linux-kernel - Re: [PATCH] kconfig: allow to pass optional flag into as-instr macro

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Date:   Mon, 23 Mar 2020 16:20:10 -0600
From:   "Jason A. Donenfeld" <Jason@...c4.com>
To:     Masahiro Yamada <masahiroy@...nel.org>
Cc:     linux-kbuild@...r.kernel.org, LKML <linux-kernel@...r.kernel.org>,
        clang-built-linux <clang-built-linux@...glegroups.com>
Subject: Re: [PATCH] kconfig: allow to pass optional flag into as-instr macro

On Mon, Mar 23, 2020 at 3:58 PM Masahiro Yamada <masahiroy@...nel.org> wrote:
> Commit 42d519e3d0c0 ("kbuild: Add support for 'as-instr' to be used in
> Kconfig files") introduced the Kconfig variant of as-instr.
>
> It is currently used in arch/arm64/Kconfig.
>
> arm and arm64 are simple cases because they are separated by directory,
> and GCC also provides a single architecture.
>
> Such architectures as x86, powerpc, sparc, etc. support both 32 and
> 64 bit kernel in the unified arch directory, and GCC can be biarch.
> On such architectures, Kbuild passes -m32 / -m64 flag (or a proper
> target triple for Clang) to select the target machine bit.
>
> This commit adds the second parameter to as-instr so you can optionally
> pass a compiler flag to evaluate the instruction with.
>
> One example usage of this is for the conversion of the following code
> in arch/x86/Makefile:
>
>   adx_instr := $(call as-instr,adox %r10$(comma)%r10,-DCONFIG_AS_ADX=1)
>
> This instruction code should be evaluated by the 64-bit assembler
> (r10 is a 64-bit register).
>
> If you use compiler that defaults to 32-bit for building the 64-bit
> kernel, -m64 should be passed to invoke the 64-bit assembler.
>
>   config AS_ADX
>           def_bool $(as-instr,adox %r10$(comma)%r10,$(m64-flags))
>
> Signed-off-by: Masahiro Yamada <masahiroy@...nel.org>
> ---
>
>  scripts/Kconfig.include | 5 +++--
>  1 file changed, 3 insertions(+), 2 deletions(-)
>
> diff --git a/scripts/Kconfig.include b/scripts/Kconfig.include
> index 496d11c92c97..9b8c98168ba9 100644
> --- a/scripts/Kconfig.include
> +++ b/scripts/Kconfig.include
> @@ -31,9 +31,10 @@ cc-option = $(success,$(CC) -Werror $(CLANG_FLAGS) $(1) -S -x c /dev/null -o /de
>  # Return y if the linker supports <flag>, n otherwise
>  ld-option = $(success,$(LD) -v $(1))
>
> -# $(as-instr,<instr>)
> +# $(as-instr,<instr>,[<flag>])
>  # Return y if the assembler supports <instr>, n otherwise
> -as-instr = $(success,printf "%b\n" "$(1)" | $(CC) $(CLANG_FLAGS) -c -x assembler -o /dev/null -)
> +# You can pass optional <flag> to evaluate <instr> with
> +as-instr = $(success,printf "%b\n" "$(1)" | $(CC) $(CLANG_FLAGS) $(2) -c -x assembler -o /dev/null -)
>
>  # check if $(CC) and $(LD) exist
>  $(error-if,$(failure,command -v $(CC)),compiler '$(CC)' not found)
> --
> 2.17.1

This looks fine to me, and might be useful down the line in the
future. But for the ADX situation, I wound up just going with the
bicompatible register:

https://git.zx2c4.com/linux-dev/commit/?h=jd/kconfig-assembler-support&id=c81d9c743d058ef9013fbf8661d071ac9d574472

+# binutils >= 2.23
+config AS_ADX
+ def_bool $(as-instr,adox %eax$(comma)%eax)

This seems to work. But it also doesn't matter much, as it gets
removed anyway during the binutils 2.23 bump:

https://git.zx2c4.com/linux-dev/commit/?h=jd/kconfig-assembler-support&id=c95d6f2ce88722eeb86d54f206bd7599bdc51e1d