linux-kernel - Re: mainline build failure of powerpc allmodconfig for prom_init

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Date:   Tue, 19 Jul 2022 23:35:33 +1000
From:   Michael Ellerman <mpe@...erman.id.au>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Sudip Mukherjee <sudipm.mukherjee@...il.com>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Paul Mackerras <paulus@...ba.org>,
        Kees Cook <keescook@...omium.org>,
        linuxppc-dev <linuxppc-dev@...ts.ozlabs.org>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        linux-hardening@...r.kernel.org
Subject: Re: mainline build failure of powerpc allmodconfig for prom_init_check

Linus Torvalds <torvalds@...ux-foundation.org> writes:
> On Sun, Jul 17, 2022 at 9:41 PM Michael Ellerman <mpe@...erman.id.au> wrote:
>>
>> >         li 4,254                 #,
>>
>> Here we load 254 into r4, which is the 2nd parameter to memset (c).
>
> I love how even powerpc people know that "4" is bogus, and have to
> make it clear that it means "r4".

I wouldn't say it's bogus, I was just translating from asm to English :)

But I agree it's preferable to use a proper register name rather than a
bare integer. I never write asm using bare integers, I always use r4 or
%r4, because as you say it's too easy to get mixed up otherwise.

When looking at generated code I usually use objdump -d output, which
uses the "r4" syntax.

> It's not even an IBM thing. S390 uses perfectly sane register syntax,
> and calls things '%r4" etc.

as accepts that syntax if you tell it to.

We use that syntax in some of our newer inline asm blocks.

> The human-written asm files have those #define's in headers just to
> make things slightly more legible, because apparently the assembler
> doesn't even *accept* the sane names.

I would like to switch to using %rX everywhere and get rid of those
defines, but it's never seemed like it's worth the churn. We have ~48K
lines of asm in arch/powerpc.

cheers