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Date: Wed, 21 Feb 2018 17:03:25 +0100
From: Arnd Bergmann <arnd@...db.de>
To: Masahiro Yamada <yamada.masahiro@...ionext.com>
Cc: Rich Felker <dalias@...c.org>, Kernel Hardening <kernel-hardening@...ts.openwall.com>, 
	X86 ML <x86@...nel.org>, Paul Mackerras <paulus@...ba.org>, "H. Peter Anvin" <hpa@...or.com>, 
	sparclinux <sparclinux@...r.kernel.org>, Sam Ravnborg <sam@...nborg.org>, 
	Yoshinori Sato <ysato@...rs.sourceforge.jp>, Jonathan Corbet <corbet@....net>, 
	Richard Weinberger <richard@....at>, Linux-sh list <linux-sh@...r.kernel.org>, 
	Ingo Molnar <mingo@...hat.com>, Emese Revfy <re.emese@...il.com>, Kees Cook <keescook@...omium.org>, 
	uml-devel <user-mode-linux-devel@...ts.sourceforge.net>, 
	Linux Kbuild mailing list <linux-kbuild@...r.kernel.org>, 
	Peter Oberparleiter <oberpar@...ux.vnet.ibm.com>, Jeff Dike <jdike@...toit.com>, 
	linuxppc-dev <linuxppc-dev@...ts.ozlabs.org>, user-mode-linux-user@...ts.sourceforge.net, 
	Thomas Gleixner <tglx@...utronix.de>, Michal Marek <michal.lkml@...kovi.net>, 
	Ulf Magnusson <ulfalizer@...il.com>, Greg Kroah-Hartman <gregkh@...uxfoundation.org>, 
	Randy Dunlap <rdunlap@...radead.org>, 
	"open list:DOCUMENTATION" <linux-doc@...r.kernel.org>, 
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>, 
	Linus Torvalds <torvalds@...ux-foundation.org>, "David S. Miller" <davem@...emloft.net>
Subject: Re: [PATCH 00/23] kconfig: move compiler capability tests to Kconfig

On Wed, Feb 21, 2018 at 1:57 PM, Masahiro Yamada
<yamada.masahiro@...ionext.com> wrote:
> 2018-02-21 19:52 GMT+09:00 Arnd Bergmann <arnd@...db.de>:
>> On Wed, Feb 21, 2018 at 11:20 AM, Masahiro Yamada
>> <yamada.masahiro@...ionext.com> wrote:
>>> 2018-02-21 18:56 GMT+09:00 Arnd Bergmann <arnd@...db.de>:
>>>> On Wed, Feb 21, 2018 at 8:38 AM, Masahiro Yamada
>>>> <yamada.masahiro@...ionext.com> wrote:
>>>>> 2018-02-20 0:18 GMT+09:00 Ulf Magnusson <ulfalizer@...il.com>:
>
> 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
conditionals.

       Arnd

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