x86, build: Change code16gcc.h from a C header to an assembly header

By changing code16gcc.h from a C header to an assembly header and use
the -Wa,... option to gcc to force it to be added to the assembly
input, we can avoid the problems with gcc reordering code bits on us.

If we have -m16, we still use it, of course.

Suggested-by: Kevin O'Connor <kevin@koconnor.net>
Signed-off-by: H. Peter Anvin <hpa@linux.intel.com>
Link: http://lkml.kernel.org/n/tip-xw8ibgdemucl9fz3i1bymu6w@git.kernel.org

1 files changed, 3 insertions, 6 deletions

diff --git a/arch/x86/Makefile b/arch/x86/Makefile
index 602f57e590b5..a98cc9064d8b 100644
--- a/arch/x86/Makefile
+++ b/arch/x86/Makefile
@@ -15,12 +15,9 @@ endif
 # that way we can complain to the user if the CPU is insufficient.
 #
 # The -m16 option is supported by GCC >= 4.9 and clang >= 3.5. For
-# older versions of GCC, we need to play evil and unreliable tricks to
-# attempt to ensure that our asm(".code16gcc") is first in the asm
-# output.
-CODE16GCC_CFLAGS := -m32 -include $(srctree)/arch/x86/boot/code16gcc.h \
-		    $(call cc-option, -fno-toplevel-reorder,\
-		      $(call cc-option, -fno-unit-at-a-time))
+# older versions of GCC, include an *assembly* header to make sure that
+# gcc doesn't play any games behind our back.
+CODE16GCC_CFLAGS := -m32 -Wa,$(srctree)/arch/x86/boot/code16gcc.h
 M16_CFLAGS	 := $(call cc-option, -m16, $(CODE16GCC_CFLAGS))
 
 REALMODE_CFLAGS	:= $(M16_CFLAGS) -g -Os -D__KERNEL__ \