linux-kernel - Re: [PATCH v4 resend 3/7] arm64/fpsimd: Don't warn when EFI execution context is preemptible

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Message-ID: <aQohO07DpxlriQfJ@willie-the-truck>
Date: Tue, 4 Nov 2025 15:52:27 +0000
From: Will Deacon <will@...nel.org>
To: Ard Biesheuvel <ardb+git@...gle.com>
Cc: linux-efi@...r.kernel.org, linux-kernel@...r.kernel.org,
	linux-arm-kernel@...ts.infradead.org,
	Ard Biesheuvel <ardb@...nel.org>,
	Mark Rutland <mark.rutland@....com>,
	Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
	Peter Zijlstra <peterz@...radead.org>,
	Catalin Marinas <catalin.marinas@....com>,
	Mark Brown <broonie@...nel.org>,
	Pierre Gondois <Pierre.Gondois@....com>,
	Sami Mujawar <Sami.Mujawar@....com>
Subject: Re: [PATCH v4 resend 3/7] arm64/fpsimd: Don't warn when EFI
 execution context is preemptible

On Wed, Oct 15, 2025 at 10:56:38PM +0200, Ard Biesheuvel wrote:
> From: Ard Biesheuvel <ardb@...nel.org>
> 
> Kernel mode FP/SIMD no longer requires preemption to be disabled, so
> only warn on uses of FP/SIMD from preemptible context if the fallback
> path is taken for cases where kernel mode NEON would not be allowed
> otherwise.
> 
> Signed-off-by: Ard Biesheuvel <ardb@...nel.org>
> ---
>  arch/arm64/kernel/fpsimd.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/arm64/kernel/fpsimd.c b/arch/arm64/kernel/fpsimd.c
> index e3f8f51748bc..3d848c89604e 100644
> --- a/arch/arm64/kernel/fpsimd.c
> +++ b/arch/arm64/kernel/fpsimd.c
> @@ -1934,11 +1934,11 @@ void __efi_fpsimd_begin(void)
>  	if (!system_supports_fpsimd())
>  		return;
>  
> -	WARN_ON(preemptible());
> -
>  	if (may_use_simd()) {
>  		kernel_neon_begin();
>  	} else {
> +		WARN_ON(preemptible());
> +

Given that may_use_simd() returns false on systems without support for
fpsimd, I wonder whether moving this WARN_ON() actually helps with
anything. That is, you probably shouldn't be calling __efi_fpsimd_begin()
from preemptible code regardless, no?

Will