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SubjectRe: [PATCH] x86/build: Specify stack alignment for clang
El Sat, Jun 10, 2017 at 09:03:59AM +0200 Ingo Molnar ha dit:

>
> * Matthias Kaehlcke <mka@chromium.org> wrote:
>
> > 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 for 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>
> > ---
> > arch/x86/Makefile | 9 ++++++---
> > 1 file changed, 6 insertions(+), 3 deletions(-)
> >
> > diff --git a/arch/x86/Makefile b/arch/x86/Makefile
> > index 5851411e60fb..a32badbe87ad 100644
> > --- a/arch/x86/Makefile
> > +++ b/arch/x86/Makefile
> > @@ -27,7 +27,8 @@ REALMODE_CFLAGS := $(M16_CFLAGS) -g -Os -D__KERNEL__ \
> > -mno-mmx -mno-sse \
> > $(call cc-option, -ffreestanding) \
> > $(call cc-option, -fno-stack-protector) \
> > - $(call cc-option, -mpreferred-stack-boundary=2)
> > + $(call cc-option, -mpreferred-stack-boundary=2) \
> > + $(call cc-option, -mstack-alignment=2)
> > export REALMODE_CFLAGS
> >
> > # BITS is used as extension for files which are available in a 32 bit
> > @@ -64,8 +65,9 @@ ifeq ($(CONFIG_X86_32),y)
> > # with nonstandard options
> > KBUILD_CFLAGS += -fno-pic
> >
> > - # prevent gcc from keeping the stack 16 byte aligned
> > + # prevent the compiler from keeping the stack 16 byte aligned
> > KBUILD_CFLAGS += $(call cc-option,-mpreferred-stack-boundary=2)
> > + KBUILD_CFLAGS += $(call cc-option,-mstack-alignment=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:
> > @@ -97,8 +99,9 @@ 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.
> > + # Align the stack to 8 bytes if supported.
> > KBUILD_CFLAGS += $(call cc-option,-mpreferred-stack-boundary=3)
> > + KBUILD_CFLAGS += $(call cc-option,-mstack-alignment=3)
> >
> > # Use -mskip-rax-setup if supported.
> > KBUILD_CFLAGS += $(call cc-option,-mskip-rax-setup)
>
> That's really ugly and the duplicated options are repeated. A cleaner solution
> would be introduce a variable that stores the option, and use that later on. Also
> describe the variable, pointing out that -mpreferred-stack-boundary is a GCC
> option, while -mstack-alignment is a Clang one.

Thanks for your comments, I will send out an updated patch shortly.

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