linux-kernel - Re: [PATCH] ARM: do not assemble iwmmxt.S with LLVM toolchain

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Date:   Wed, 15 Apr 2020 14:58:05 +0200
From:   Arnd Bergmann <arnd@...db.de>
To:     Ard Biesheuvel <ardb@...nel.org>
Cc:     Kees Cook <keescook@...omium.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Kristof Beyls <Kristof.Beyls@....com>,
        Stephen Hines <srhines@...gle.com>,
        Luis Lozano <llozano@...gle.com>,
        Russell King - ARM Linux admin <linux@...linux.org.uk>,
        Jian Cai <caij2003@...il.com>,
        Linus Walleij <linus.walleij@...aro.org>,
        Peter Smith <Peter.Smith@....com>,
        Stefan Agner <stefan@...er.ch>,
        David Howells <dhowells@...hat.com>,
        Mauro Carvalho Chehab <mchehab+samsung@...nel.org>,
        Manoj Gupta <manojgupta@...gle.com>,
        Benjamin Gaignard <benjamin.gaignard@...aro.org>,
        "Joel Fernandes (Google)" <joel@...lfernandes.org>,
        clang-built-linux <clang-built-linux@...glegroups.com>,
        Ilie Halip <ilie.halip@...il.com>,
        Masahiro Yamada <masahiroy@...nel.org>,
        Krzysztof Kozlowski <krzk@...nel.org>,
        Bartosz Golaszewski <bgolaszewski@...libre.com>,
        Sami Tolvanen <samitolvanen@...gle.com>,
        "Eric W. Biederman" <ebiederm@...ssion.com>,
        "Steven Rostedt (VMware)" <rostedt@...dmis.org>,
        Jian Cai <jiancai@...gle.com>,
        Doug Anderson <armlinux@...isordat.com>,
        Dan Williams <dan.j.williams@...el.com>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Patrick Bellasi <patrick.bellasi@....com>,
        Masami Hiramatsu <mhiramat@...nel.org>,
        Tejun Heo <tj@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH] ARM: do not assemble iwmmxt.S with LLVM toolchain

On Wed, Apr 15, 2020 at 12:32 PM Ard Biesheuvel <ardb@...nel.org> wrote:
>
> On Tue, 14 Apr 2020 at 22:53, Kees Cook <keescook@...omium.org> wrote:
> >
> > I don't know if this will help, but I feel like folks might be talking
> > past each other a little here. I see two primary issues that are
> > colliding, and I just want to call them out specifically...
>
> To reiterate my point: I strongly prefer minor asm surgery over
> elaborate Kconfig plumbing if it means we can retain the functionality
> even when using LLVM tools. In particular, the use of macros to
> implement missing ISA support should be considered before any other
> solution, as these are already being used widely across architectures
> to fill in such gaps.

+1

> > What's a good middle ground here? For VLAs, I ended up getting akpm's
> > help by having him add -Wvla to his local builds and send nag emails
> > to people when they added VLAs. That's not really a thing here, but it
> > seems like there should be a way to avoid losing ground (in this case,
> > it's the erosion of attention: repeated known-issue warnings means the
> > CI gets ignored for the warnings on newly added issues). It does seem
> > to me like adding the negative depends is a reasonable first step: it
> > marks what hard things need fixing later without losing coverage for
> > things that can be more easily fixed now with available resources.
> >
> > For the specific iwmmxt.S case, perhaps the solution is the suggested
> > changes? I imagine it should be possible to do a binary diff to see zero
> > changes before/after.
>
> This code has been around since 2004. It has never been possible to
> assemble it with Clang's assembler. So the only thing this patch gives
> you is the ability to switch from a .config where IWMMXT was disabled
> by hand to one where it gets disabled automatically by Kconfig.
>
> So what hard-won ground are we losing here? Did IWMMXT recently get
> enabled in a defconfig that you care about?

I mainly care about the build testing aspect here, it seems we are getting
close to having the clang integrated assembler working with all .S files
in the kernel (it used to work for none), and I'd like to do randconfig and
allmodconfig tests that include these as well. Disabling the option works
for me, but your suggestion with the added macros is clearly better.

        Arnd