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

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Date:   Thu, 23 Jul 2020 13:06:57 +0200
From:   Sedat Dilek <sedat.dilek@...il.com>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     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>, 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,
        Clang-Built-Linux ML <clang-built-linux@...glegroups.com>,
        Arnd Bergmann <arnd@...db.de>,
        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 11:17 AM Thomas Gleixner <tglx@...utronix.de> wrote:
>
> Thomas Gleixner <tglx@...utronix.de> writes:
> > Nick Desaulniers <ndesaulniers@...gle.com> writes:
> >
> > I'm glad I looked myself at this.
> >
> >> We also don't want to swap the use of "=q" with "=r". For 64b, it
> >> doesn't matter. For 32b, it's possible that a 32b register without a 8b
> >> lower alias (i.e. ESI, EDI, EBP) is selected which the assembler will
> >> then reject.
> >
> > The above is really garbage.
> >
> > We don't want? It's simply not possible to do so, because ...
> >
> > 64b,32b,8b. For heavens sake is it too much asked to write a changelog
> > with understandable wording instead of ambiguous abbreviations?
> >
> > There is no maximum character limit for changelogs.
>
> Gah. Hit send too fast.
>
> >> With this, Clang can finally build an i386 defconfig.
>
> With what? I can't find anything which explains the solution at the
> conceptual level. Sigh.
>

Hi,

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

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?

Doing some grep+ping:

$ git grep "ARCH=i386"
Documentation/kbuild/headers_install.rst:  make headers_install
ARCH=i386 INSTALL_HDR_PATH=/usr
tools/testing/ktest/examples/crosstests.conf:MAKE_CMD = make ARCH=i386
tools/testing/ktest/sample.conf:#MAKE_CMD = CC=i386-gcc AS=i386-as
make ARCH=i386

i386-gcc / i386-as - someone uses that?

Again my question (I did not do a diff):

$ make headers_install ARCH=i386 INSTALL_HDR_PATH=/usr
$ make headers_install ARCH=x86 INSTALL_HDR_PATH=/usr

...should generate the same?

To come back to "i386" again:

$ git grep i386 | grep ARCH

...reveals in top-level Makefile [2]:

376: # Additional ARCH settings for x86
377: ifeq ($(ARCH),i386)
378:        SRCARCH := x86

For me this means:

ARCH=i386 make ...
ARCH=x86 make ...

...should result in the same .config, but for what reason CONFIG_64BIT
is dropped when "ARCH=i386" is used.

Coming to a conclusion:

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.

Thanks.

Regards,
- Sedat -

[1] https://github.com/ClangBuiltLinux/linux/issues/194
[2] https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/Makefile#n376