linux-kernel - Re: [PATCH v3 20/21] arm64/fpu: Enforce task-context only for generic kernel mode FPU

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Message-ID: <lEQKYlJspHGzmd8DyuKKcpZege3zgX8WlhnHUM044EmEhtaElIqZrX-cZ5ApNx9ylcn8fS1JjzCPcmEP72WXnn4H0JrsFYge3Jba-YngKrs=@protonmail.com>
Date: Thu, 09 Oct 2025 04:35:36 +0000
From: Jari Ruusu <jariruusu@...tonmail.com>
To: Ard Biesheuvel <ardb+git@...gle.com>
Cc: "linux-arm-kernel@...ts.infradead.org" <linux-arm-kernel@...ts.infradead.org>, "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>, "linux-crypto@...r.kernel.org" <linux-crypto@...r.kernel.org>, "herbert@...dor.apana.org.au" <herbert@...dor.apana.org.au>, "ebiggers@...nel.org" <ebiggers@...nel.org>, Ard Biesheuvel <ardb@...nel.org>
Subject: Re: [PATCH v3 20/21] arm64/fpu: Enforce task-context only for generic kernel mode FPU

Ard Biesheuvel wrote:
> So enforce that kernel_fpu_begin() can only be called from task context,
> and [redundantly] disable preemption. This removes the need for users of
> this API to provide a kernel mode FP/SIMD state after a future patch
> that makes that compulsory for preemptible task context.
[snip]
> --- a/arch/arm64/include/asm/fpu.h
> +++ b/arch/arm64/include/asm/fpu.h
[snip]
> +static inline void kernel_fpu_begin(void)
> +{
> +     BUG_ON(!in_task());
> +     preempt_disable();
                ^^^^^^^------this looks okay
> +     kernel_neon_begin();
> +}
> +
> +static inline void kernel_fpu_end(void)
> +{
> +     kernel_neon_end();
> +     preempt_disable();
                ^^^^^^^------this looks wrong
> +}
 
Maybe that second one should be preempt_enable()

--
Jari Ruusu  4096R/8132F189 12D6 4C3A DCDA 0AA4 27BD  ACDF F073 3C80 8132 F189