linux-kernel - Re: [PATCH v2 2/7] arm64: fpsimd: Make sure SVE setup is complete before SIMD is used

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Date:   Fri, 10 Jan 2020 11:51:32 +0000
From:   Catalin Marinas <catalin.marinas@....com>
To:     Suzuki K Poulose <suzuki.poulose@....com>
Cc:     linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        will@...nel.org, maz@...nel.org, mark.rutland@....com,
        dave.martin@....com, ard.biesheuvel@...aro.org,
        christoffer.dall@....com, Will Deacon <will.deacon@....com>
Subject: Re: [PATCH v2 2/7] arm64: fpsimd: Make sure SVE setup is complete
 before SIMD is used

On Tue, Dec 17, 2019 at 06:33:57PM +0000, Suzuki K Poulose wrote:
> In-kernel users of NEON rely on may_use_simd() to check if the SIMD
> can be used. However, we must initialize the SVE before SIMD can
> be used. Add a sanity check to make sure that we have completed the
> SVE setup before anyone uses the SIMD.
> 
> Cc: Ard Biesheuvel <ard.biesheuvel@...aro.org>
> Cc: Mark Rutland <mark.rutland@....com>
> Cc: Will Deacon <will.deacon@....com>
> Cc: Catalin Marinas <catalin.marinas@....com>
> Signed-off-by: Suzuki K Poulose <suzuki.poulose@....com>
> ---
> Discussion here : https://lkml.kernel.org/r/20191014145204.GS27757@arm.com

Re-reading this thread, I think the conclusion was more towards having a
WARN_ON in system_supports_fpsimd() (or may_use_simd()). We don't expect
code to start using neon before the SMP is initialised (other than
early_initcall(), the rest run after the secondary CPUs are brought up).

-- 
Catalin