Messages in this thread | | | Subject | Re: [PATCH 1/3] arm64: cpufeature: Fix the type of no FP/SIMD capability | From | Suzuki K Poulose <> | Date | Tue, 15 Oct 2019 10:44:23 +0100 |
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On 14/10/2019 17:57, Ard Biesheuvel wrote: > On Mon, 14 Oct 2019 at 17:50, Dave P Martin <Dave.Martin@arm.com> wrote: >> >> On Mon, Oct 14, 2019 at 04:45:40PM +0100, Suzuki K Poulose wrote: >>> >>> >>> 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 ? >> >> Could do, at least as an experiment. >> >> Ard, do you have any thoughts on this? >> > > All in-kernel NEON code checks whether the NEON is usable, so I'd > expect that check to return 'false' if it is too early in the boot for > the NEON to be used at all.
Ok. That implies, we need a check to make sure SVE set up is complete, which we don't at the moment, as we default to assume FP/SIMD is available.
"system_can_use_fpsimd()" instead of the "system_supports_fpsimd() where the former should indicate:
system_supports_fpsimd() && sve_setup_complete()
Where the sve_setup_complete() can itself be a static key, initialized very early if we have !CONFIG_SVE. Otherwise, set from sve_setup().
Thoughts ? Suzuki
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