linux-kernel - Re: [PATCH] arm64: lse: fix LSE atomics with LLVM's integrated assembler

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Message-ID: <240f81a1-4fe5-0ff3-f97a-0c9aa6b68e03@arm.com>
Date:   Wed, 9 Oct 2019 11:03:16 +0100
From:   Robin Murphy <robin.murphy@....com>
To:     Ard Biesheuvel <ard.biesheuvel@...aro.org>
Cc:     Sami Tolvanen <samitolvanen@...gle.com>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Kees Cook <keescook@...omium.org>,
        Catalin Marinas <catalin.marinas@....com>,
        LKML <linux-kernel@...r.kernel.org>,
        clang-built-linux <clang-built-linux@...glegroups.com>,
        Andrew Murray <andrew.murray@....com>,
        Will Deacon <will@...nel.org>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>
Subject: Re: [PATCH] arm64: lse: fix LSE atomics with LLVM's integrated
 assembler

On 2019-10-08 10:03 pm, Ard Biesheuvel wrote:
> On Tue, 8 Oct 2019 at 18:19, Robin Murphy <robin.murphy@....com> wrote:
>>
>> On 08/10/2019 16:22, Sami Tolvanen wrote:
>>> On Mon, Oct 7, 2019 at 2:46 PM 'Nick Desaulniers' via Clang Built
>>> Linux <clang-built-linux@...glegroups.com> wrote:
>>>> I'm worried that one of these might lower to LSE atomics without
>>>> ALTERNATIVE guards by blanketing all C code with `-march=armv8-a+lse`.
>>>
>>> True, that's a valid concern. I think adding the directive to each
>>> assembly block is the way forward then, assuming the maintainers are
>>> fine with that.
>>
>> It's definitely a valid concern in principle, but in practice note that
>> lse.h ends up included in ~99% of C files, so the extension is enabled
>> more or less everywhere already.
>>
> 
> lse.h currently does
> 
> __asm__(".arch_extension        lse");
> 
> which instructs the assembler to permit the use of LSE opcodes, but it
> does not instruct the compiler to emit them, so this is not quite the
> same thing.

Derp, of course it isn't. And IIRC we can't just pass the option through 
with -Wa either because at least some versions of GCC emit an explicit 
.arch directive at the top of the output. Oh well; sorry for the 
distraction.

Robin.