linux-kernel - Re: [PATCH v8 02/10] Makefile: Prepare for using macros for inline asm

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Date:   Wed, 26 Sep 2018 10:58:11 +0200
From:   Rasmus Villemoes <linux@...musvillemoes.dk>
To:     Nadav Amit <namit@...are.com>, Ingo Molnar <mingo@...hat.com>
Cc:     linux-kernel@...r.kernel.org, x86@...nel.org,
        Sam Ravnborg <sam@...nborg.org>,
        Michal Marek <michal.lkml@...kovi.net>,
        Thomas Gleixner <tglx@...utronix.de>,
        "H. Peter Anvin" <hpa@...or.com>, linux-kbuild@...r.kernel.org
Subject: Re: [PATCH v8 02/10] Makefile: Prepare for using macros for inline
 asm

On 2018-09-18 23:28, Nadav Amit wrote:

> diff --git a/arch/x86/Makefile b/arch/x86/Makefile
> index 8f6e7eb8ae9f..944fa3bc9376 100644
> --- a/arch/x86/Makefile
> +++ b/arch/x86/Makefile
> @@ -214,8 +214,8 @@ ifdef CONFIG_X86_64
>  KBUILD_LDFLAGS += $(call ld-option, -z max-page-size=0x200000)
>  endif
>  
> -# Speed up the build
> -KBUILD_CFLAGS += -pipe
> +# We cannot use -pipe flag since we give an additional .s file to the compiler
> +#KBUILD_CFLAGS += -pipe

Is this really necessary? The gas manual says that one can use -- to
name stdin, though that's probably a typo and should just be - . Doing

gcc -pipe -Wa,foo.s -Wa,-

does seem to work as expected (and would also make it possible to append
some .s file should that ever be required).
>  
> +archmacros:
> +	$(Q)$(MAKE) $(build)=arch/x86/kernel arch/x86/kernel/macros.s
> +
> +ASM_MACRO_FLAGS = -Wa,arch/x86/kernel/macros.s
> +export ASM_MACRO_FLAGS
> +KBUILD_CFLAGS += $(ASM_MACRO_FLAGS)
How does this affect what gets rebuilt when one of the asm/foo.h files
going into macros.s changes? Does that cause a global rebuild because
everything depends on macros.s, or do we still only rebuild the files
that actually include asm/foo.h?

Rasmus