linux-kernel - Re: [GIT PULL] x86/build changes for v4.17

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Message-ID: <20180404093007.GI4082@hirez.programming.kicks-ass.net>
Date:   Wed, 4 Apr 2018 11:30:07 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Matthias Kaehlcke <mka@...omium.org>
Cc:     Ingo Molnar <mingo@...nel.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Andrew Morton <akpm@...ux-foundation.org>,
        James Y Knight <jyknight@...gle.com>,
        Chandler Carruth <chandlerc@...gle.com>,
        Stephen Hines <srhines@...gle.com>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Kees Cook <keescook@...gle.com>,
        Guenter Roeck <groeck@...omium.org>,
        Greg Hackmann <ghackmann@...gle.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Subject: Re: [GIT PULL] x86/build changes for v4.17

On Tue, Apr 03, 2018 at 11:06:58AM -0700, Matthias Kaehlcke wrote:

> Yes, Chrome OS R67 (currently dev, soon beta) will ship a kernel built
> with Clang for multiple x86 Chromebooks.

But there are still _known_ miscompilations....

> Given that it takes time for distributions to roll out new compiler
> versions I would like to ask for a longer period of 'exemption' from
> asm-goto for Clang, at least if it isn't an actual burden for the
> kernel, like preventing important features from being added. An ideal
> time would be after the next-next LTS version, if this is considered
> too far out, after the next LTS version would be the second best time
> IMO. Let me be clear, this is *not* to delay the implementation of
> asm-goto, but to facilitate the use of Clang-built kernels by other
> projects and distributions, as well as automated builds of upstream
> kernels with Clang, without requiring necessarily the very latest
> version of Clang or extra patches.

I don't think that's sane or realistic, given that the very latest clang
is _known_ to miscompile the kernel. How can you want to support older
compilers that are therefore also known to not work correctly.

Next LTS is still a fair way out, if we take LTS release to be
every ~5 releases, the next one would be ~.19, that's still 3 releases
hence. That's a _long_ time.

I don't see the point in waiting that long for a compiler that doesn't
work even without asm-goto.