On Thu, Jul 8, 2021 at 7:43 AM 'Nick Desaulniers' via Clang Built Linux <clang-built-linux@xxxxxxxxxxxxxxxx> wrote: > > We get constant feedback that the command line invocation of make is too > long. CROSS_COMPILE is helpful when a toolchain has a prefix of the > target triple, or is an absolute path outside of $PATH, but it's mostly > redundant for a given ARCH. > > If CROSS_COMPILE is not set, simply set --target= for CLANG_FLAGS, > KBUILD_CFLAGS, and KBUILD_AFLAGS based on $ARCH. > > Previously, we'd cross compile via: > $ ARCH=arm64 CROSS_COMPILE=aarch64-linxu-gnu make LLVM=1 LLVM_IAS=1 > Now: > $ ARCH=arm64 make LLVM=1 LLVM_IAS=1 > > Link: https://github.com/ClangBuiltLinux/linux/issues/1399 > Suggested-by: Arnd Bergmann <arnd@xxxxxxxxxx> > Suggested-by: Nathan Chancellor <nathan@xxxxxxxxxx> > Signed-off-by: Nick Desaulniers <ndesaulniers@xxxxxxxxxx> > --- > Changes RFC -> v1: > * Rebase onto linux-kbuild/for-next > * Keep full target triples since missing the gnueabi suffix messes up > 32b ARM. Drop Fangrui's sugguested by tag. Update commit message to > drop references to arm64. > * Flush out TODOS. > * Add note about -EL/-EB, -m32/-m64. > * Add note to Documentation/. > > Documentation/kbuild/llvm.rst | 5 +++++ > scripts/Makefile.clang | 38 +++++++++++++++++++++++++++++++++-- > 2 files changed, 41 insertions(+), 2 deletions(-) When I was considering a similar idea, my plan was to implement this in Kconfig instead of in Makefile because that will pass the compiler information in one direction (only from Kconfig to Kbuild), but that is not so important. We can change it later if needed. I did not complete it because I was investigating some issues (especially [3] below), but maybe that is something we don't care about. Can you address [2] below at least? If we do not have any concern, I can merge it. It is likely so because people are only discussing "We want to omit omit CROSS_COMPILE". [1] explicit target triple for native builds The default target of my distro clang is x86_64-pc-linux-gnu. $ clang --version Ubuntu clang version 11.0.0-2 Target: x86_64-pc-linux-gnu Thread model: posix InstalledDir: /usr/bin So, previously, the kernel was built with implied --target=x86_64-pc-linux-gnu. With this patch, --target=x86_64-linux-gnu will be explicitly specified. The same applies to native-builds of other architectures. For example, when a user builds the arm64 kernel on an arm64 server, --target=aarch64-linux-gnu is explicitly forced. I guess, this is a good direction because the produced code will be more deterministic, irrespective of the Clang's configuration. [2] 32/64-bit configuration is toggled in Kconfig time. Initially, you submitted only arm64. Maybe, it was intentional since arm64 is the simplest case. In the kernel tree, arch/arm and arch/arm64 are very special cases where 32-bit and 64-bit are separated by directory. Some of the other architectures are bi-arch, and 32-bit/64-bit is specified by CONFIG_64BIT in Kconfig time. When Makefiles are being parsed, we actually do not know whether the user is planning to configure the kernel for 32-bit or 64-bit because CONFIG_64BIT is not specified at this point. ARCH=x86 + CONFIG_64BIT=y will build the x86_64 kernel, and ARCH=x86 + CONFIG_64BIT=n will build the i386 kernel. Then, you may wonder else ifeq ($(ARCH),x86) CLANG_FLAGS += --target=x86_64-linux-gnu ... works? Yes, it does fortunately. -m32/-m64 takes precedence over the {x86_64,i386} part of the target triple. As far as I tested, clang --target=x86_64-linux-gnu -m32 produced i386 code. Interestingly, clang --target=i386-linux-gnu -m64 produced x86_64 code. We must rely on this behavior of Clang because --target (which is contained in CLANG_FLAGS) must be specified before the Kconfig time. Then, a user can toggle CONFIG_64BIT any time from menuconfig etc. With this in mind, using $(ARCH) as if-else switches is pointless. $(SRCARCH) is the only meaningful input. else ifeq ($(ARCH),i386) CLANG_FLAGS += --target=i686-linux-gnu else ifeq ($(ARCH),x86) CLANG_FLAGS += --target=x86_64-linux-gnu else ifeq ($(ARCH),x86_64) CLANG_FLAGS += --target=x86_64-linux-gnu should be replaced with: else ifeq ($(SRCARCH),x86_64) CLANG_FLAGS += --target=x86_64-linux-gnu Some architectures are not only bi-arch, but also bi-endian. You hardcoded 64bit little endian for ppc: else ifeq ($(ARCH),powerpc) CLANG_FLAGS += --target=powerpc64le-linux-gnu But, we must rely on the fact that clang --target=powerpc64le-linux-gnu -mbig-endian -m32 produces big-endian 32-bit code. This makes the "64le" part meaningless. This should be noted. Otherwise it is difficult to understand why --target=x86_64-linux-gnu works fine with building the i386 kernel. [3] User-space compilation This does not matter to the kernel itself, but Kbuild compiles some userspace programs for the target architecture. See the samples/ directory for example. Another example is net/bpfilter/Makefile, which embeds the user mode helper when CONFIG_BPFILTER_UMH=y. For this purpose, Kconfig checks if $(CC) is capable of linking the userspace. (CONFIG_CC_CAN_LINK). When cross-building with Clang, I cannot see CONFIG_CC_CAN_LINK set. If we care about CONFIG_CC_CAN_LINK, probably, --sysroot or something should be set according to: https://clang.llvm.org/docs/CrossCompilation.html This is an existing issue, but I have no time for looking into this. On debian systems, sysroot for cross-compilation are located in /usr/aarch64-linux-gnu, /usr/arm-linux-gnueabi, /usr/arm-linux-gnueabihf, /usr/i686-linux-gnu/ etc. but I do not know if it is the same across distros. [4] What is the best target if we hard-code it? Currently, we require the correct CROSS_COMPILE is provided by users. The target might impact the performance or the ABI. It was difficult for me to define which one is better than another. For example for ARCH=arm, which is better --target=arm-linux-gnueabi or --target=arm-lnux-gnueabihf or something we don't care about? > diff --git a/Documentation/kbuild/llvm.rst b/Documentation/kbuild/llvm.rst > index b18401d2ba82..80c63dd9a6d1 100644 > --- a/Documentation/kbuild/llvm.rst > +++ b/Documentation/kbuild/llvm.rst > @@ -46,6 +46,11 @@ example: :: > > clang --target=aarch64-linux-gnu foo.c > > +When both ``LLVM=1`` and ``LLVM_IAS=1`` are used, ``CROSS_COMPILE`` becomes > +unnecessary and can be inferred from ``ARCH``. Example: :: > + > + ARCH=arm64 make LLVM=1 LLVM_IAS=1 > + > LLVM Utilities > -------------- > > diff --git a/scripts/Makefile.clang b/scripts/Makefile.clang > index 297932e973d4..a79088797a50 100644 > --- a/scripts/Makefile.clang > +++ b/scripts/Makefile.clang > @@ -1,6 +1,40 @@ > -ifneq ($(CROSS_COMPILE),) > +# Individual arch/{arch}/Makfiles should use -EL/-EB to set intended endianness > +# and -m32/-m64 to set word size based on Kconfigs instead of relying on the > +# target triple. > +ifeq ($(CROSS_COMPILE),) > +ifneq ($(LLVM),) > +ifeq ($(LLVM_IAS),1) > +ifeq ($(ARCH),arm) > +CLANG_FLAGS += --target=arm-linux-gnueabi > +else ifeq ($(ARCH),arm64) > +CLANG_FLAGS += --target=aarch64-linux-gnu > +else ifeq ($(ARCH),hexagon) > +CLANG_FLAGS += --target=hexagon-linux-gnu > +else ifeq ($(ARCH),i386) > +CLANG_FLAGS += --target=i686-linux-gnu > +else ifeq ($(ARCH),m68k) > +CLANG_FLAGS += --target=m68k-linux-gnu > +else ifeq ($(ARCH),mips) > +CLANG_FLAGS += --target=mipsel-linux-gnu > +else ifeq ($(ARCH),powerpc) > +CLANG_FLAGS += --target=powerpc64le-linux-gnu > +else ifeq ($(ARCH),riscv) > +CLANG_FLAGS += --target=riscv64-linux-gnu > +else ifeq ($(ARCH),s390) > +CLANG_FLAGS += --target=s390x-linux-gnu > +else ifeq ($(ARCH),x86) > +CLANG_FLAGS += --target=x86_64-linux-gnu > +else ifeq ($(ARCH),x86_64) > +CLANG_FLAGS += --target=x86_64-linux-gnu > +else > +$(error Specify CROSS_COMPILE or add '--target=' option to scripts/Makefile.clang) > +endif # ARCH > +endif # LLVM_IAS > +endif # LLVM > +else > CLANG_FLAGS += --target=$(notdir $(CROSS_COMPILE:%-=%)) > -endif > +endif # CROSS_COMPILE > + > ifeq ($(LLVM_IAS),1) > CLANG_FLAGS += -integrated-as > else > -- > 2.32.0.93.g670b81a890-goog > > -- > You received this message because you are subscribed to the Google Groups "Clang Built Linux" group. > To unsubscribe from this group and stop receiving emails from it, send an email to clang-built-linux+unsubscribe@xxxxxxxxxxxxxxxx. > To view this discussion on the web visit https://groups.google.com/d/msgid/clang-built-linux/20210707224310.1403944-3-ndesaulniers%40google.com. -- Best Regards Masahiro Yamada
