powerpc VDSO files being unnecessarily rebuilt

[Masahiro Yamada]

On Thu, Dec 17, 2020 at 6:23 PM Masahiro Yamada <masahiroy at kernel.org> wrote:
>
> On Thu, Dec 17, 2020 at 11:56 AM Michael Ellerman <mpe at ellerman.id.au> wrote:
> >
> > Hi all,
> >
> > Since the merge of the C VDSO I see we are repeatedly rebuilding some
> > files in the VDSO, eg:
> >
> >   $ make V=2
> >   make[1]: Entering directory '/home/michael/linux/build~'
> >     GEN     Makefile
> >     CALL    /home/michael/linux/scripts/checksyscalls.sh - due to target missing
> >     CALL    /home/michael/linux/scripts/atomic/check-atomics.sh - due to target missing
> >     CHK     include/generated/compile.h
> >     CC      arch/powerpc/kernel/vdso64/vgettimeofday.o - due to vgettimeofday.o not in $(targets)
> >
> > This then causes multiple other files to be rebuilt.
> >
> > So the obvious fix is to add it to targets:
> >
> > diff --git a/arch/powerpc/kernel/vdso64/Makefile b/arch/powerpc/kernel/vdso64/Makefile
> > index d365810a689a..5386532866ce 100644
> > --- a/arch/powerpc/kernel/vdso64/Makefile
> > +++ b/arch/powerpc/kernel/vdso64/Makefile
> > @@ -5,6 +5,7 @@ ARCH_REL_TYPE_ABS := R_PPC_JUMP_SLOT|R_PPC_GLOB_DAT|R_PPC_ADDR32|R_PPC_ADDR24|R_
> >  include $(srctree)/lib/vdso/Makefile
> >
> >  obj-vdso64 = sigtramp.o gettimeofday.o datapage.o cacheflush.o note.o getcpu.o
> > +targets := $(obj-vdso64) vdso64.so.dbg
> >
> >  ifneq ($(c-gettimeofday-y),)
> >    CFLAGS_vgettimeofday.o += -include $(c-gettimeofday-y)
> > @@ -13,11 +14,11 @@ ifneq ($(c-gettimeofday-y),)
> >    CFLAGS_vgettimeofday.o += -DDISABLE_BRANCH_PROFILING
> >    CFLAGS_vgettimeofday.o += -ffreestanding -fasynchronous-unwind-tables
> >    CFLAGS_REMOVE_vgettimeofday.o = $(CC_FLAGS_FTRACE)
> > +  targets += vgettimeofday.o
> >  endif
> >
> >  # Build rules
> >
> > -targets := $(obj-vdso64) vdso64.so.dbg
> >  obj-vdso64 := $(addprefix $(obj)/, $(obj-vdso64))
> >
> >  GCOV_PROFILE := n
> >
> >
> > But then I see it still rebuilt:
> >
> >   CC      arch/powerpc/kernel/vdso64/vgettimeofday.o - due to command line change
> >
> >
> > I'm not changing the command line, and AFAICS the .cmd file is not
> > changing either:
> >
> >   $ make V=2
> >   ...
> >     CC      arch/powerpc/kernel/vdso64/vgettimeofday.o - due to command line change
> >
> >   $ sha256sum build\~/arch/powerpc/kernel/vdso64/vgettimeofday.o
> >   7f635546bc2768c7b929d3de1724d83285f3cd54394fcd7104f8b1301d689d65  build~/arch/powerpc/kernel/vdso64/vgettimeofday.o
> >
> >   $ make V=2
> >   ...
> >     CC      arch/powerpc/kernel/vdso64/vgettimeofday.o - due to command line change
> >
> >   $ sha256sum build\~/arch/powerpc/kernel/vdso64/vgettimeofday.o
> >   7f635546bc2768c7b929d3de1724d83285f3cd54394fcd7104f8b1301d689d65  build~/arch/powerpc/kernel/vdso64/vgettimeofday.o
> >
> >
> > So any hints on what I'm missing here?
> >
> > cheers
>
>
> This is because PPC builds the vdso twice
> with different command arguments.
>
>
> First time:
>
> vdso_prepare: prepare0
>          $(if $(CONFIG_VDSO32),$(Q)$(MAKE) \
>                  $(build)=arch/powerpc/kernel/vdso32
> include/generated/vdso32-offsets.h)
>          $(if $(CONFIG_PPC64),$(Q)$(MAKE) \
>                  $(build)=arch/powerpc/kernel/vdso64
> include/generated/vdso64-offsets.h)
>
>
> Second time:
>    from  arch/powerpc/kernel/Makefile
>
>
>
>
>
> For the first build, -Werror is missing because
> Kbuild directly descends into arch/powerpc/kernel/vdso[32,64]/.
>
>
> For the second build,
>
> arch/powerpc/Kbuild appends the following:
>
> subdir-ccflags-$(CONFIG_PPC_WERROR) := -Werror
>


This is very dangerous because the vdso offsets are wrong.

I think ppc followed arm64 implementation.

Let me figure out how to fix arm64 vdso.



-- 
Best Regards
Masahiro Yamada