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

[Dave Martin <Dave.Martin@arm.com>]

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.

But IIUC it doesn't make a difference for the only real systems that
this feature is relevant to.

[...]

Cheers
---Dave