linux-kernel - Re: [RFC PATCH 1/2] x86/fpu: make kernel-mode FPU reliably usable in softirqs

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Message-ID: <20250228035924.GC5588@sol.localdomain>
Date: Thu, 27 Feb 2025 19:59:24 -0800
From: Eric Biggers <ebiggers@...nel.org>
To: x86@...nel.org
Cc: linux-crypto@...r.kernel.org, linux-kernel@...r.kernel.org,
	Ard Biesheuvel <ardb@...nel.org>,
	Ben Greear <greearb@...delatech.com>,
	Xiao Liang <shaw.leon@...il.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
	Dave Hansen <dave.hansen@...ux.intel.com>,
	Andy Lutomirski <luto@...nel.org>,
	"Jason A . Donenfeld" <Jason@...c4.com>
Subject: Re: [RFC PATCH 1/2] x86/fpu: make kernel-mode FPU reliably usable in
 softirqs

On Wed, Feb 19, 2025 at 09:13:24PM -0800, Eric Biggers wrote:
> To comply with the requirements of local_bh_disable and local_bh_enable,
> this change also removes support for kernel-mode FPU in hardirq context
> or with hardirqs disabled.  This should not be a problem, though.  There
> does not appear to be any use case for kernel-mode FPU in such contexts,
> and notably arm64 and riscv already have these same conditions.

I found a problem with this assumption: the system suspend and resume code calls
kernel_fpu_begin() and kernel_fpu_end() with hardirqs disabled.  See
__save_processor_state() and __restore_processor_state() in
arch/x86/power/cpu.c.  That triggers the WARN_ON_FPU(!irq_fpu_usable()).

I think there are two directions we could go with this: either choose a solution
that keeps kernel_fpu_begin() usable with hardirqs disabled; or change
__save_processor_state() and __restore_processor_state() to save/restore the FPU
registers directly, e.g. via save_fpregs_to_fpstate() and
restore_fpregs_from_fpstate().  (Kernel-mode FPU isn't actually being used in
this case, so a more direct save/restore might make sense here.)

- Eric