Messages in this thread | | | From | Masahiro Yamada <> | Date | Wed, 13 Oct 2021 12:02:16 +0900 | Subject | Re: [PATCH 2/3] arm64: vdso32: lazily invoke COMPAT_CC |
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On Wed, Oct 13, 2021 at 8:46 AM Nick Desaulniers <ndesaulniers@google.com> wrote: > > When running the following command without arm-linux-gnueabi-gcc in > one's $PATH, the following warning is observed: > > $ ARCH=arm64 CROSS_COMPILE_COMPAT=arm-linux-gnueabi- make -j72 LLVM=1 mrproper > make[1]: arm-linux-gnueabi-gcc: No such file or directory > > This is because KCONFIG is not run for mrproper, so CONFIG_CC_IS_CLANG > is not set, and we end up eagerly evaluating various variables that try > to invoke CC_COMPAT. > > This is a similar problem to what was observed in > commit 3ec8a5b33dea ("kbuild: do not export LDFLAGS_vmlinux") > > Cc: Masahiro Yamada <masahiroy@kernel.org> > Reported-by: Lucas Henneman <henneman@google.com> > Signed-off-by: Nick Desaulniers <ndesaulniers@google.com>
There are two ways to fix it:
[1]: sink the error message to /dev/null (as in commit dc960bfeedb01cf832c5632ed1f3daed4416b142) [2]: use a recursively-expanded variable as you did.
"Simple variable (:=) vs recursive variable (=)" is a trade-off.
Please be careful about the cost when you try the [2] approach.
Simple variables are immediately expanded while parsing Makefile. There are 7 call-sites for cc32-option, hence the compiler is invoked 7 times for building vdso32, 0 times for cleaning. (Since 57fd251c789647552d32d2fc51bedd4f90d70f9f, try-run is no-op for 'make clean').
Recursive variables are expanded every time they are used.
IIUC, if_changed expands the command line 3 times. There are 2 objects (note.o and vgettimeofday.o) There are 7 call-sites for cc32-option.
So, the compiler is invoked 42 (3 * 2 * 7) times for building vdso32.
> --- > arch/arm64/kernel/vdso32/Makefile | 8 ++++---- > 1 file changed, 4 insertions(+), 4 deletions(-) > > diff --git a/arch/arm64/kernel/vdso32/Makefile b/arch/arm64/kernel/vdso32/Makefile > index 89299a26638b..d24b12318f4c 100644 > --- a/arch/arm64/kernel/vdso32/Makefile > +++ b/arch/arm64/kernel/vdso32/Makefile > @@ -38,12 +38,12 @@ cc32-disable-warning = $(call try-run,\ > # As a result we set our own flags here. > > # KBUILD_CPPFLAGS and NOSTDINC_FLAGS from top-level Makefile > -VDSO_CPPFLAGS := -DBUILD_VDSO -D__KERNEL__ -nostdinc -isystem $(shell $(CC_COMPAT) -print-file-name=include) > +VDSO_CPPFLAGS = -DBUILD_VDSO -D__KERNEL__ -nostdinc -isystem $(shell $(CC_COMPAT) -print-file-name=include) > VDSO_CPPFLAGS += $(LINUXINCLUDE) > > # Common C and assembly flags > # From top-level Makefile > -VDSO_CAFLAGS := $(VDSO_CPPFLAGS) > +VDSO_CAFLAGS = $(VDSO_CPPFLAGS) > ifneq ($(shell $(CC_COMPAT) --version 2>&1 | head -n 1 | grep clang),) > VDSO_CAFLAGS += --target=$(notdir $(CROSS_COMPILE_COMPAT:%-=%)) > endif > @@ -73,7 +73,7 @@ VDSO_CAFLAGS += -DDISABLE_BRANCH_PROFILING > VDSO_CAFLAGS += $(call cc32-option,-march=armv8-a -D__LINUX_ARM_ARCH__=8,\ > -march=armv7-a -D__LINUX_ARM_ARCH__=7) > > -VDSO_CFLAGS := $(VDSO_CAFLAGS) > +VDSO_CFLAGS = $(VDSO_CAFLAGS) > VDSO_CFLAGS += -DENABLE_COMPAT_VDSO=1 > # KBUILD_CFLAGS from top-level Makefile > VDSO_CFLAGS += -Wall -Wundef -Wstrict-prototypes -Wno-trigraphs \ > @@ -108,7 +108,7 @@ else > VDSO_CFLAGS += -marm > endif > > -VDSO_AFLAGS := $(VDSO_CAFLAGS) > +VDSO_AFLAGS = $(VDSO_CAFLAGS) > VDSO_AFLAGS += -D__ASSEMBLY__ > > # From arm vDSO Makefile > -- > 2.33.0.882.g93a45727a2-goog >
-- Best Regards Masahiro Yamada
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