Messages in this thread | | | Subject | Re: [PATCH 1/3] arm64: cpufeature: Fix the type of no FP/SIMD capability | From | Suzuki K Poulose <> | Date | Mon, 14 Oct 2019 16:45:40 +0100 |
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On 14/10/2019 15:52, Dave Martin wrote: > On Fri, Oct 11, 2019 at 06:28:43PM +0100, Suzuki K Poulose wrote: >> >> >> On 11/10/2019 15:21, Dave Martin wrote: >>> On Fri, Oct 11, 2019 at 01:13:18PM +0100, Suzuki K Poulose wrote: > 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. >>> >>> I don't see why waiting for the random defective early CPU to come up is >>> better than waiting for all the early CPUs to come up and then deciding. >>> >>> Kernel-mode NEON aside, the status of this cap should not matter until >>> we enter userspace for the first time. >>> >>> The only issue is if e.g., crypto drivers that can use kernel-mode NEON >>> probe for it before all early CPUs are up, and so cache the wrong >>> decision. The current approach doesn't cope with that anyway AFAICT. >> >> This approach does in fact. With LOCAL_CPU scope, the moment a defective >> CPU turns up, we mark the "capability" and thus the kernel cannot use >> the neon then onwards, unlike the existing case where we have time till >> we boot all the CPUs (even when the boot CPU may be defective). > > I guess that makes sense. > > I'm now wondering what happens if anything tries to use kernel-mode NEON > before SVE is initialised -- which doesn't happen until cpufeatures > configures the system features. > > I don't think your proposed change makes anything worse here, but it may > need looking into.
We could throw in a WARN_ON() in kernel_neon() to make sure that the SVE is initialised ?
Suzuki
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