From mboxrd@z Thu Jan  1 00:00:00 1970
From: Nicholas Piggin <npiggin@gmail.com>
Date: Fri, 15 Oct 2021 08:02:00 +0000
Subject: Re: [PATCH v2 06/13] asm-generic: Use HAVE_FUNCTION_DESCRIPTORS to define associated stubs
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To: Andrew Morton <akpm@linux-foundation.org>, Arnd Bergmann <arnd@arndb.de>, Benjamin Herrenschmidt <benh@kernel.crashing.org>, Christophe Leroy <christophe.leroy@csgroup.eu>, Helge Deller <deller@gmx.de>, Greg Kroah-Hartman <gregkh@linuxfoundation.org>, "James E.J. Bottomley" <James.Bottomley@HansenPartnership.com>, Kees Cook <keescook@chromium.org>, Michael Ellerman <mpe@ellerman.id.au>, Paul Mackerras <paulus@samba.org>
Cc: linux-arch@vger.kernel.org, linux-ia64@vger.kernel.org, linux-kernel@vger.kernel.org, linux-mm@kvack.org, linux-parisc@vger.kernel.org, linuxppc-dev@lists.ozlabs.org

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