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

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Message-ID: <CAK8P3a2BOQt5Y0B4271WqaJvvWqfxHxD3S6r1i1xONvb+CJ5Dg@mail.gmail.com>
Date:   Wed, 21 Feb 2018 10:56:24 +0100
From:   Arnd Bergmann <arnd@...db.de>
To:     Masahiro Yamada <yamada.masahiro@...ionext.com>
Cc:     Ulf Magnusson <ulfalizer@...il.com>, Rich Felker <dalias@...c.org>,
        Linux-sh list <linux-sh@...r.kernel.org>,
        Kernel Hardening <kernel-hardening@...ts.openwall.com>,
        Paul Mackerras <paulus@...ba.org>,
        "H. Peter Anvin" <hpa@...or.com>,
        sparclinux <sparclinux@...r.kernel.org>,
        Sam Ravnborg <sam@...nborg.org>,
        uml-devel <user-mode-linux-devel@...ts.sourceforge.net>,
        Yoshinori Sato <ysato@...rs.sourceforge.jp>,
        Jonathan Corbet <corbet@....net>, X86 ML <x86@...nel.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Ingo Molnar <mingo@...hat.com>,
        Emese Revfy <re.emese@...il.com>,
        Kees Cook <keescook@...omium.org>,
        Linux Kbuild mailing list <linux-kbuild@...r.kernel.org>,
        Peter Oberparleiter <oberpar@...ux.vnet.ibm.com>,
        Jeff Dike <jdike@...toit.com>,
        user-mode-linux-user@...ts.sourceforge.net,
        Thomas Gleixner <tglx@...utronix.de>,
        Michal Marek <michal.lkml@...kovi.net>,
        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>,
        Richard Weinberger <richard@....at>,
        linuxppc-dev <linuxppc-dev@...ts.ozlabs.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 8:38 AM, Masahiro Yamada
<yamada.masahiro@...ionext.com> wrote:
> 2018-02-20 0:18 GMT+09:00 Ulf Magnusson <ulfalizer@...il.com>:
>
>>>
>>> I'm not happy that we in one context can reference CONFIG variables
>>> directly, but inside the $(call ...) and $(shell ...) needs the $ prefix.
>>> But I could not come up with something un-ambigious where this could be avoided.
>>
>> I think we should be careful about allowing references to config
>> symbols. It mixes up the parsing and evaluation phases, since $() is
>> expanded during parsing (which I consider a feature and think is
>> needed to retain sanity).
>>
>> Patch 06/23 removes the last existing instance of symbol references in
>> strings by getting rid of 'option env'. That's an improvement to me.
>> We shouldn't add it back.
>
>
> This is really important design decision,
> so I'd like to hear a little more from experts.
>
>
> For example, x86 allows users to choose sub-arch, either 'i386' or 'x86_64'.
>
> https://github.com/torvalds/linux/blob/v4.16-rc2/arch/x86/Kconfig#L4
>
>
>
> If the user toggles CONFIG_64BIT,
> the bi-arch compiler will work in a slightly different mode
> (at least, back-end parts)
>
> So, my question is, is there a case,
>
> $(cc-option, -m32 -foo) is y, but
> $(cc-option, -m64 -foo) is n  ?
> (or vice versa)
>
>
> If the answer is yes, $(cc-option -foo) would have to be re-calculated
> every time CONFIG_64BIT is toggled.
>
> This is what I'd like to avoid, though.

The -m32/-m64 trick (and -mbig-endian/-mlittle-endian on other architectures
as well as a couple of other flags) only works if the compiler is configured to
support it. In other cases (e.g. big-endian xtensa), the kernel always
detects what the compiler does and silently configures itself to match
using Makefile logic.

On x86, compilers are usually built as bi-arch, but you can build one that
only allows one of them.

I can see two reasonable ways out:

- we don't use  $(cc-option -foo) in a case like this, and instead require the
  user to have a matching toolchain.
- we could make the 32/64 selection on x86 a 'choice' statement where
  each option depends on both the ARCH= variable and the
  $(cc-option, -m32)/ $(cc-option, -m64) output.

       Arnd