linux-kernel - Re: [PATCH v2] x86/boot: define CC_HAVE_ASM

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Message-ID: <d98d47ea-b639-8afd-2851-1f7ceaf66fd0@zytor.com>
Date:   Mon, 1 Oct 2018 10:38:33 -0700
From:   "H. Peter Anvin" <hpa@...or.com>
To:     ndesaulniers@...gle.com, bp@...en8.de, mingo@...hat.com,
        tglx@...utronix.de
Cc:     x86@...nel.org,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        Masahiro Yamada <yamada.masahiro@...ionext.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Matthias Kaehlcke <mka@...omium.org>,
        Kees Cook <keescook@...omium.org>,
        Cao jin <caoj.fnst@...fujitsu.com>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] x86/boot: define CC_HAVE_ASM_GOTO

On 09/27/18 13:47, ndesaulniers@...gle.com wrote:
> Early prototypes of Clang with asm goto support produce 6 instances of
> the following warning:
> 
> In file included from arch/x86/boot/compressed/misc.h:20:
> In file included from ./include/linux/elf.h:5:
> In file included from ./arch/x86/include/asm/elf.h:8:
> In file included from ./include/linux/thread_info.h:38:
> In file included from ./arch/x86/include/asm/thread_info.h:53:
> ./arch/x86/include/asm/cpufeature.h:150:2: warning: "Compiler lacks
> ASM_GOTO support. Add -D __BPF_TRACING__ to your compiler arguments"
> [-W#warnings]
> your compiler arguments"
>  ^
> 
> Since 6 files under arch/x86/boot/compressed/ include
> arch/x86/boot/compressed/misc.h AND
> arch/x86/boot/compressed/Makefile happens to redefine KBUILD_CFLAGS,
> which set these variables in the top level MAKEFILE.
> 

The Right Thing[TM] would probably be to split the CFLAGS into those
that are independent of x86_64 vs i386. After all, there is no
fundamental reason we couldn't want to use asm goto in the setup or
real-mode code in the future.  In addition to BIOS entry code there is
the EFI32 code.

	-hpa