linux-kernel - Re: Clang arm64 build is broken

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Message-ID: <CAAeHK+xDpESgjA-Y7JbOAXSzE_yUjz=zcb9carbLxceB3BN=6A@mail.gmail.com>
Date:   Tue, 22 May 2018 18:31:35 +0200
From:   Andrey Konovalov <andreyknvl@...gle.com>
To:     Nick Desaulniers <ndesaulniers@...gle.com>
Cc:     Marc Zyngier <marc.zyngier@....com>,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will.deacon@....com>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        kvmarm@...ts.cs.columbia.edu, LKML <linux-kernel@...r.kernel.org>,
        Kostya Serebryany <kcc@...gle.com>,
        Dmitry Vyukov <dvyukov@...gle.com>,
        Alexander Potapenko <glider@...gle.com>,
        christoffer.dall@....com,
        Ard Biesheuvel <ard.biesheuvel@...aro.org>
Subject: Re: Clang arm64 build is broken

On Mon, May 14, 2018 at 6:24 PM, Nick Desaulniers
<ndesaulniers@...gle.com> wrote:
> On Fri, Apr 20, 2018 at 7:59 AM Andrey Konovalov <andreyknvl@...gle.com>
> wrote:
>> On Fri, Apr 20, 2018 at 10:13 AM, Marc Zyngier <marc.zyngier@....com>
> wrote:
>> >> The issue is that
>> >> clang doesn't know about the "S" asm constraint. I reported this to
>> >> clang [2], and hopefully this will get fixed. In the meantime, would
>> >> it possible to work around using the "S" constraint in the kernel?
>> >
>> > I have no idea, I've never used clang to build the kernel. Clang isn't
>> > really supported to build the arm64 kernel anyway (as you mention
>> > below), and working around clang deficiencies would mean that we leave
>> > with the workaround forever. I'd rather enable clang once it is at
>> > feature parity with GCC.
>
>> The fact that there are some existing issues with building arm64
>> kernel with clang doesn't sound like a good justification for adding
>> new issues :)
>
>> However in this case I do believe that this is more of a bug in clang
>> that should be fixed.
>
> Just to follow up with this thread;
>
> Support for "S" constraints is being (re-)added to Clang in:
> https://reviews.llvm.org/D46745

Hi Nick!

I can confirm that the latest clang (which includes this patch) is
able to build the kernel with CONFIG_KVM enabled.

Thanks!