linux-kernel - Re: [RFC PATCH] arm64/fpsimd: Don't disable softirq when touching FPSIMD/SVE state

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Message-ID: <899713e0-2d32-04a0-b2f2-3493f7299033@arm.com>
Date:   Thu, 11 Apr 2019 16:58:41 +0100
From:   Julien Grall <julien.grall@....com>
To:     Dave Martin <Dave.Martin@....com>
Cc:     linux-arm-kernel@...ts.infradead.org, bigeasy@...utronix.de,
        linux-rt-users@...r.kernel.org, catalin.marinas@....com,
        will.deacon@....com, ard.biesheuvel@...aro.org,
        linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH] arm64/fpsimd: Don't disable softirq when touching
 FPSIMD/SVE state

Hi Dave,

On 4/5/19 4:07 PM, Dave Martin wrote:
> On Fri, Apr 05, 2019 at 10:02:45AM +0100, Julien Grall wrote:
>>>> +#ifdef CONFIG_KERNEL_MODE_NEON
>>>> +
>>>>   /*
>>>>    * may_use_simd - whether it is allowable at this time to issue SIMD
>>>>    *                instructions or access the SIMD register file
>>>> diff --git a/arch/arm64/kernel/fpsimd.c b/arch/arm64/kernel/fpsimd.c
>>>> index 5ebe73b69961..b7e5dac26190 100644
>>>> --- a/arch/arm64/kernel/fpsimd.c
>>>> +++ b/arch/arm64/kernel/fpsimd.c
>>>> @@ -90,7 +90,8 @@
>>>>    * To prevent this from racing with the manipulation of the task's FPSIMD state
>>>>    * from task context and thereby corrupting the state, it is necessary to
>>>>    * protect any manipulation of a task's fpsimd_state or TIF_FOREIGN_FPSTATE
>>>> - * flag with local_bh_disable() unless softirqs are already masked.
>>>> + * flag with kernel_neon_{disable, enable}. This will still allow softirqs to
>>>> + * run but prevent them to use FPSIMD.
>>>>    *
>>>>    * For a certain task, the sequence may look something like this:
>>>>    * - the task gets scheduled in; if both the task's fpsimd_cpu field
>>>> @@ -142,6 +143,9 @@ extern void __percpu *efi_sve_state;
>>>>   #endif /* ! CONFIG_ARM64_SVE */
>>>> +static void kernel_neon_disable(void);
>>>> +static void kernel_neon_enable(void);
>>>
>>> I find these names a bit confusing: _disable() actualy enables FPSIMD/SVE
>>> context access for the current context (i.e., makes it safe).
>>>
>>> Since these also disable/enable preemption, perhaps we can align them
>>> with the existing get_cpu()/put_cpu(), something like:
>>>
>>> void get_cpu_fpsimd_context();
>>> vold put_cpu_fpsimd_context();
>>>
>>> If you consider it's worth adding the checking helper I alluded to
>>> above, it could then be called something like:
>>>
>>> bool have_cpu_fpsimd_context();
>>
>> I am not sure where you suggested a checking helper above. Do you refer to
>> exposing kernel_neon_busy even for !CONFIG_KERNEL_MODE_NEON?
> 
> Hmmm, looks like I got my reply out of order here.
> 
> I meant the helper (if any) to check
> !preemptible() && !__this_cpu_read(kernel_neon_busy).

I guess you are using && instead of || because some of the caller may 
not call get_cpu_fpsimd_context() before but still disable preemption, 
right?

Wouldn't it be better to have all the user calling 
get_cpu_fpsimd_context() and put_cpu_fpsimd_context()?

This has the advantage to uniformize how the way FPSIMD is protected and 
also...

> 
> Looks like you inferred what I meant later on anyway.
> 
>>
>>>
>>>> +
>>>>   /*
>>>>    * Call __sve_free() directly only if you know task can't be scheduled
>>>>    * or preempted.
>>>> @@ -213,11 +217,11 @@ static void sve_free(struct task_struct *task)
>>>>    * thread_struct is known to be up to date, when preparing to enter
>>>>    * userspace.
>>>>    *
>>>> - * Softirqs (and preemption) must be disabled.
>>>> + * Preemption must be disabled.
>>>
>>> [*] That's not enough: we need to be in kernel_neon_disable()...
>>> _enable() (i.e., kernel_neon_busy needs to be true to prevent softirqs
>>> messing with the FPSIMD state).
>>
>> How about not mentioning preemption at all and just say:
>>
>> "The fpsimd context should be acquired before hand".
>>
>> This would help if we ever decide to protect critical section differently.
> 
> Yes, or even better, name the function used to do this (i.e.,
> kernel_neon_disable() or get_cpu_fpsimd_context() or whatever it's
> called).

... would make the comments simpler because we would have only one 
possible case to care.

Cheers,

-- 
Julien Grall