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Subject: [PATCH v2] arch/x86: Propagate $(CLANG_FLAGS) to $(REALMODE_FLAGS)
To:     Nathan Chancellor <natechancellor@gmail.com>, hpa@zytor.com
Cc:     x86@kernel.org, linux-kernel@vger.kernel.org,
        clang-built-linux@googlegroups.com,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
        John Millikin <john@john-millikin.com>
References: <cceb074c-861c-d716-5e19-834a8492f245@gmail.com>
 <37DE7046-5096-4C0C-A96D-FD168A849DFD@zytor.com>
 <20201226075347.GA1977841@ubuntu-m3-large-x86>
From:   John Millikin <jmillikin@gmail.com>
Message-ID: <3b793c42-8983-4502-1f9e-729cc0ebec47@gmail.com>
Date:   Sat, 26 Dec 2020 17:41:25 +0900
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When cross-compiling with Clang, the `$(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>
---
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