Re: reasoning beehind prohibiting VFP/NEON on AArch32

[Ard Biesheuvel <ard.biesheuvel@linaro.org>]

On 12 May 2018 at 23:11, Michael Zimmermann <sigmaepsilon92@gmail.com> wrote:
> For AArch32 the spec says in 2.3.5.3:
>> Floating point, SIMD, vector operations and other instruction set
> extensions must not
> be used.
>
> For AArch64 the spec says in 2.3.6.4:
>> Floating point and SIMD instructions may be used.
>
> So is there a reason why AArch32 is not allowed to use Floating point
> operations?
> I'd understand if this restriction was limited to runtime services only but
> I don't see how it makes sense for boot services.
>
> I've written a patch which adds NEON support to FrameBufferBltLib to
> increase the rendering performance(by a lot actually) for 24bit displays
> and thought about sending it to the mailing list - that's why the question
> came up.
>

The reason for the difference between AArch64 and the other EFI
architectures is that AArch64 does not have a softfloat ABI, so it is
impossible to compile floating point code [portably] without enabling
VFP/NEON. This is why AArch64 is the exception here.

Currently, the AArch32 CPU context structure [EFI_SYSTEM_CONTEXT_ARM]
does not cover VFP/NEON registers, and so they are not
preserved/restored when an interrupt is taken. This means you cannot
use VFP/NEON registers in an event handler or you will corrupt the
VFP/NEON state of the interrupted context.