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Date:   Sat, 26 Dec 2020 20:31:22 -0700
From:   Nathan Chancellor <natechancellor@gmail.com>
To:     John Millikin <jmillikin@gmail.com>
Cc:     hpa@zytor.com, x86@kernel.org, linux-kernel@vger.kernel.org,
        clang-built-linux@googlegroups.com,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
        John Millikin <john@john-millikin.com>
Subject: Re: [PATCH v2] arch/x86: Propagate $(CLANG_FLAGS) to
 $(REALMODE_FLAGS)
Message-ID: <20201227033122.GA1993664@ubuntu-m3-large-x86>
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 <20201226075347.GA1977841@ubuntu-m3-large-x86>
 <3b793c42-8983-4502-1f9e-729cc0ebec47@gmail.com>
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On Sat, Dec 26, 2020 at 05:41:25PM +0900, John Millikin wrote:
> When cross-compiling with Clang, the `$(CLANG_FLAGS)' variable
> contains additional flags needed to build C and assembly sources
> for the target platform. Normally this variable is automatically
> included in `$(KBUILD_CFLAGS)' by via the top-level Makefile.
> 
> The x86 real-mode makefile builds `$(REALMODE_CFLAGS)' from a
> plain assignment and therefore drops the Clang flags. This causes
> Clang to not recognize x86-specific assembler directives:
> 
> ? arch/x86/realmode/rm/header.S:36:1: error: unknown directive
> ? .type real_mode_header STT_OBJECT ; .size real_mode_header, .-real_mode_header
> ? ^
> 
> Explicit propagation of `$(CLANG_FLAGS)' to `$(REALMODE_CFLAGS)',
> which is inherited by real-mode make rules, fixes cross-compilation
> with Clang for x86 targets.
> 
> Relevant flags:
> 
> * `--target' sets the target architecture when cross-compiling. This
> ? flag must be set for both compilation and assembly (`KBUILD_AFLAGS')
> ? to support architecture-specific assembler directives.
> 
> * `-no-integrated-as' tells clang to assemble with GNU Assembler
> ? instead of its built-in LLVM assembler. This flag is set by default
> ? unless `LLVM_IAS=1' is set, because the LLVM assembler can't yet
> ? parse certain GNU extensions.
> 
> Signed-off-by: John Millikin <john@john-millikin.com>

Reviewed-by: Nathan Chancellor <natechancellor@gmail.com>

> ---
> Changes in v2:
> ? - Reworded the commit message to highlight that this is for
> ??? cross-compilation.
> ? - Removed the `ifdef CONFIG_CC_IS_CLANG' guard.
> 
> ?arch/x86/Makefile | 1 +
> ?1 file changed, 1 insertion(+)
> 
> diff --git a/arch/x86/Makefile b/arch/x86/Makefile
> index 7116da3980be..412b849063ec 100644
> --- a/arch/x86/Makefile
> +++ b/arch/x86/Makefile
> @@ -33,6 +33,7 @@ REALMODE_CFLAGS += -ffreestanding
> ?REALMODE_CFLAGS += -fno-stack-protector
> ?REALMODE_CFLAGS += $(call __cc-option, $(CC), $(REALMODE_CFLAGS), -Wno-address-of-packed-member)
> ?REALMODE_CFLAGS += $(call __cc-option, $(CC), $(REALMODE_CFLAGS), $(cc_stack_align4))
> +REALMODE_CFLAGS += $(CLANG_FLAGS)
> ?export REALMODE_CFLAGS
> ?
> ?# BITS is used as extension for files which are available in a 32 bit
> -- 
> 2.29.2
> 
>