linux-kernel - Re: [PATCH 2/3] arm64: vdso32: lazily invoke COMPAT

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Date:   Wed, 13 Oct 2021 12:02:16 +0900
From:   Masahiro Yamada <masahiroy@...nel.org>
To:     Nick Desaulniers <ndesaulniers@...gle.com>
Cc:     Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will@...nel.org>, llvm@...ts.linux.dev,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-arm-kernel <linux-arm-kernel@...ts.infradead.org>,
        Vincenzo Frascino <vincenzo.frascino@....com>,
        Lucas Henneman <henneman@...gle.com>
Subject: Re: [PATCH 2/3] arm64: vdso32: lazily invoke COMPAT_CC

On Wed, Oct 13, 2021 at 8:46 AM Nick Desaulniers
<ndesaulniers@...gle.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@...nel.org>
> Reported-by: Lucas Henneman <henneman@...gle.com>
> Signed-off-by: Nick Desaulniers <ndesaulniers@...gle.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