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Date: Wed, 21 Feb 2018 17:03:25 +0100
From: Arnd Bergmann <>
To: Masahiro Yamada <>
Cc: Rich Felker <>, Kernel Hardening <>, 
	X86 ML <>, Paul Mackerras <>, "H. Peter Anvin" <>, 
	sparclinux <>, Sam Ravnborg <>, 
	Yoshinori Sato <>, Jonathan Corbet <>, 
	Richard Weinberger <>, Linux-sh list <>, 
	Ingo Molnar <>, Emese Revfy <>, Kees Cook <>, 
	uml-devel <>, 
	Linux Kbuild mailing list <>, 
	Peter Oberparleiter <>, Jeff Dike <>, 
	linuxppc-dev <>,, 
	Thomas Gleixner <>, Michal Marek <>, 
	Ulf Magnusson <>, Greg Kroah-Hartman <>, 
	Randy Dunlap <>, 
	"open list:DOCUMENTATION" <>, 
	Linux Kernel Mailing List <>, 
	Linus Torvalds <>, "David S. Miller" <>
Subject: Re: [PATCH 00/23] kconfig: move compiler capability tests to Kconfig

On Wed, Feb 21, 2018 at 1:57 PM, Masahiro Yamada
<> wrote:
> 2018-02-21 19:52 GMT+09:00 Arnd Bergmann <>:
>> On Wed, Feb 21, 2018 at 11:20 AM, Masahiro Yamada
>> <> wrote:
>>> 2018-02-21 18:56 GMT+09:00 Arnd Bergmann <>:
>>>> On Wed, Feb 21, 2018 at 8:38 AM, Masahiro Yamada
>>>> <> wrote:
>>>>> 2018-02-20 0:18 GMT+09:00 Ulf Magnusson <>:
> Hmm, I think I can implement those somehow.
> But, I hope we do not have many instances like this...
> If you know more naive cases, please share your knowledge.

One case that comes to mind would be architecture level selection on 32-bit
ARM, which is roughly this (I probably have some details wrong, but you
get the idea):

- older compilers don't support the latest architecture setting (-march=armv8
  or -march=armv7ve)
- newer compilers no longer support really old architectures (-march=armv4)
- setting -mthumb requires setting one of  -march=armv7-a, armv7ve, armv7-m or
  armv8 if the compiler doesn't default to those
- on a compiler that defaults to -marm, setting -march=armv7-m requires
  setting -mthumb (IIRC)
- really old compilers only support OABI, but not EABI
- newer compilers no longer support OABI
- mthumb requires EABI
- armv6 and higher are subtly broken with OABI, but only when using
  certain inline assembly with 64-bit arguments in register pairs.

I think we just shouldn't try to capture all of the above correctly in Kconfig


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