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SubjectRe: [PATCH 09/16] arm64: capabilities: Introduce strict features based on local CPU
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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@arm.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

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