4.4-stable review patch. If anyone has any objections, please let me know.
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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: Nathan Chancellor natechancellor@gmail.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)