Re: [PATCH 3/3] x86/fpu: Don't support kernel-mode FPU when irqs_disabled()

Ingo Molnar
2025-05-17T07:09:06Z

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

> From: Eric Biggers <ebiggers@google.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