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

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Date:   Tue, 8 Oct 2019 23:03:24 +0200
From:   Ard Biesheuvel <ard.biesheuvel@...aro.org>
To:     Robin Murphy <robin.murphy@....com>
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 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.