linux-kernel - Re: [PATCH v2 06/13] asm-generic: Use HAVE_FUNCTION

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Date:   Fri, 15 Oct 2021 18:02:00 +1000
From:   Nicholas Piggin <npiggin@...il.com>
To:     Andrew Morton <akpm@...ux-foundation.org>,
        Arnd Bergmann <arnd@...db.de>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Christophe Leroy <christophe.leroy@...roup.eu>,
        Helge Deller <deller@....de>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        "James E.J. Bottomley" <James.Bottomley@...senPartnership.com>,
        Kees Cook <keescook@...omium.org>,
        Michael Ellerman <mpe@...erman.id.au>,
        Paul Mackerras <paulus@...ba.org>
Cc:     linux-arch@...r.kernel.org, linux-ia64@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        linux-parisc@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org
Subject: Re: [PATCH v2 06/13] asm-generic: Use HAVE_FUNCTION_DESCRIPTORS to
 define associated stubs

Excerpts from Christophe Leroy's message of October 15, 2021 4:24 pm:
> 
> 
> Le 15/10/2021 à 08:16, Nicholas Piggin a écrit :
>> Excerpts from Christophe Leroy's message of October 14, 2021 3:49 pm:
>>> Replace HAVE_DEREFERENCE_FUNCTION_DESCRIPTOR by
>>> HAVE_FUNCTION_DESCRIPTORS and use it instead of
>>> 'dereference_function_descriptor' macro to know
>>> whether an arch has function descriptors.
>>>
>>> To limit churn in one of the following patches, use
>>> an #ifdef/#else construct with empty first part
>>> instead of an #ifndef in asm-generic/sections.h
>> 
>> Is it worth putting this into Kconfig if you're going to
>> change it? In any case
> 
> That was what I wanted to do in the begining but how can I do that in 
> Kconfig ?
> 
> #ifdef __powerpc64__
> #if defined(_CALL_ELF) && _CALL_ELF == 2
> #define PPC64_ELF_ABI_v2
> #else
> #define PPC64_ELF_ABI_v1
> #endif
> #endif /* __powerpc64__ */
> 
> #ifdef PPC64_ELF_ABI_v1
> #define HAVE_DEREFERENCE_FUNCTION_DESCRIPTOR 1

We have ELFv2 ABI / function descriptors iff big-endian so you could 
just select based on that.

I have a patch that makes the ABI version configurable which cleans
some of this up a bit, but that can be rebased on your series if we
ever merge it. Maybe just add BUILD_BUG_ONs in the above ifdef block
to ensure CONFIG_HAVE_FUNCTION_DESCRIPTORS was set the right way, so
I don't forget.

Thanks,
Nick