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

[Nick Desaulniers]

On Mon, Oct 7, 2019 at 1:14 PM 'Sami Tolvanen' via Clang Built Linux
<clang-built-linux@xxxxxxxxxxxxxxxx> wrote:
>
> Unlike gcc, clang considers each inline assembly block to be independent
> and therefore, when using the integrated assembler for inline assembly,
> any preambles that enable features must be repeated in each block.
>
> Instead of changing all inline assembly blocks that use LSE, this change
> adds -march=armv8-a+lse to KBUILD_CFLAGS, which works with both clang
> and gcc.
>
> Signed-off-by: Sami Tolvanen <samitolvanen@xxxxxxxxxx>

Thanks Sami, looks like this addresses:
Link: https://github.com/ClangBuiltLinux/linux/issues/671
I tried adding `.arch armv8-a+lse` directives to all of the inline asm:
https://github.com/ClangBuiltLinux/linux/issues/573#issuecomment-535098996
though that quickly ran aground in some failed assertion using the
alternatives system that I don't quite yet understand.

One thing to be careful about is that blankets the entire kernel in
`+lse`, allowing LSE atomics to be selected at any point.  The
assembler directive in the header (or per inline asm block) is more
fine grained.  I'm not sure that they would be guarded by the
alternative system.  Maybe that's not an issue, but it is the reason I
tried to localize the assembler directive first.

Note that Clang really wants the target arch specified by either:
1. command line argument (as in this patch)
2. per function function attribute
3. per asm statement assembler directive

1 is the smallest incision.

> ---
>  arch/arm64/Makefile          | 2 ++
>  arch/arm64/include/asm/lse.h | 2 --
>  2 files changed, 2 insertions(+), 2 deletions(-)
>
> diff --git a/arch/arm64/Makefile b/arch/arm64/Makefile
> index 84a3d502c5a5..7a7c0cb8ed60 100644
> --- a/arch/arm64/Makefile
> +++ b/arch/arm64/Makefile
> @@ -36,6 +36,8 @@ lseinstr := $(call as-instr,.arch_extension lse,-DCONFIG_AS_LSE=1)
>  ifeq ($(CONFIG_ARM64_LSE_ATOMICS), y)
>    ifeq ($(lseinstr),)
>  $(warning LSE atomics not supported by binutils)
> +  else
> +KBUILD_CFLAGS  += -march=armv8-a+lse
>    endif
>  endif
>
> diff --git a/arch/arm64/include/asm/lse.h b/arch/arm64/include/asm/lse.h
> index 80b388278149..8603a9881529 100644
> --- a/arch/arm64/include/asm/lse.h
> +++ b/arch/arm64/include/asm/lse.h
> @@ -14,8 +14,6 @@
>  #include <asm/atomic_lse.h>
>  #include <asm/cpucaps.h>
>
> -__asm__(".arch_extension       lse");
> -
>  extern struct static_key_false cpu_hwcap_keys[ARM64_NCAPS];
>  extern struct static_key_false arm64_const_caps_ready;
>
> --
> 2.23.0.581.g78d2f28ef7-goog
>
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-- 
Thanks,
~Nick Desaulniers