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

[Suzuki K Poulose]

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@xxxxxxx>
> > ---
> >   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