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

From: Ard Biesheuvel
Date: Tue Oct 08 2019 - 17:03:38 EST

On Tue, 8 Oct 2019 at 18:19, Robin Murphy <robin.murphy@xxxxxxx> 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@xxxxxxxxxxxxxxxx> 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.