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Commit a9cfccee authored by H. Peter Anvin's avatar H. Peter Anvin
Browse files

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: default avatarKevin O'Connor <kevin@koconnor.net>
Signed-off-by: default avatarH. Peter Anvin <hpa@linux.intel.com>
Link: http://lkml.kernel.org/n/tip-xw8ibgdemucl9fz3i1bymu6w@git.kernel.org
parent a9358bc3

arch/x86/Makefile

+3 −6
Original line number Diff line number Diff line
@@ -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__ \

arch/x86/boot/code16gcc.h

+10 −14
Original line number Diff line number Diff line 
/*

 * code16gcc.h

 *

 * This file is -include'd when compiling 16-bit C code.

 * Note: this asm() needs to be emitted before gcc emits any code.

 * Depending on gcc version, this requires -fno-unit-at-a-time or

 * -fno-toplevel-reorder.

 *

 * Hopefully gcc will eventually have a real -m16 option so we can

 * drop this hack long term.

 */

#

# code16gcc.h

#

# This file is added to the assembler via -Wa when compiling 16-bit C code.

# This is done this way instead via asm() to make sure gcc does not reorder

# things around us.

#

# gcc 4.9+ has a real -m16 option so we can drop this hack long term.

#



#ifndef __ASSEMBLY__

asm(".code16gcc");

#endif
 
	.code16gcc