[PATCH v2 01/20] arm64: Revert support for generic kernel mode FPU

[Ard Biesheuvel]

On Thu, 2 Oct 2025 at 09:23, Mark Brown <broonie at kernel.org> wrote:
>
> On Wed, Oct 01, 2025 at 11:02:03PM +0200, Ard Biesheuvel wrote:
>
> > However, dropping that flag allows the compiler to use FPU and SIMD
> > registers in other ways too, and for this reason, arm64 only permits
> > doing so in strictly controlled contexts, i.e., isolated compilation
> > units that get called from inside a kernel_neon_begin() and
> > kernel_neon_end() pair.
>
> > The users of the generic kernel mode FPU API lack such strict checks,
> > and this may result in userland FP/SIMD state to get corrupted, given
> > that touching FP/SIMD registers outside of a kernel_neon_begin/end pair
> > does not fault, but silently operates on the userland state without
> > preserving it.
>
> Oh dear, that's nasty - I didn't see the patch when it was going in:
>

Actually, there is a check, it just wasn't wired up correctly by the
amdgpu driver, due to the fact that it wraps kernel_fpu_begin()/end()
calls into its own API, which are therefore always made from a
compilation unit where it is supported.

The trick is to #include <linux/fpu.h> into the definition of their
own wrapper API, so that using /that/ from FP/SIMD code also triggers
a build error.

So I'll drop this patch.