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

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Date:   Mon, 14 Oct 2019 16:45:40 +0100
From:   Suzuki K Poulose <suzuki.poulose@....com>
To:     Dave Martin <Dave.Martin@....com>
Cc:     mark.rutland@....com, catalin.marinas@....com, will@...nel.org,
        linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH 1/3] arm64: cpufeature: Fix the type of no FP/SIMD
 capability



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 ?

Suzuki