linux-kernel - Re: [PATCH v3 11/11] x86: support i386 with Clang

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Date:   Thu, 23 Jul 2020 13:42:16 +0200
From:   Arnd Bergmann <arnd@...db.de>
To:     Sedat Dilek <sedat.dilek@...il.com>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Dennis Zhou <dennis@...nel.org>, Tejun Heo <tj@...nel.org>,
        Christoph Lameter <cl@...ux.com>,
        "the arch/x86 maintainers" <x86@...nel.org>,
        "H. Peter Anvin" <hpa@...or.com>,
        Al Viro <viro@...iv.linux.org.uk>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Peter Zijlstra <peterz@...radead.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Clang-Built-Linux ML <clang-built-linux@...glegroups.com>,
        David Woodhouse <dwmw2@...radead.org>,
        Dmitry Golovin <dima@...ovin.in>,
        Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [PATCH v3 11/11] x86: support i386 with Clang

On Thu, Jul 23, 2020 at 1:07 PM Sedat Dilek <sedat.dilek@...il.com> wrote:
> On Thu, Jul 23, 2020 at 11:17 AM Thomas Gleixner <tglx@...utronix.de> wrote:
> > Thomas Gleixner <tglx@...utronix.de> writes:
>
> I have applied this patch-series v3 but some basics of "i386" usage
> are not clear to me when I wanted to test it and give some feedback.
>
> [1] is the original place in CBL where this was reported and I have
> commented on this.
>
> Beyond some old cruft in i386_defconfig like non-existent
> "CONFIG_CRYPTO_AES_586" I have some fundamental questions:
>
> What means "ARCH=i386" and where it is used (for)?
>
> I can do:
>
> $ ARCH=x86 make V=1 -j3 $MAKE_OPTS i386_defconfig
> $ make V=1 -j3 $MAKE_OPTS i386_defconfig
>
> ...which results in the same .config.
>
> Whereas when I do:
>
> $ ARCH=i386 make V=1 -j3 $MAKE_OPTS i386_defconfig
>
> ...drops CONFIG_64BIT line entirely.
>
> But "# CONFIG_64BIT is not set" is explicitly set in
> arch/x86/configs/i386_defconfig but gets dropped.
>
> Unsure if above is the same like:
> $ ARCH=i386 make V=1 -j3 $MAKE_OPTS defconfig

The logic was introduced when arch/i386 and arch/x86_64 got
merged into arch/x86, to stay compatible with the original behavior
that would produce a 32-bit or 64-bit kernel depending on which
machine you are running on.

There are probably not a lot of people building kernels on 32-bit
machines any more (real 32-bit machines are really slow compared
to modern ones, and 64-bit machines running 32-bit distros usually
want a 64-bit kernel), so it could in theory be changed.

It will certainly break someone's workflow though, so nobody has
proposed actually changing it so far.

> When generating via "make ... i386_defconfig" modern gcc-9 and and a
> snapshot version of clang-11 build both with:
>
> $ ARCH=x86 make V=1 -j3 $MAKE_OPTS
> ... -march=i686 -mtune=generic ...
>
> Checking generated .config reveals:
>
> CONFIG_M686=y
>
> So, I guess modern compilers do at least support "i686" as lowest CPU?

i686 compiler support goes back to the 1990s, and the kernel now
requires at least gcc-4.9 from 2014, so yes.

> Nick D. says:
> > I usually test with make ... i386_defconfig.
>
> Can you enlighten a bit?
>
> Of course, I can send a patch to remove the "CONFIG_CRYPTO_AES_586=y"
> line from i386_defconfig.

The "i386" in i386_defconfig is just a synonym for x86-32, it does not
imply a particular CPU generation. The original i386 is no longer supported,
i486sx (barely) is and in practice most 32-bit Linux code gets compiled
for some variant of i586 or i686 variant but run on 64-bit hardware.

      Arnd