Re: reasoning beehind prohibiting VFP/NEON on AArch32

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

On 13 May 2018 at 12:58, Michael Zimmermann <sigmaepsilon92@gmail.com> wrote:
>> 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.
> But isn't that only necessary if you assume that interrupt-handlers use VFP
> registers?

Event handlers are called from the timer interrupt handler. So unless
you want to restrict use of the NEON to non-event handler context
(which is not generally possible for libraries), you will need to
raise the TPL to avoid any interruptions.

> afaik on ARM <TPL_HIGH_LEVEL events are never called from the timer
> interrupt handler so basically if you're going to be interrupted during VFP
> operations no other <TPL_HIGH_LEVEL code should ever run.
>
> Please correct me if I'm misunderstanding something.

I don't follow. Your NEON code running at TPL_APPLICATION may be
interrupted at any time by event handlers running at higher TPL
levels. If such code uses the NEON, it will corrupt your register
file.



> On Sun, May 13, 2018 at 12:16 PM Ard Biesheuvel <ard.biesheuvel@linaro.org>
> wrote:
>
>> On 13 May 2018 at 11:48, Michael Zimmermann <sigmaepsilon92@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.biesheuvel@linaro.org>
>> > wrote:
>> >
>> >> 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.