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

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Date:   Tue, 2 Oct 2018 07:01:19 +0900
From:   Masahiro Yamada <yamada.masahiro@...ionext.com>
To:     Nadav Amit <namit@...are.com>
Cc:     Rasmus Villemoes <linux@...musvillemoes.dk>,
        Ingo Molnar <mingo@...hat.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        X86 ML <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 mailing list <linux-kbuild@...r.kernel.org>
Subject: Re: [PATCH v8 02/10] Makefile: Prepare for using macros for inline asm

Hi.



2018年9月27日(木) 2:57 Nadav Amit <namit@...are.com>:
>
> at 1:58 AM, Rasmus Villemoes <linux@...musvillemoes.dk> wrote:
>
> > 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,-
>
> Good idea. I didn’t think of it. Yes, this can do the trick. I’ll do it in
> v9.
>
> > 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?
>
> There will not be a global rebuild. Any source file that uses the header
> files that are included in macros.S will be rebuilt.
>
> But your question actually raises two issues:
>
> 1. If macros.S changes, there *should* be a global rebuild,


Looking at this series, I can see this rule:
"macros and inline functions that calls them are placed in the same header".


For example,
REFCOUNT_CHECK_LE_ZERO is defined in <asm/refcount.h>,
and REFCOUNT_CHECK_LE_ZERO is invoked via refcount_sub_and_test() etc.
in the same header.

Therefore, all users of REFCOUNT_CHECK_LE_ZERO must have included
<asm/refcount.h>

This means all C files using REFCOUNT_CHECK_LE_ZERO
will be appropriately recompiled as long as the rule above is observed.



> and currently
> there wouldn’t be one. Do you happen to know what would be the appropriate
> way to trigger one?
>
> 2. Someone might mistakenly use the macros through inline assembly without
> including the header file.

Right, it is possible to use REFCOUNT_CHECK_LE_ZERO directly.
If this happens, my assumption breaks.

It would be unlikely to happen, though...


> It would be better to detect such cases and fail
> the build. I may be able to create another asm macro in the C part of the
> header that would cause the build to fail. But let me know if you any
> better idea.
>
> Regards,
> Nadav
>


-- 
Best Regards
Masahiro Yamada