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

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Message-ID: <20191014155009.GM24047@e103592.cambridge.arm.com>
Date:   Mon, 14 Oct 2019 15:50:11 +0000
From:   Dave P Martin <Dave.Martin@....com>
To:     Suzuki Poulose <Suzuki.Poulose@....com>
CC:     Mark Rutland <Mark.Rutland@....com>,
        Catalin Marinas <Catalin.Marinas@....com>,
        "will@...nel.org" <will@...nel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-arm-kernel@...ts.infradead.org" 
        <linux-arm-kernel@...ts.infradead.org>,
        Ard Biesheuvel <ard.biesheuvel@...aro.org>
Subject: Re: [PATCH 1/3] arm64: cpufeature: Fix the type of no FP/SIMD
 capability

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@...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.
> >>>
> >>> 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?

Cheers
---Dave
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