linux-kernel - Re: clang asm-goto support (Was Re: [PATCH v2] x86/retpoline: Add clang support)

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Date:   Wed, 14 Feb 2018 11:44:08 +0100 (CET)
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Peter Zijlstra <peterz@...radead.org>
cc:     Chandler Carruth <chandlerc@...gle.com>,
        "Yatsina, Marina" <marina.yatsina@...el.com>,
        Kees Cook <keescook@...gle.com>,
        David Woodhouse <dwmw2@...radead.org>,
        "Kreitzer, David L" <david.l.kreitzer@...el.com>,
        "Grischenko, Andrei L" <andrei.l.grischenko@...el.com>,
        "rnk@...gle.com" <rnk@...gle.com>,
        LLVM Developers <llvm-dev@...ts.llvm.org>,
        "ehsan@...illa.com" <ehsan@...illa.com>,
        "Tayree, Coby" <coby.tayree@...el.com>,
        Matthias Braun <matze@...unis.de>,
        Dean Michael Berris <dean.berris@...il.com>,
        James Y Knight <jyknight@...gle.com>,
        Guenter Roeck <linux@...ck-us.net>, X86 ML <x86@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Alan Cox <gnomes@...rguk.ukuu.org.uk>,
        Rik van Riel <riel@...hat.com>,
        Andi Kleen <ak@...ux.intel.com>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Tom Lendacky <thomas.lendacky@....com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Jiri Kosina <jikos@...nel.org>,
        Andy Lutomirski <luto@...capital.net>,
        "Hansen, Dave" <dave.hansen@...el.com>,
        Tim Chen <tim.c.chen@...ux.intel.com>,
        Greg Kroah-Hartman <gregkh@...ux-foundation.org>,
        Paul Turner <pjt@...gle.com>,
        Stephen Hines <srhines@...gle.com>,
        Nick Desaulniers <ndesaulniers@...gle.com>
Subject: Re: clang asm-goto support (Was Re: [PATCH v2] x86/retpoline: Add
 clang support)

On Wed, 14 Feb 2018, Peter Zijlstra wrote:
> On Wed, Feb 14, 2018 at 08:41:43AM +0000, Chandler Carruth wrote:
> > Marina, Kees:
> > 
> > I think the combination of LKML and the LLVM developer's list is probably
> > too large a set of folks to have a (somewhat) lengthy exploration of
> > options and how to go about this and find a good path forward.
> 
> As one of the original authors of the jump-label implementation I'm
> saying there are no options. We're not going to do a second parallel
> implementation. If you want to build the kernel, you get to implement
> asm-goto, teh end.

Amen to that.

As x86 maintainer I'm really interested and willing to support llvm/clang
for x86 and the kernel in general, but asm-goto is not longer negotiable.

asm-goto is in the kernel since Oct 2010 and the relevant llvm/clang bug
#9295 is open since Feb 2011. All clang/llvm folks came up with in those 7
years is occasional handwaving about better solutions.

We really try to support old tools and tools which lack features for quite
a long time, but that's not a free pass for eternity.

That said, I'm not willing to wait another 7 years and I'm going to pull
the plug on compilers w/o asm-goto support soon whether llvm/clang gets
their act together or not.

Hint: GCC is not the only major compiler which supports asm-goto.

Thanks,

	tglx