linux-kernel - Re: [PATCH] arch/x86: Propagate $(CLANG_FLAGS) to $(REALMODE

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Date:   Sat, 26 Dec 2020 16:52:03 +0900
From:   John Millikin <jmillikin@...il.com>
To:     hpa@...or.com, x86@...nel.org, linux-kernel@...r.kernel.org
Cc:     clang-built-linux@...glegroups.com,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        John Millikin <john@...n-millikin.com>
Subject: Re: [PATCH] arch/x86: Propagate $(CLANG_FLAGS) to $(REALMODE_FLAGS)

On 12/26/20 16:35, hpa@...or.com wrote:
> Why is CLANG_FLAGS non-null when unused? It would be better to centralize that.
CLANG_FLAGS normally propagates through inclusion in the default KBUILD_CFLAGS and KBUILD_AFLAGS, set in `/Makefile':

    # Makefile
    KBUILD_CFLAGS += $(CLANG_FLAGS)
    KBUILD_AFLAGS += $(CLANG_FLAGS)
    export CLANG_FLAGS

This default can be overridden by explicit assignment, as is done in some of the arch/x86 makefiles:

    # arch/x86/realmode/rm/Makefile
    KBUILD_CFLAGS    := $(REALMODE_CFLAGS) -D_SETUP -D_WAKEUP \
               -I$(srctree)/arch/x86/boot
    KBUILD_AFLAGS    := $(KBUILD_CFLAGS) -D__ASSEMBLY__
    KBUILD_CFLAGS    += -fno-asynchronous-unwind-tables

Since REALMODE_CFLAGS is being built up from a plain assignment, the Clang flags get lost. As a result Clang fails to compile the real-mode code when cross-compiling for an x86 target.

    arch/x86/realmode/rm/header.S:36:1: error: unknown directive
    .type real_mode_header STT_OBJECT ; .size real_mode_header, .-real_mode_header
    ^
    arch/x86/realmode/rm/header.S:36:37: error: unknown directive
    .type real_mode_header STT_OBJECT ; .size real_mode_header, .-real_mode_header
                                        ^
    arch/x86/realmode/rm/header.S:41:62: error: unknown directive
    .globl end_signature ; ; end_signature: ; .long 0x65a22c82 ; .type end_signature STT_OBJECT ; .size end_signature, .-end_signature
                                                                 ^
    arch/x86/realmode/rm/header.S:41:95: error: unknown directive
    .globl end_signature ; ; end_signature: ; .long 0x65a22c82 ; .type end_signature STT_OBJECT ; .size end_signature, .-end_signature
                                                                                                ^

This patch allows the Clang-specific flags to propagate through the REALMODE_CFLAGS variable set in `arch/x86/Makefile' and consumed by certain arch/x86 targets, which fixes cross-compilation of x86 kernels.