Messages in this thread | | | Subject | Re: [PATCH 1/3] arm64: cpufeature: Fix the type of no FP/SIMD capability | From | Suzuki K Poulose <> | Date | Fri, 11 Oct 2019 13:13:18 +0100 |
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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@kernel.org> >> Cc: Mark Rutland <mark.rutland@arm.com> >> Cc: Catalin Marinas <catalin.marinas@arm.com> >> Signed-off-by: Suzuki K Poulose <suzuki.poulose@arm.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
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