linux-kernel - Re: [PATCH 3/3] x86/fpu: Don't support kernel-mode FPU when irqs

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Message-ID: <aCg2DSYp0nakwX3l@gmail.com>
Date: Sat, 17 May 2025 09:09:01 +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 3/3] x86/fpu: Don't support kernel-mode FPU when
 irqs_disabled()


* Eric Biggers <ebiggers@...nel.org> wrote:

> From: Eric Biggers <ebiggers@...gle.com>
> 
> Make irq_fpu_usable() return false when irqs_disabled().  That makes the
> irqs_disabled() checks in kernel_fpu_begin_mask() and kernel_fpu_end()
> unnecessary, so also remove those.
> 
> Rationale:
> 
> - There's no known use case for kernel-mode FPU when irqs_disabled().

Except EFI?

>   arm64 and riscv already disallow kernel-mode FPU when irqs_disabled().
>   __save_processor_state() previously did expect kernel_fpu_begin() and
>   kernel_fpu_end() to work when irqs_disabled(), but this was a
>   different use case and not actual kernel-mode FPU use.
> 
> - This is more efficient, since one call to irqs_disabled() replaces two
>   irqs_disabled() and one in_hardirq().

This is noise compared to the overhead of saving/restoring vector CPU 
context ...

> - This fixes irq_fpu_usable() to correctly return false during CPU
>   initialization.  Incorrectly returning true caused the SHA-256 library
>   code, which is called when loading AMD microcode, to take a
>   SIMD-optimized code path too early, causing a crash.  By correctly
>   returning false from irq_fpu_usable(), the generic SHA-256 code
>   correctly gets used instead.  (Note: SIMD-optimized SHA-256 doesn't
>   get enabled until subsys_initcall, but CPU hotplug can happen later.)

Alternatively we could set in_kernel_fpu during CPU bootstrap, and 
clear it once we know the FPU is usable? This is only a relatively 
short early boot period, with no scheduling, right?

Thanks,

	Ingo