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: <aQom3XPDmPTkq47d@willie-the-truck>
Date: Tue, 4 Nov 2025 16:16:29 +0000
From: Will Deacon <will@...nel.org>
To: Ard Biesheuvel <ardb@...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, Nov 04, 2025 at 05:00:34PM +0100, Ard Biesheuvel wrote:
> 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.

Sorry, I missed the check _right at the top of the hunk_!

> 
> > 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.

Ignore me. I was reviewing this against the upstream
arch_efi_call_virt_setup/teardown() functions rather than in the context
of the changes made later in this series (specifically patch 5).

Will