Re: [PATCH v4 resend 3/7] arm64/fpsimd: Don't warn when EFI execution context is preemptible

Ard Biesheuvel
2025-11-04T16:00:47Z

On Tue, 4 Nov 2025 at 16:52, Will Deacon <will@kernel.org> wrote:
>
> On Wed, Oct 15, 2025 at 10:56:38PM +0200, Ard Biesheuvel wrote:
> > From: Ard Biesheuvel <ardb@kernel.org>
> >
> > Kernel mode FP/SIMD no longer requires preemption to be disabled, so
> > only warn on uses of FP/SIMD from preemptible context if the fallback
> > path is taken for cases where kernel mode NEON would not be allowed
> > otherwise.
> >
> > Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
> > ---
> > arch/arm64/kernel/fpsimd.c | 4 ++--
> > 1 file changed, 2 insertions(+), 2 deletions(-)
> >
> > diff --git a/arch/arm64/kernel/fpsimd.c b/arch/arm64/kernel/fpsimd.c
> > index e3f8f51748bc..3d848c89604e 100644
> > --- a/arch/arm64/kernel/fpsimd.c
> > +++ b/arch/arm64/kernel/fpsimd.c
> > @@ -1934,11 +1934,11 @@ void __efi_fpsimd_begin(void)
> > if (!system_supports_fpsimd())
> > return;
> >
> > - WARN_ON(preemptible());
> > -
> > if (may_use_simd()) {
> > kernel_neon_begin();
> > } else {
> > + WARN_ON(preemptible());
> > +
>
> Given that may_use_simd() returns false on systems without support for
> fpsimd, I wonder whether moving this WARN_ON() actually helps with
> anything.

This code is only reachable if system_supports_fpsimd() returns true,
so may_use_simd() will only return false here when running in
hardirq/NMI context.

> That is, you probably shouldn't be calling __efi_fpsimd_begin()
> from preemptible code regardless, no?
>

Not sure I follow you here: __efi_fpsimd_begin() is generally called
from preemptible code, and it is the EFI runtime wrappers themselves
that disable preemption - that is the whole point of this series.