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

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Message-ID: <20191011113620.GG27757@arm.com>
Date:   Fri, 11 Oct 2019 12:36:20 +0100
From:   Dave Martin <Dave.Martin@....com>
To:     Suzuki K Poulose <suzuki.poulose@....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

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.

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().

>  		.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.)

>  		.matches = has_no_fpsimd,
>  	},

Cheers
---Dave