linux-kernel - Re: [PATCH 09/16] arm64: capabilities: Introduce strict features based on local CPU

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Date:   Tue, 30 Jan 2018 11:25:51 +0000
From:   Suzuki K Poulose <Suzuki.Poulose@....com>
To:     Dave Martin <Dave.Martin@....com>
Cc:     linux-arm-kernel@...ts.infradead.org, mark.rutland@....com,
        ckadabi@...eaurora.org, ard.biesheuvel@...aro.org,
        marc.zyngier@....com, catalin.marinas@....com, will.deacon@....com,
        linux-kernel@...r.kernel.org, jnair@...iumnetworks.com
Subject: Re: [PATCH 09/16] arm64: capabilities: Introduce strict features
 based on local CPU

On 26/01/18 12:12, Dave Martin wrote:
> On Tue, Jan 23, 2018 at 12:28:02PM +0000, Suzuki K Poulose wrote:
>> Add type for features that are detected on individual CPUs,
>> rather than on a system wide safe features. This behavior
> 
> feature
> 
>> is similar to that of a strict cpu erratum, where a later
>> CPU is not allowed to boot if the system doesn't posses it.
>>
>> Use this for software prefetching capability.
>>
>> Signed-off-by: Suzuki K Poulose <suzuki.poulose@....com>
>> ---
>>   arch/arm64/include/asm/cpufeature.h | 7 +++++++
>>   arch/arm64/kernel/cpufeature.c      | 2 +-
>>   2 files changed, 8 insertions(+), 1 deletion(-)
>>
>> diff --git a/arch/arm64/include/asm/cpufeature.h b/arch/arm64/include/asm/cpufeature.h
>> index a621d2184227..4c3d6987acfc 100644
>> --- a/arch/arm64/include/asm/cpufeature.h
>> +++ b/arch/arm64/include/asm/cpufeature.h
>> @@ -118,6 +118,13 @@ extern struct arm64_ftr_reg arm64_ftr_reg_ctrel0;
>>    */
>>   #define ARM64_CPUCAP_BOOT_SYSTEM_FEATURE	\
>>   	(ARM64_CPUCAP_SCOPE_SYSTEM | ARM64_CPUCAP_LATE_CPU_SAFE_TO_HAVE)
>> +/*
>> + * CPU feature detected at boot time based on feature of one or more CPUs.
>> + * It is not safe for a late CPU to have this feature, when the system doesn't
>> + * have it. But it is safe to miss the feature if the system has it.
>> + */
>> +#define ARM64_CPUCAP_STRICT_CPU_LOCAL_FEATURE	\
>> +	(ARM64_CPUCAP_SCOPE_LOCAL_CPU | ARM64_CPUCAP_LATE_CPU_SAFE_TO_MISS)
> 
> "STRICT" seem quite odd here, since we never require all CPUs to have
> the feature.  The case we forbid is when the boot-time decision is that
> the system doesn't tolerate this feature.  So this feels erratum-like.
> 
>>   struct arm64_cpu_capabilities {
>>   	const char *desc;
>> diff --git a/arch/arm64/kernel/cpufeature.c b/arch/arm64/kernel/cpufeature.c
>> index 7ae5cf9092d0..111f6c4b4cd7 100644
>> --- a/arch/arm64/kernel/cpufeature.c
>> +++ b/arch/arm64/kernel/cpufeature.c
>> @@ -951,7 +951,7 @@ static const struct arm64_cpu_capabilities arm64_features[] = {
>>   	{
>>   		.desc = "Software prefetching using PRFM",
>>   		.capability = ARM64_HAS_NO_HW_PREFETCH,
>> -		.type = ARM64_CPUCAP_BOOT_SYSTEM_FEATURE,
>> +		.type = ARM64_CPUCAP_STRICT_CPU_LOCAL_FEATURE,
>>   		.matches = has_no_hw_prefetch,
> 
> For ARM64_HAS_NO_HW_PREFETCH this is more describing an implementation
> option that only affects performance -- in that case it's not obvious
> that we should be strict at all.
> 
> This suggests ARM64_CPUCAP_SCOPE_LOCAL_CPU |
> 	ARM64_CPUCAP_LATE_CPU_SAFE_TO_HAVE |
> 	ARM64_CPUCAP_LATE_CPU_SAFE_TO_MISS.

You're right. This is more like a WEAK feature we add for DBM. I will switch it.

Cheers
Suzuki