Commit-ID: a9cfccee6604854aebc70215610b9788667f4fec
Gitweb: http://git.kernel.org/tip/a9cfccee6604854aebc70215610b9788667f4fec
Author: H. Peter Anvin <hpa@xxxxxxxxxxxxxxx>
AuthorDate: Wed, 4 Jun 2014 13:16:48 -0700
Committer: H. Peter Anvin <hpa@xxxxxxxxxxxxxxx>
CommitDate: Wed, 4 Jun 2014 13:16:48 -0700
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@xxxxxxxxxxxx>
Signed-off-by: H. Peter Anvin <hpa@xxxxxxxxxxxxxxx>
Link: http://lkml.kernel.org/n/tip-xw8ibgdemucl9fz3i1bymu6w@xxxxxxxxxxxxxx
---
arch/x86/Makefile | 9 +++------
arch/x86/boot/code16gcc.h | 24 ++++++++++--------------
2 files changed, 13 insertions(+), 20 deletions(-)
diff --git a/arch/x86/Makefile b/arch/x86/Makefile
index 602f57e..a98cc90 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__ \
diff --git a/arch/x86/boot/code16gcc.h b/arch/x86/boot/code16gcc.h
index d93e480..5ff4265 100644
--- 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
--
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