linux-kernel - Re: [PATCH] x86/fpu: Fix irq_fpu_usable() to return false during CPU onlining

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Message-ID: <aCrsiPd3u1-tEVd0@gmail.com>
Date: Mon, 19 May 2025 10:32:08 +0200
From: Ingo Molnar <mingo@...nel.org>
To: Eric Biggers <ebiggers@...nel.org>
Cc: x86@...nel.org, linux-kernel@...r.kernel.org,
	linux-crypto@...r.kernel.org, linux-pm@...r.kernel.org,
	Borislav Petkov <bp@...en8.de>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ayush Jain <Ayush.Jain3@....com>,
	Herbert Xu <herbert@...dor.apana.org.au>,
	Ard Biesheuvel <ardb@...nel.org>
Subject: Re: [PATCH] x86/fpu: Fix irq_fpu_usable() to return false during CPU
 onlining

> void fpu__init_cpu(void)
> {
>        fpu__init_cpu_generic();
>        fpu__init_cpu_xstate();
> +
> +       /* Start allowing kernel-mode FPU: */
> +       this_cpu_write(kernel_fpu_allowed, true);
> }

BTW., this is the chunk that fixes the crypto crash, right? If yes, 
then could you please split this from the main patch, with the main 
patch setting kernel_fpu_allowed very early, which should make the main 
patch an identity transformation with no (expected) change in behavior.

Likewise, the cpu_disable_common change should similarly replicate the 
current code, and should only be changed in the second patch.

Phasing it in like that should improve bisectability, for the off 
chance of some regression.

Thanks,

	Ingo