| From foo@baz Wed Nov 21 18:50:25 CET 2018 |
| From: Matthias Kaehlcke <mka@chromium.org> |
| Date: Wed, 21 Jun 2017 16:28:05 -0700 |
| Subject: x86/build: Specify stack alignment for clang |
| |
| From: Matthias Kaehlcke <mka@chromium.org> |
| |
| commit d77698df39a512911586834d303275ea5fda74d0 upstream. |
| |
| For gcc stack alignment is configured with -mpreferred-stack-boundary=N, |
| clang has the option -mstack-alignment=N for that purpose. Use the same |
| alignment as with gcc. |
| |
| If the alignment is not specified clang assumes an alignment of |
| 16 bytes, as required by the standard ABI. However as mentioned in |
| d9b0cde91c60 ("x86-64, gcc: Use -mpreferred-stack-boundary=3 if |
| supported") the standard kernel entry on x86-64 leaves the stack |
| on an 8-byte boundary, as a consequence clang will keep the stack |
| misaligned. |
| |
| Signed-off-by: Matthias Kaehlcke <mka@chromium.org> |
| Acked-by: Ingo Molnar <mingo@kernel.org> |
| Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com> |
| Signed-off-by: Nick Desaulniers <ndesaulniers@google.com> |
| Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org> |
| --- |
| arch/x86/Makefile | 26 +++++++++++++++++++++----- |
| 1 file changed, 21 insertions(+), 5 deletions(-) |
| |
| --- a/arch/x86/Makefile |
| +++ b/arch/x86/Makefile |
| @@ -11,6 +11,14 @@ else |
| KBUILD_DEFCONFIG := $(ARCH)_defconfig |
| endif |
| |
| +# For gcc stack alignment is specified with -mpreferred-stack-boundary, |
| +# clang has the option -mstack-alignment for that purpose. |
| +ifneq ($(call cc-option, -mpreferred-stack-boundary=4),) |
| + cc_stack_align_opt := -mpreferred-stack-boundary |
| +else ifneq ($(call cc-option, -mstack-alignment=4),) |
| + cc_stack_align_opt := -mstack-alignment |
| +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. |
| # |
| @@ -28,7 +36,7 @@ REALMODE_CFLAGS := $(M16_CFLAGS) -g -Os |
| |
| REALMODE_CFLAGS += $(call __cc-option, $(CC), $(REALMODE_CFLAGS), -ffreestanding) |
| REALMODE_CFLAGS += $(call __cc-option, $(CC), $(REALMODE_CFLAGS), -fno-stack-protector) |
| -REALMODE_CFLAGS += $(call __cc-option, $(CC), $(REALMODE_CFLAGS), -mpreferred-stack-boundary=2) |
| +REALMODE_CFLAGS += $(call __cc-option, $(CC), $(REALMODE_CFLAGS), $(cc_stack_align_opt)=2) |
| export REALMODE_CFLAGS |
| |
| # BITS is used as extension for files which are available in a 32 bit |
| @@ -65,8 +73,10 @@ ifeq ($(CONFIG_X86_32),y) |
| # with nonstandard options |
| KBUILD_CFLAGS += -fno-pic |
| |
| - # prevent gcc from keeping the stack 16 byte aligned |
| - KBUILD_CFLAGS += $(call cc-option,-mpreferred-stack-boundary=2) |
| + # Align the stack to the register width instead of using the default |
| + # alignment of 16 bytes. This reduces stack usage and the number of |
| + # alignment instructions. |
| + KBUILD_CFLAGS += $(call cc-option,$(cc_stack_align_opt)=2) |
| |
| # Disable unit-at-a-time mode on pre-gcc-4.0 compilers, it makes gcc use |
| # a lot more stack due to the lack of sharing of stacklots: |
| @@ -98,8 +108,14 @@ else |
| KBUILD_CFLAGS += $(call cc-option,-mno-80387) |
| KBUILD_CFLAGS += $(call cc-option,-mno-fp-ret-in-387) |
| |
| - # Use -mpreferred-stack-boundary=3 if supported. |
| - KBUILD_CFLAGS += $(call cc-option,-mpreferred-stack-boundary=3) |
| + # By default gcc and clang use a stack alignment of 16 bytes for x86. |
| + # However the standard kernel entry on x86-64 leaves the stack on an |
| + # 8-byte boundary. If the compiler isn't informed about the actual |
| + # alignment it will generate extra alignment instructions for the |
| + # default alignment which keep the stack *mis*aligned. |
| + # Furthermore an alignment to the register width reduces stack usage |
| + # and the number of alignment instructions. |
| + KBUILD_CFLAGS += $(call cc-option,$(cc_stack_align_opt)=3) |
| |
| # Use -mskip-rax-setup if supported. |
| KBUILD_CFLAGS += $(call cc-option,-mskip-rax-setup) |