linux-kernel - Re: [PATCH] arm64: kvm: use -fno-jump-tables with clang

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Date:   Wed, 23 May 2018 20:57:32 +0200
From:   Andrey Konovalov <andreyknvl@...gle.com>
To:     Nick Desaulniers <ndesaulniers@...gle.com>
Cc:     Marc Zyngier <marc.zyngier@....com>,
        Sami Tolvanen <samitolvanen@...gle.com>,
        christoffer.dall@....com,
        Takahiro Akashi <takahiro.akashi@...aro.org>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        kvmarm@...ts.cs.columbia.edu, LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] arm64: kvm: use -fno-jump-tables with clang

On Wed, May 23, 2018 at 7:47 PM, Nick Desaulniers
<ndesaulniers@...gle.com> wrote:
> On Wed, May 23, 2018 at 4:54 AM Andrey Konovalov <andreyknvl@...gle.com>
> wrote:
>> On Tue, May 22, 2018 at 8:28 PM, Nick Desaulniers
>> <ndesaulniers@...gle.com> wrote:
>> > On Fri, May 18, 2018 at 11:13 AM Marc Zyngier <marc.zyngier@....com>
> wrote:
>> >> > - you have checked that with a released version of the compiler, you
>> >
>> > On Tue, May 22, 2018 at 10:58 AM Andrey Konovalov <andreyknvl@...gle.com
>
>> > wrote:
>> >> Tested-by: Andrey Konovalov <andreyknvl@...gle.com>
>> >
>> > Hi Andrey,
>> > Thank you very much for this report.  Can you confirm as well the
> version
>> > of Clang that you were using?
>
>> I'm on 86852a40 ("[InstCombine] Calloc-ed strings optimizations").
>
>> > If it's not a binary release (built from
>> > source), would you be able to re-confirm with a released version?
>
>> Sure. Which release should I try and how do I get it?
>
> Maybe clang-6.0 as the latest release (though I suspect you may run into
> the recently-fixed-in-clang-7.0 "S" constraint bug that you reported).

Yes, and also into the "support for "r" prefixed variables in ARM
inline assembly" issue.

Tested on upstream commit ded4c39e (before both issues were
introduced) with -fno-jump-tables patch applied using clang 6.0.

Same result, the patch helps.

>
> I've had luck on debian based distributions installing from:
> http://apt.llvm.org/
>
> (These can be added to your /etc/apt/sources.list, then a `sudo apt update`
> and `sudo apt install clang-6.0`)
>
> If you're not able to add remote repositories (some employers block this ;)
> ), then you can find releases for download for a few different platforms:
> https://releases.llvm.org/
>
> For example, a quick:
> $ mkdir llvm-6.0
> $ cd !$
> $ wget
> https://releases.llvm.org/6.0.0/clang+llvm-6.0.0-x86_64-linux-gnu-debian8.tar.xz
> $ tar xvf clang+llvm-6.0.0-x86_64-linux-gnu-debian8.tar.xz
> $ ./clang+llvm-6.0.0-x86_64-linux-gnu-debian8/bin/clang-6.0 -v
> clang version 6.0.0 (tags/RELEASE_600/final)
> Target: x86_64-unknown-linux-gnu
> Thread model: posix
> InstalledDir: .../llvm-6.0/./clang+llvm-6.0.0-x86_64-linux-gnu-debian8/bin
> Found candidate GCC installation: ...
> Candidate multilib: .;@m64
> Selected multilib: .;@m64
>
> Seems to work.
> --
> Thanks,
> ~Nick Desaulniers