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

diff --git a/arch/x86/Makefile b/arch/x86/Makefile
index 602f57e590b57fafd8dcf461c9a5468842d9c29e..a98cc9064d8be6751f6e8afd46e5ea35d3d2147c 100644 (file)
--- 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__ \

diff --git a/arch/x86/boot/code16gcc.h b/arch/x86/boot/code16gcc.h
index d93e48010b619e52006b13c068f7ffc82c5a96de..5ff426535397c18afec4049ce4681b802dd3b7ee 100644 (file)
--- a/arch/x86/boot/code16gcc.h
+++ b/arch/x86/boot/code16gcc.h
@@ -1,15 +1,11 @@
-/*
- * 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