linux-kernel - Re: [PATCH v4 3/3] arm64/fpsimd: Don't disable softirq when touching FPSIMD/SVE state

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Message-ID: <4fddb4e5-6691-c393-d7f8-15cfc6fe7c4d@arm.com>
Date:   Tue, 7 May 2019 11:52:18 +0100
From:   Julien Grall <julien.grall@....com>
To:     Dave Martin <Dave.Martin@....com>
Cc:     julien.thierry@....com, marc.zyngier@....com,
        catalin.marinas@....com, ard.biesheuvel@...aro.org,
        will.deacon@....com, linux-kernel@...r.kernel.org,
        christoffer.dall@....com, james.morse@....com,
        suzuki.poulose@....com, linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH v4 3/3] arm64/fpsimd: Don't disable softirq when touching
 FPSIMD/SVE state

Hi Dave,

On 4/26/19 4:31 PM, Dave Martin wrote:
> On Fri, Apr 26, 2019 at 04:06:02PM +0100, Julien Grall wrote:
>> Hi,
>>
>> On 26/04/2019 15:52, Dave Martin wrote:
>>> On Fri, Apr 26, 2019 at 03:37:40PM +0100, Julien Grall wrote:
>>>> When the kernel is compiled with CONFIG_KERNEL_MODE_NEON, some part of
>>>> the kernel may be able to use FPSIMD/SVE. This is for instance the case
>>>> for crypto code.
>>>>
>>>> Any use of FPSIMD/SVE in the kernel are clearly marked by using the
>>>> function kernel_neon_{begin, end}. Furthermore, this can only be used
>>>> when may_use_simd() returns true.
>>>>
>>>> The current implementation of may_use_simd() allows softirq to use
>>>> FPSIMD/SVE unless it is currently in use (i.e kernel_neon_busy is true).
>>>> When in use, softirqs usually fall back to a software method.
>>>>
>>>> At the moment, as a softirq may use FPSIMD/SVE, softirqs are disabled
>>>> when touching the FPSIMD/SVE context. This has the drawback to disable
>>>> all softirqs even if they are not using FPSIMD/SVE.
>>>>
>>>> Since a softirq is supposed to check may_use_simd() anyway before
>>>> attempting to use FPSIMD/SVE, there is limited reason to keep softirq
>>>> disabled when touching the FPSIMD/SVE context. Instead, we can simply
>>>> disable preemption and mark the FPSIMD/SVE context as in use by setting
>>>> CPU's kernel_neon_busy flag.
>>>
>>> fpsimd_context_busy?
>>
>> Yes.
>>
>>>
>>>> Two new helpers {get, put}_cpu_fpsimd_context is introduced to mark the
>>>> area using FPSIMD/SVE context and uses them in replacement of
>>>
>>> Paragraph mangled during edit?
>>
>> Possibly, I will update it.
>>
>>>
>>> -> "are introduced ... and they are used to replace ..."
>>>
>>>> local_bh_{disable, enable}. The functions kernel_neon_{begin, end} are
>>>> also re-implemented to use the new helpers.
>>>>
>>>> Additionally, double-underscored versions of the helpers are provided to
>>>> be used in function called with interrupt masked. They are used for
>>>> sanity and also help to mark place where the FPSIMD context can be
>>>> manipulate freely.
>>>
>>> For the benefit of other readers, this should be more explicit.  Also,
>>> the distinction between the normal and __ helpers is that the latter
>>> can be caller with preemption disabled.
>>>
>>> To clarify the impact, we can say something like
>>>
>>> "These are only relevant on paths where irqs are disabled anyway, so
>>> they are not needed for correctness in the current code. Let's use them
>>> anyway though: this marks the critical sections clearly and will help
>>> to avoid mistakes during future maintenance."
>>
>> How about the following commit message?
>>
>>      arm64/fpsimd: Don't disable softirq when touching FPSIMD/SVE state
>>
>>      When the kernel is compiled with CONFIG_KERNEL_MODE_NEON, some part of
>>      the kernel may be able to use FPSIMD/SVE. This is for instance the case
>>      for crypto code.
>>
>>      Any use of FPSIMD/SVE in the kernel are clearly marked by using the
>>      function kernel_neon_{begin, end}. Furthermore, this can only be used
>>      when may_use_simd() returns true.
>>
>>      The current implementation of may_use_simd() allows softirq to use
>>      FPSIMD/SVE unless it is currently in use (i.e kernel_neon_busy is true).
>>      When in use, softirqs usually fall back to a software method.
>>
>>      At the moment, as a softirq may use FPSIMD/SVE, softirqs are disabled
>>      when touching the FPSIMD/SVE context. This has the drawback to disable
>>      all softirqs even if they are not using FPSIMD/SVE.
>>
>>      Since a softirq is supposed to check may_use_simd() anyway before
>>      attempting to use FPSIMD/SVE, there is limited reason to keep softirq
>>      disabled when touching the FPSIMD/SVE context. Instead, we can simply
>>      disable preemption and mark the FPSIMD/SVE context as in use by setting
>>      CPU's fpsimd_context_busy flag.
>>
>>      Two new helpers {get, put}_cpu_fpsimd_context are introduced to mark
>>      the area using FPSIMD/SVE context and they are used to replace
>>      local_bh_{disable, enable}. The functions kernel_neon_{begin, end} are
>>      also re-implemented to use the new helpers.
>>
>>      Additionally, double-underscored versions of the helpers are provided to
>>      called when preemption is already disabled. These are only relevant on
>>      paths where irqs are disabled anyway, so they are not needed for
>>      correctness in the current code. Let's use them anyway though: this
>>      marks critical sections clearly and will help to avoid mistakes during
>>      future maintenance.
> 
> Looks good to me.
> 
> Reviewed-by: Dave Martin <Dave.Martin@....com>
> 
> (For the diff as well as the commit message, obviously.)

Thank you! I will resend the series once rc1 has been cut.

Cheers,

-- 
Julien Grall