linux-kernel - Re: [PATCH 00/23] kconfig: move compiler capability tests to Kconfig

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Message-ID: <CAK8P3a3-eufj=HM4Mi+KaSLxuoPwtw0P4EfmtYfVcbnR0mVwAw@mail.gmail.com>
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