powerpc VDSO files being unnecessarily rebuilt

[Masahiro Yamada]

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





-- 
Best Regards
Masahiro Yamada