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

[Eric Biggers <ebiggers@kernel.org>]

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