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

[Mark Brown]

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:

Reviewed-by: Mark Brown <broonie@xxxxxxxxxx>

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