linux-kernel - Re: [tip:x86/urgent] x86/mm/kaslr: Use the _ASM_MUL macro for multiplication to work around Clang incompatibility

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Message-ID: <B5C90399-63CA-4F70-8D09-CFB5E55A0D42@zytor.com>
Date:   Sun, 07 May 2017 08:42:52 -0700
From:   hpa@...or.com
To:     Peter Zijlstra <peterz@...radead.org>,
        Michael Davidson <md@...gle.com>
CC:     Linus Torvalds <torvalds@...ux-foundation.org>,
        Matthias Kaehlcke <mka@...omium.org>,
        Ingo Molnar <mingo@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>, grundler@...omium.org,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Greg Hackmann <ghackmann@...gle.com>,
        Kees Cook <keescook@...omium.org>,
        "linux-tip-commits@...r.kernel.org" 
        <linux-tip-commits@...r.kernel.org>
Subject: Re: [tip:x86/urgent] x86/mm/kaslr: Use the _ASM_MUL macro for multiplication to work around Clang incompatibility

On May 6, 2017 1:16:35 AM PDT, Peter Zijlstra <peterz@...radead.org> wrote:
>On Fri, May 05, 2017 at 01:36:34PM -0700, Michael Davidson wrote:
>
>> There are a few lingering places in the kernel which use variable
>> length arrays in structs (eg the raid10 driver) which don't build
>with
>> clang and that is about it.
>
>So the other point I raised is lack of asm goto (and asm flags output).
>
>Without that our static key infrastructure reverts to runtime branches
>and affects performance.
>
>> So, while I completely understand the resistance to adding arbitrary
>> hacks to the kernel just to support another compiler it is important
>> to also understand just how close things are to "just working".
>
>Reading up on the LLVM thread on asm goto they appear to want to
>provide
>an intrinsic to allow doing the patchable branch thing. That would be
>fairly limiting, and the proposal I've seen doesn't even cover the two
>(or rather 4) states of patchable branches we have in the kernel.
>
>Not to mention that such an intrinsic doesn't even begin to cover all
>the other (perhaps creative) uses we have got asm goto used.
>
>But my main point is that we'd have to rewrite and maintain _two_
>versions of the static key infrastructure if we were to support LLVM's
>intrinsic and the GCC asm goto. That is a very undesirable place to be.
>
>So while they'll say they support the feature, I'll say its worthless
>since I'm not inclined to support their variant of it. As is, I'm not
>getting the feeling the LLVM team really cares about Linux.

This has been a common problem with LLVM: they say that they will provide feature compatibility with gcc, but then they say this or that gcc extension "doesn't make sense" and is something they don't want to support.
-- 
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