Recent fixes for an embargoed hardware security vulnerability failed to link with ld.lld (LLVM's linker). [0] To be fair, our documentation mentions ``CC=clang`` foremost with ``LLVM=1`` being buried "below the fold." We want to encourage the use of ``LLVM=1`` rather than just ``CC=clang``. Make that suggestion "above the fold" and "front and center" in our docs. While here, the following additional changes were made: - remove the bit about CROSS_COMPILE setting --target=, that's no longer true. - Add ARCH=loongarch to the list of maintained targets (though we're still working on getting defconfig building cleanly at the moment; we're pretty close). - Bump ARCH=powerpc from CC=clang to LLVM=1 status. - Promote ARCH=riscv from being Maintained to being Supported. Android is working towards supporting RISC-V, and we have excellent support from multiple companies in this regard. - Note that the toolchain distribution on kernel.org has been built with profile data from kernel builds. - Note how to use ccache with clang. Link: https://github.com/ClangBuiltLinux/linux/issues/1907 [0] Reviewed-by: Nathan Chancellor <nathan@xxxxxxxxxx> Signed-off-by: Nick Desaulniers <ndesaulniers@xxxxxxxxxx> --- Changes in v3: - Note why kernel.org toolchains are shiny. - Add note about ccache to doc and commit message. - Link to v2: https://lore.kernel.org/r/20230824-docs-v2-1-848abf651184@xxxxxxxxxx Changes in v2: - Commit message changes: - Put SOB tag above the fold. - Mention PPC change in commit message. - Fix typos in commit message. - Add Nathan's RB tag to commit message. - Fix kernel doc warnings - Reword s390 example under `The LLVM= argument`. - Reword and add examples to `The LLVM_IAS= argument`. - Link to v1: https://lore.kernel.org/r/20230824-docs-v1-1-67e061278b8f@xxxxxxxxxx --- Documentation/kbuild/llvm.rst | 124 +++++++++++++++++++++++++++--------------- 1 file changed, 80 insertions(+), 44 deletions(-) diff --git a/Documentation/kbuild/llvm.rst b/Documentation/kbuild/llvm.rst index c3851fe1900d..384622dd36b7 100644 --- a/Documentation/kbuild/llvm.rst +++ b/Documentation/kbuild/llvm.rst @@ -25,50 +25,38 @@ objects <https://www.aosabook.org/en/llvm.html>`_. Clang is a front-end to LLVM that supports C and the GNU C extensions required by the kernel, and is pronounced "klang," not "see-lang." -Clang ------ - -The compiler used can be swapped out via ``CC=`` command line argument to ``make``. -``CC=`` should be set when selecting a config and during a build. :: - - make CC=clang defconfig - - make CC=clang +Building with LLVM +------------------ -Cross Compiling ---------------- +Invoke ``make`` via:: -A single Clang compiler binary will typically contain all supported backends, -which can help simplify cross compiling. :: - - make ARCH=arm64 CC=clang CROSS_COMPILE=aarch64-linux-gnu- + make LLVM=1 -``CROSS_COMPILE`` is not used to prefix the Clang compiler binary, instead -``CROSS_COMPILE`` is used to set a command line flag: ``--target=<triple>``. For -example: :: +to compile for the host target. For cross compiling:: - clang --target=aarch64-linux-gnu foo.c + make LLVM=1 ARCH=arm64 -LLVM Utilities --------------- +The LLVM= argument +------------------ -LLVM has substitutes for GNU binutils utilities. They can be enabled individually. -The full list of supported make variables:: +LLVM has substitutes for GNU binutils utilities. They can be enabled +individually. The full list of supported make variables:: make CC=clang LD=ld.lld AR=llvm-ar NM=llvm-nm STRIP=llvm-strip \ OBJCOPY=llvm-objcopy OBJDUMP=llvm-objdump READELF=llvm-readelf \ HOSTCC=clang HOSTCXX=clang++ HOSTAR=llvm-ar HOSTLD=ld.lld -To simplify the above command, Kbuild supports the ``LLVM`` variable:: - - make LLVM=1 +``LLVM=1`` expands to the above. If your LLVM tools are not available in your PATH, you can supply their location using the LLVM variable with a trailing slash:: make LLVM=/path/to/llvm/ -which will use ``/path/to/llvm/clang``, ``/path/to/llvm/ld.lld``, etc. +which will use ``/path/to/llvm/clang``, ``/path/to/llvm/ld.lld``, etc. The +following may also be used:: + + PATH=/path/to/llvm:$PATH make LLVM=1 If your LLVM tools have a version suffix and you want to test with that explicit version rather than the unsuffixed executables like ``LLVM=1``, you @@ -78,31 +66,72 @@ can pass the suffix using the ``LLVM`` variable:: which will use ``clang-14``, ``ld.lld-14``, etc. +To support combinations of out of tree paths with version suffixes, we +recommend:: + + PATH=/path/to/llvm/:$PATH make LLVM=-14 + ``LLVM=0`` is not the same as omitting ``LLVM`` altogether, it will behave like -``LLVM=1``. If you only wish to use certain LLVM utilities, use their respective -make variables. +``LLVM=1``. If you only wish to use certain LLVM utilities, use their +respective make variables. + +The same value used for ``LLVM=`` should be set for each invocation of ``make`` +if configuring and building via distinct commands. ``LLVM=`` should also be set +as an environment variable when running scripts that will eventually run +``make``. -The integrated assembler is enabled by default. You can pass ``LLVM_IAS=0`` to -disable it. +Cross Compiling +--------------- -Omitting CROSS_COMPILE +A single Clang compiler binary (and corresponding LLVM utilities) will +typically contain all supported back ends, which can help simplify cross +compiling especially when ``LLVM=1`` is used. If you use only LLVM tools, +``CROSS_COMPILE`` or target-triple-prefixes become unnecessary. Example:: + + make LLVM=1 ARCH=arm64 + +As an example of mixing LLVM and GNU utilities, for a target like ``ARCH=s390`` +which does not yet have ``ld.lld`` or ``llvm-objcopy`` support, you could +invoke ``make`` via:: + + make LLVM=1 ARCH=s390 LD=s390x-linux-gnu-ld.bfd \ + OBJCOPY=s390x-linux-gnu-objcopy + +This example will invoke ``s390x-linux-gnu-ld.bfd`` as the linker and +``s390x-linux-gnu-objcopy``, so ensure those are reachable in your ``$PATH``. + +``CROSS_COMPILE`` is not used to prefix the Clang compiler binary (or +corresponding LLVM utilities) as is the case for GNU utilities when ``LLVM=1`` +is not set. + +The LLVM_IAS= argument ---------------------- -As explained above, ``CROSS_COMPILE`` is used to set ``--target=<triple>``. +Clang can assemble assembler code. You can pass ``LLVM_IAS=0`` to disable this +behavior and have Clang invoke the corresponding non-integrated assembler +instead. Example:: + + make LLVM=1 LLVM_IAS=0 + +``CROSS_COMPILE`` is necessary when cross compiling and ``LLVM_IAS=0`` +is used in order to set ``--prefix=`` for the compiler to find the +corresponding non-integrated assembler (typically, you don't want to use the +system assembler when targeting another architecture). Example:: -If ``CROSS_COMPILE`` is not specified, the ``--target=<triple>`` is inferred -from ``ARCH``. + make LLVM=1 ARCH=arm LLVM_IAS=0 CROSS_COMPILE=arm-linux-gnueabi- -That means if you use only LLVM tools, ``CROSS_COMPILE`` becomes unnecessary. -For example, to cross-compile the arm64 kernel:: +Ccache +------ - make ARCH=arm64 LLVM=1 +``ccache`` can be used with ``clang`` to improve subsequent builds, (though +KBUILD_BUILD_TIMESTAMP_ should be set to a deterministic value between builds +in order to avoid 100% cache misses, see Reprocible_builds_ for more info): -If ``LLVM_IAS=0`` is specified, ``CROSS_COMPILE`` is also used to derive -``--prefix=<path>`` to search for the GNU assembler and linker. :: + KBUILD_BUILD_TIMESTAMP='' make LLVM=1 CC="ccache clang" - make ARCH=arm64 LLVM=1 LLVM_IAS=0 CROSS_COMPILE=aarch64-linux-gnu- +.. _KBUILD_BUILD_TIMESTAMP: kbuild.html#kbuild-build-timestamp +.. _Reproducible_builds: reproducible-builds.html#timestamps Supported Architectures ----------------------- @@ -135,14 +164,17 @@ yet. Bug reports are always welcome at the issue tracker below! * - hexagon - Maintained - ``LLVM=1`` + * - loongarch + - Maintained + - ``LLVM=1`` * - mips - Maintained - ``LLVM=1`` * - powerpc - Maintained - - ``CC=clang`` + - ``LLVM=1`` * - riscv - - Maintained + - Supported - ``LLVM=1`` * - s390 - Maintained @@ -171,7 +203,11 @@ Getting Help Getting LLVM ------------- -We provide prebuilt stable versions of LLVM on `kernel.org <https://kernel.org/pub/tools/llvm/>`_. +We provide prebuilt stable versions of LLVM on `kernel.org +<https://kernel.org/pub/tools/llvm/>`_. These have been optimized with profile +data for building Linux kernels, which should improve kernel build times +relative to other distributions of LLVM. + Below are links that may be useful for building LLVM from source or procuring it through a distribution's package manager. --- base-commit: 93f5de5f648d2b1ce3540a4ac71756d4a852dc23 change-id: 20230824-docs-c17a5de7f103 Best regards, -- Nick Desaulniers <ndesaulniers@xxxxxxxxxx>
