On 13 May 2018 at 11:48, Michael Zimmermann <sigmaepsilo...@gmail.com> wrote:
> So basically using them should be safe as long as you're in
> EfiGetCurrentTpl() < TPL_HIGH_LEVEL, right?

No, the other way around. You should raise the TPL to TPL_HIGH_LEVEL
to prevent being interrupted by something that may corrupt the NEON
registers.

> Also, it'd probably be trivial to add  VFP/NEON regs to
> EFI_SYSTEM_CONTEXT_ARM though that wouldn't help when writing apps for
> existing uefi platforms.

EFI_SYSTEM_CONTEXT_ARM is covered by the UEFI spec, so that is not
going to change.

> On Sun, May 13, 2018 at 9:32 AM Ard Biesheuvel <ard.biesheu...@linaro.org>
> wrote:
>
>> On 12 May 2018 at 23:11, Michael Zimmermann <sigmaepsilo...@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.
_______________________________________________
edk2-devel mailing list
edk2-devel@lists.01.org
https://lists.01.org/mailman/listinfo/edk2-devel

Reply via email to