Re: [PATCH 1/3] arm64: cpufeature: Fix the type of no FP/SIMD capability

[Suzuki K Poulose]

Hi Dave

On 11/10/2019 12:36, Dave Martin wrote:
> On Thu, Oct 10, 2019 at 06:15:15PM +0100, Suzuki K Poulose wrote:
> > The NO_FPSIMD capability is defined with scope SYSTEM, which implies
> > that the "absence" of FP/SIMD on at least one CPU is detected only
> > after all the SMP CPUs are brought up. However, we use the status
> > of this capability for every context switch. So, let us change
> > the scop to LOCAL_CPU to allow the detection of this capability
> > as and when the first CPU without FP is brought up.
> >
> > Also, the current type allows hotplugged CPU to be brought up without
> > FP/SIMD when all the current CPUs have FP/SIMD and we have the userspace
> > up. Fix both of these issues by changing the capability to
> > BOOT_RESTRICTED_LOCAL_CPU_FEATURE.
> >
> > Fixes: 82e0191a1aa11abf ("arm64: Support systems without FP/ASIMD")
> > Cc: Will Deacon <will@xxxxxxxxxx>
> > Cc: Mark Rutland <mark.rutland@xxxxxxx>
> > Cc: Catalin Marinas <catalin.marinas@xxxxxxx>
> > Signed-off-by: Suzuki K Poulose <suzuki.poulose@xxxxxxx>
> > ---
> >   arch/arm64/kernel/cpufeature.c | 2 +-
> >   1 file changed, 1 insertion(+), 1 deletion(-)
> >
> > diff --git a/arch/arm64/kernel/cpufeature.c b/arch/arm64/kernel/cpufeature.c
> > index 9323bcc40a58..0f9eace6c64b 100644
> > --- a/arch/arm64/kernel/cpufeature.c
> > +++ b/arch/arm64/kernel/cpufeature.c
> > @@ -1361,7 +1361,7 @@ static const struct arm64_cpu_capabilities arm64_features[] = {
> >   	{
> >   		/* FP/SIMD is not implemented */
> >   		.capability = ARM64_HAS_NO_FPSIMD,
> > -		.type = ARM64_CPUCAP_SYSTEM_FEATURE,
> > +		.type = ARM64_CPUCAP_BOOT_RESTRICTED_CPU_LOCAL_FEATURE,
>
> ARM64_HAS_NO_FPSIMD is really a disability, not a capability.
>
> Although we have other things that smell like this (CPU errata for
> example), I wonder whether inverting the meaning in the case would
> make the situation easier to understand.

Yes, it is indeed a disability, more on that below.

> So, we'd have ARM64_HAS_FPSIMD, with a minimum (signed) feature field
> value of 0.  Then this just looks like an ARM64_CPUCAP_SYSTEM_FEATURE
> IIUC.  We'd just need to invert the sense of the check in
> system_supports_fpsimd().

This is particularly something we want to avoid with this patch. We want
to make sure that we have the up-to-date status of the disability right
when it happens. i.e, a CPU without FP/SIMD is brought up. With SYSTEM_FEATURE
you have to wait until we bring all the CPUs up. Also, for HAS_FPSIMD,
you must wait until all the CPUs are up, unlike the negated capability.

> >   		.min_field_value = 0,
>
> (Does .min_field_value == 0 make sense, or is it even used?  I thought
> only the default has_cpuid_feature() match logic uses that.)

True, it is not used for this particular case.

Cheers
Suzuki