From: Ard Biesheuvel <ard.biesheuvel@linaro.org>
To: Michael Zimmermann <sigmaepsilon92@gmail.com>
Cc: "edk2-devel@lists.01.org" <edk2-devel@lists.01.org>,
Leif Lindholm <leif.lindholm@linaro.org>
Subject: Re: reasoning beehind prohibiting VFP/NEON on AArch32
Date: Sun, 13 May 2018 12:16:47 +0200 [thread overview]
Message-ID: <CAKv+Gu9NoofOV3oFQwWU8emquFxcp1npTNtnNiXhvtDgXC4ajg@mail.gmail.com> (raw)
In-Reply-To: <CAN9vWDLGMsTBBKjp1VEheBWuf1cz=sxNtQv5TT_YcNV916Gfqw@mail.gmail.com>
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.
next prev parent reply other threads:[~2018-05-13 10:16 UTC|newest]
Thread overview: 7+ messages / expand[flat|nested] mbox.gz Atom feed top
2018-05-12 21:11 reasoning beehind prohibiting VFP/NEON on AArch32 Michael Zimmermann
2018-05-13 7:32 ` Ard Biesheuvel
2018-05-13 9:48 ` Michael Zimmermann
2018-05-13 10:16 ` Ard Biesheuvel [this message]
2018-05-13 10:58 ` Michael Zimmermann
2018-05-13 11:39 ` Ard Biesheuvel
2018-05-13 12:49 ` Michael Zimmermann
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