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

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Message-ID: <CAMj1kXHJ8QWLE=awDUpNa0Cf_1YXMQaW-Se1NvvavOocshDkTQ@mail.gmail.com>
Date: Tue, 4 Nov 2025 17:00:34 +0100
From: Ard Biesheuvel <ardb@...nel.org>
To: Will Deacon <will@...nel.org>
Cc: Ard Biesheuvel <ardb+git@...gle.com>, linux-efi@...r.kernel.org, 
	linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org, 
	Mark Rutland <mark.rutland@....com>, Sebastian Andrzej Siewior <bigeasy@...utronix.de>, 
	Peter Zijlstra <peterz@...radead.org>, Catalin Marinas <catalin.marinas@....com>, 
	Mark Brown <broonie@...nel.org>, Pierre Gondois <Pierre.Gondois@....com>, 
	Sami Mujawar <Sami.Mujawar@....com>
Subject: Re: [PATCH v4 resend 3/7] arm64/fpsimd: Don't warn when EFI execution
 context is preemptible

On Tue, 4 Nov 2025 at 16:52, Will Deacon <will@...nel.org> wrote:
>
> On Wed, Oct 15, 2025 at 10:56:38PM +0200, Ard Biesheuvel wrote:
> > From: Ard Biesheuvel <ardb@...nel.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@...nel.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.