros are generated to add
>> it to arm64_cpu_capabilities_arm64_features.

> I agree it's appropriate to mark ID_AA64DFR0_EL1.HPMN0 as an
> UnsignedEnum. It follows the usual ID scheme per ARM DDI 0487 L.a
> section D24.1.3, and zero means not present, so it must be unsigned.

> Likewise, the value renames (UNPREDICTABLE => NI and DEF => IMP) look
> fine to me.

>> Signed-off-by: Colton Lewis <coltonlewis@...gle.com>

> I have one minor nit below, but either way:

> Acked-by: Mark Rutland <mark.rutland@....com>

Thank you Mark

>> ---
>>   arch/arm64/kernel/cpufeature.c | 8 ++++++++
>>   arch/arm64/tools/cpucaps       | 1 +
>>   arch/arm64/tools/sysreg        | 6 +++---
>>   3 files changed, 12 insertions(+), 3 deletions(-)

>> diff --git a/arch/arm64/kernel/cpufeature.c  
>> b/arch/arm64/kernel/cpufeature.c
>> index b34044e20128..73a7dac4b6f6 100644
>> --- a/arch/arm64/kernel/cpufeature.c
>> +++ b/arch/arm64/kernel/cpufeature.c
>> @@ -548,6 +548,7 @@ static const struct arm64_ftr_bits ftr_id_mmfr0[] = {
>>   };

>>   static const struct arm64_ftr_bits ftr_id_aa64dfr0[] = {
>> +	ARM64_FTR_BITS(FTR_HIDDEN, FTR_STRICT, FTR_LOWER_SAFE,  
>> ID_AA64DFR0_EL1_HPMN0_SHIFT, 4, 0),
>>   	S_ARM64_FTR_BITS(FTR_HIDDEN, FTR_STRICT, FTR_LOWER_SAFE,  
>> ID_AA64DFR0_EL1_DoubleLock_SHIFT, 4, 0),
>>   	ARM64_FTR_BITS(FTR_HIDDEN, FTR_NONSTRICT, FTR_LOWER_SAFE,  
>> ID_AA64DFR0_EL1_PMSVer_SHIFT, 4, 0),
>>   	ARM64_FTR_BITS(FTR_HIDDEN, FTR_STRICT, FTR_LOWER_SAFE,  
>> ID_AA64DFR0_EL1_CTX_CMPs_SHIFT, 4, 0),
>> @@ -2896,6 +2897,13 @@ static const struct arm64_cpu_capabilities  
>> arm64_features[] = {
>>   		.matches = has_cpuid_feature,
>>   		ARM64_CPUID_FIELDS(ID_AA64MMFR0_EL1, FGT, FGT2)
>>   	},
>> +	{
>> +		.desc = "FEAT_HPMN0",

> Minor nit, but we can drop the "FEAT_" prefix here, for consistency with
> other features (e.g. E0PD, FPMR).

> Mark.

Will do.

>> +		.type = ARM64_CPUCAP_SYSTEM_FEATURE,
>> +		.capability = ARM64_HAS_HPMN0,
>> +		.matches = has_cpuid_feature,
>> +		ARM64_CPUID_FIELDS(ID_AA64DFR0_EL1, HPMN0, IMP)
>> +	},
>>   #ifdef CONFIG_ARM64_SME
>>   	{
>>   		.desc = "Scalable Matrix Extension",
>> diff --git a/arch/arm64/tools/cpucaps b/arch/arm64/tools/cpucaps
>> index 10effd4cff6b..5b196ba21629 100644
>> --- a/arch/arm64/tools/cpucaps
>> +++ b/arch/arm64/tools/cpucaps
>> @@ -39,6 +39,7 @@ HAS_GIC_CPUIF_SYSREGS
>>   HAS_GIC_PRIO_MASKING
>>   HAS_GIC_PRIO_RELAXED_SYNC
>>   HAS_HCR_NV1
>> +HAS_HPMN0
>>   HAS_HCX
>>   HAS_LDAPR
>>   HAS_LPA2
>> diff --git a/arch/arm64/tools/sysreg b/arch/arm64/tools/sysreg
>> index 8a8cf6874298..d29742481754 100644
>> --- a/arch/arm64/tools/sysreg
>> +++ b/arch/arm64/tools/sysreg
>> @@ -1531,9 +1531,9 @@ EndEnum
>>   EndSysreg

>>   Sysreg	ID_AA64DFR0_EL1	3	0	0	5	0
>> -Enum	63:60	HPMN0
>> -	0b0000	UNPREDICTABLE
>> -	0b0001	DEF
>> +UnsignedEnum	63:60	HPMN0
>> +	0b0000	NI
>> +	0b0001	IMP
>>   EndEnum
>>   UnsignedEnum	59:56	ExtTrcBuff
>>   	0b0000	NI
>> --
>> 2.50.0.727.gbf7dc18ff4-goog