[PATCH 07/24] arm64: ilp32: add documentation on the ILP32 ABI for ARM64

Dr. Philipp Tomsich
2018-05-24T12:24:19Z

Yury & Pavel,

> On 24 May 2018, at 14:15, Yury Norov <ynorov@caviumnetworks.com> wrote:
>
> Hi Pavel,
>
> On Wed, May 23, 2018 at 04:06:20PM +0200, Pavel Machek wrote:
>> On Wed 2018-05-16 11:18:52, Yury Norov wrote:
>>> Based on Andrew Pinski's patch-series.
>>>
>>> Signed-off-by: Yury Norov <ynorov@caviumnetworks.com>
>>
>> So Andrew's signoff should be here?
>
> Yes it should, but it lost since v4. I'll restore it.
>
>>> ---
>>> Documentation/arm64/ilp32.txt | 45 +++++++++++++++++++++++++++++++++++
>>> 1 file changed, 45 insertions(+)
>>> create mode 100644 Documentation/arm64/ilp32.txt
>>>
>>> diff --git a/Documentation/arm64/ilp32.txt b/Documentation/arm64/ilp32.txt
>>> new file mode 100644
>>> index 000000000000..d0fd5109c4b2
>>> --- /dev/null
>>> +++ b/Documentation/arm64/ilp32.txt
>>> @@ -0,0 +1,45 @@
>>> +ILP32 AARCH64 SYSCALL ABI
>>> +=========================
>>> +
>>> +This document describes the ILP32 syscall ABI and where it differs
>>> +from the generic compat linux syscall interface.
>>
>> I was hoping to learn what ILP32 is / what is it good for, but no,
>> this does not tell me... it would be good to do a short explanation
>> here, and maybe reference it from cover letter of the series...
>> Pavel
>
> ILP32 is ABI acronym that means "Integers, Longs and Pointers are 32-bit".
> And LP64 means "Longs and Pointers are 64-bit?.

Just a nitpick: ILP32 is in fact just the memory model, but calling from ILP32
code into the Linux kernel requires modifications to the syscall-ABI due to
datastructure layout changing (every time a pointer or a ?long? is present in
a structure). As such structures are passed between the userspace and the
kernel (and also due to the fact that time_t is an ?unsigned long? in the C
language standard), modifications to the syscall ABI in Linux are needed to
support ILP32 processes calling into the kernel.

Things get a bit more involved, as the final consensus was to pass 64bit
quantities in the lower half of 2 64bit registers instead of as a single register:
this makes the way (on AArch64) that an ILP32 process calls into the kernel
more dissimilar from a LP64 process calling the same syscall.

What this rambling boils down to is: ?ILP32" is the memory model, whereas
this series deals with the ?Linux/AArch64 syscall ABI for ILP32 processes?.

Thanks,
Phil.

>
> There's AN490 - "ILP32 for AArch64 Whitepaper" from ARM which covers
> the topic:
> http://infocenter.arm.com/help/index.jsp?topic=/com.arm.doc.dai0490a/ar01s01.html
>
> And some talks:
> http://connect.linaro.org/resource/bkk16/bkk16-305b/
>
> Briefly, ILP32 is 32-bit ABI that works with AARCH64 instruction set. It looks
> better in some performance tests, and is useful for compatibility with 32-bit
> legacy code.
>
> If you're more familiar with x86 terminology, in ARM world LP64 corresponds
> to x86_64, AARCH32_EL0 corresponds to x86_32, and ILP32 corresponds to x32
> ABI.
>
> I'll add link to AN490 in next submission.
>
> Yury
>
>> --
>> (english) http://www.livejournal.com/~pavelmachek
>> (cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html