linux-kernel - Re: [PATCH 1/3] crypto: X25519 low-level primitives for ppc64le.

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Date: Thu, 16 May 2024 15:42:15 +0200
From: Andy Polyakov <appro@...ptogams.org>
To: Michael Ellerman <mpe@...erman.id.au>, Danny Tsen <dtsen@...ux.ibm.com>,
 linux-crypto@...r.kernel.org
Cc: herbert@...dor.apana.org.au, leitao@...ian.org, nayna@...ux.ibm.com,
 linux-kernel@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org,
 ltcgcw@...ux.vnet.ibm.com, dtsen@...ibm.com
Subject: Re: [PATCH 1/3] crypto: X25519 low-level primitives for ppc64le.

Hi,

>>>> +.abiversion	2
>>>
>>> I'd prefer that was left to the compiler flags.
>>
>> Problem is that it's the compiler that is responsible for providing this
>> directive in the intermediate .s prior invoking the assembler. And there
>> is no assembler flag to pass through -Wa.
> 
> Hmm, right. But none of our existing .S files include .abiversion
> directives.
> 
> We build .S files with gcc, passing -mabi=elfv2, but it seems to have no
> effect.
> 
> So all the intermediate .o's generated from .S files are not ELFv2:
> 
>    $ find .build/ -name '*.o' | xargs file | grep Unspecified
>    .build/arch/powerpc/kernel/vdso/note-64.o:                        ELF 64-bit LSB relocatable, 64-bit PowerPC or cisco 7500, Unspecified or Power ELF V1 ABI, version 1 (SYSV), not stripped

I would guess that contemporary linker is more forgiving than it was 
back then when the .abiversion directive was added. If it works now, 
then it of course can be omitted. I suppose my original remark should be 
viewed rather as "you can't replace it with a command line option" than 
"you can't make it work without it." :-)

> But the actual code follows ELFv2, because we wrote it that way, and I
> guess the linker doesn't look at the actual ABI version of the .o ?
> 
> So it currently works. But it's kind of gross that those .o files are
> not ELFv2 for an ELFv2 build.

Well, as far as passing base types and pointers to/from assembly goes, 
there are no differences between the versions. Then it's a question of 
meaning assigned to r2 and r13, but as long as you don't touch them, you 
can freely reuse the code with either ABI. With this in mind the 
abiversion directive is effectively reduced to just a marker in the .o 
file. In other words the instruction sequences by themselves are 
customarily ABI-neutral, at least in "general calculation" modules such 
as the suggested one, so that if it works 100% without the .abiversion 
directive, then it can be safely omitted.

Cheers.