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

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Date:   Fri, 11 Oct 2019 13:13:18 +0100
From:   Suzuki K Poulose <suzuki.poulose@....com>
To:     Dave Martin <Dave.Martin@....com>
Cc:     linux-arm-kernel@...ts.infradead.org, mark.rutland@....com,
        catalin.marinas@....com, linux-kernel@...r.kernel.org,
        will@...nel.org
Subject: Re: [PATCH 1/3] arm64: cpufeature: Fix the type of no FP/SIMD
 capability

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@...nel.org>
>> Cc: Mark Rutland <mark.rutland@....com>
>> Cc: Catalin Marinas <catalin.marinas@....com>
>> Signed-off-by: Suzuki K Poulose <suzuki.poulose@....com>
>> ---
>>   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