Messages in this thread Patch in this message | | | Subject | [PATCH] x86: Build 16-bit code with -m16 where possible | From | David Woodhouse <> | Date | Wed, 29 Jan 2014 12:11:25 +0000 |
| |
Both clang 3.5 and GCC 4.9 will support this (as of r207196 and r199754 respectively). Both have been tested to produce booting kernels when the 16-bit code is built with -m16. (Modulo LLVM PR3997, at least.)
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com> --- Updated comments, and retested, now that GCC supports it too. I think the two preparatory patches already made it into tip:x86/build?
Note that for clang -m16 to work you do need the patch I just posted to fix has_eflag() too. And a workaround for PR3997 as before.
arch/x86/Makefile | 16 ++++++++++++---- 1 file changed, 12 insertions(+), 4 deletions(-)
diff --git a/arch/x86/Makefile b/arch/x86/Makefile index 0f68712..0141cb0 100644 --- a/arch/x86/Makefile +++ b/arch/x86/Makefile @@ -13,14 +13,22 @@ endif # How to compile the 16-bit code. Note we always compile for -march=i386; # that way we can complain to the user if the CPU is insufficient. -REALMODE_CFLAGS := -m32 -g -Os -D__KERNEL__ -DDISABLE_BRANCH_PROFILING \ +# +# 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)) +M16_CFLAGS := -m16 + +REALMODE_CFLAGS := -g -Os -D__KERNEL__ -DDISABLE_BRANCH_PROFILING \ + $(call cc-option, $(M16_CFLAGS), $(CODE16GCC_CFLAGS)) \ -Wall -Wstrict-prototypes -march=i386 -mregparm=3 \ - -include $(srctree)/arch/x86/boot/code16gcc.h \ -fno-strict-aliasing -fomit-frame-pointer -fno-pic \ -mno-mmx -mno-sse \ $(call cc-option, -ffreestanding) \ - $(call cc-option, -fno-toplevel-reorder,\ - $(call cc-option, -fno-unit-at-a-time)) \ $(call cc-option, -fno-stack-protector) \ $(call cc-option, -mpreferred-stack-boundary=2) export REALMODE_CFLAGS -- 1.8.5.3
-- dwmw2 [unhandled content-type:application/x-pkcs7-signature] | |