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Date:   Tue, 30 Mar 2021 11:36:07 +0100
From:   Will Deacon <will@kernel.org>
To:     Ard Biesheuvel <ardb@kernel.org>
Cc:     linux-crypto@vger.kernel.org, linux-arm-kernel@lists.infradead.org,
        Dave Martin <dave.martin@arm.com>,
        Mark Brown <broonie@kernel.org>,
        Herbert Xu <herbert@gondor.apana.org.au>,
        Eric Biggers <ebiggers@kernel.org>,
        Catalin Marinas <catalin.marinas@arm.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Peter Zijlstra <peterz@infradead.org>,
        Sebastian Andrzej Siewior <bigeasy@linutronix.de>,
        Ingo Molnar <mingo@kernel.org>,
        Andy Lutomirski <luto@kernel.org>
Subject: Re: [PATCH v2 3/9] arm64: fpsimd: run kernel mode NEON with softirqs
 disabled
Message-ID: <20210330103605.GC5352@willie-the-truck>
References: <20210302090118.30666-1-ardb@kernel.org>
 <20210302090118.30666-4-ardb@kernel.org>
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On Tue, Mar 02, 2021 at 10:01:12AM +0100, Ard Biesheuvel wrote:
> Kernel mode NEON can be used in task or softirq context, but only in
> a non-nesting manner, i.e., softirq context is only permitted if the
> interrupt was not taken at a point where the kernel was using the NEON
> in task context.
> 
> This means all users of kernel mode NEON have to be aware of this
> limitation, and either need to provide scalar fallbacks that may be much
> slower (up to 20x for AES instructions) and potentially less safe, or
> use an asynchronous interface that defers processing to a later time
> when the NEON is guaranteed to be available.
> 
> Given that grabbing and releasing the NEON is cheap, we can relax this
> restriction, by increasing the granularity of kernel mode NEON code, and
> always disabling softirq processing while the NEON is being used in task
> context.
> 
> Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
> ---
>  arch/arm64/crypto/aes-modes.S      |  2 +-
>  arch/arm64/crypto/sha1-ce-core.S   |  2 +-
>  arch/arm64/crypto/sha2-ce-core.S   |  2 +-
>  arch/arm64/crypto/sha3-ce-core.S   |  4 +--
>  arch/arm64/crypto/sha512-ce-core.S |  2 +-
>  arch/arm64/include/asm/assembler.h | 28 +++++++++++++++-----
>  arch/arm64/kernel/asm-offsets.c    |  2 ++
>  arch/arm64/kernel/fpsimd.c         |  4 +--
>  8 files changed, 31 insertions(+), 15 deletions(-)

Acked-by: Will Deacon <will@kernel.org>

Will