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diff --git a/Documentation/kbuild/llvm.rst b/Documentation/kbuild/llvm.rst new file mode 100644 index 0000000000..b1d97fafdd --- /dev/null +++ b/Documentation/kbuild/llvm.rst @@ -0,0 +1,222 @@ +.. _kbuild_llvm: + +============================== +Building Linux with Clang/LLVM +============================== + +This document covers how to build the Linux kernel with Clang and LLVM +utilities. + +About +----- + +The Linux kernel has always traditionally been compiled with GNU toolchains +such as GCC and binutils. Ongoing work has allowed for `Clang +<https://clang.llvm.org/>`_ and `LLVM <https://llvm.org/>`_ utilities to be +used as viable substitutes. Distributions such as `Android +<https://www.android.com/>`_, `ChromeOS +<https://www.chromium.org/chromium-os>`_, `OpenMandriva +<https://www.openmandriva.org/>`_, and `Chimera Linux +<https://chimera-linux.org/>`_ use Clang built kernels. Google's and Meta's +datacenter fleets also run kernels built with Clang. + +`LLVM is a collection of toolchain components implemented in terms of C++ +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." + +Building with LLVM +------------------ + +Invoke ``make`` via:: + + make LLVM=1 + +to compile for the host target. For cross compiling:: + + make LLVM=1 ARCH=arm64 + +The LLVM= argument +------------------ + +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 + +``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. The +following may also be used:: + + PATH=/path/to/llvm:$PATH make LLVM=1 + +If your LLVM tools have a version suffix and you want to test with that +explicit version rather than the unsuffixed executables like ``LLVM=1``, you +can pass the suffix using the ``LLVM`` variable:: + + make LLVM=-14 + +which will use ``clang-14``, ``ld.lld-14``, etc. + +To support combinations of out of tree paths with version suffixes, we +recommend:: + + PATH=/path/to/llvm/:$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. + +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``. + +Cross Compiling +--------------- + +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 +---------------------- + +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:: + + make LLVM=1 ARCH=arm LLVM_IAS=0 CROSS_COMPILE=arm-linux-gnueabi- + + +Ccache +------ + +``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 Reproducible_builds_ for more info): + + KBUILD_BUILD_TIMESTAMP='' make LLVM=1 CC="ccache clang" + +.. _KBUILD_BUILD_TIMESTAMP: kbuild.html#kbuild-build-timestamp +.. _Reproducible_builds: reproducible-builds.html#timestamps + +Supported Architectures +----------------------- + +LLVM does not target all of the architectures that Linux supports and +just because a target is supported in LLVM does not mean that the kernel +will build or work without any issues. Below is a general summary of +architectures that currently work with ``CC=clang`` or ``LLVM=1``. Level +of support corresponds to "S" values in the MAINTAINERS files. If an +architecture is not present, it either means that LLVM does not target +it or there are known issues. Using the latest stable version of LLVM or +even the development tree will generally yield the best results. +An architecture's ``defconfig`` is generally expected to work well, +certain configurations may have problems that have not been uncovered +yet. Bug reports are always welcome at the issue tracker below! + +.. list-table:: + :widths: 10 10 10 + :header-rows: 1 + + * - Architecture + - Level of support + - ``make`` command + * - arm + - Supported + - ``LLVM=1`` + * - arm64 + - Supported + - ``LLVM=1`` + * - hexagon + - Maintained + - ``LLVM=1`` + * - loongarch + - Maintained + - ``LLVM=1`` + * - mips + - Maintained + - ``LLVM=1`` + * - powerpc + - Maintained + - ``LLVM=1`` + * - riscv + - Supported + - ``LLVM=1`` + * - s390 + - Maintained + - ``CC=clang`` + * - um (User Mode) + - Maintained + - ``LLVM=1`` + * - x86 + - Supported + - ``LLVM=1`` + +Getting Help +------------ + +- `Website <https://clangbuiltlinux.github.io/>`_ +- `Mailing List <https://lore.kernel.org/llvm/>`_: <llvm@lists.linux.dev> +- `Old Mailing List Archives <https://groups.google.com/g/clang-built-linux>`_ +- `Issue Tracker <https://github.com/ClangBuiltLinux/linux/issues>`_ +- IRC: #clangbuiltlinux on irc.libera.chat +- `Telegram <https://t.me/ClangBuiltLinux>`_: @ClangBuiltLinux +- `Wiki <https://github.com/ClangBuiltLinux/linux/wiki>`_ +- `Beginner Bugs <https://github.com/ClangBuiltLinux/linux/issues?q=is%3Aopen+is%3Aissue+label%3A%22good+first+issue%22>`_ + +.. _getting_llvm: + +Getting 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. + +- https://releases.llvm.org/download.html +- https://github.com/llvm/llvm-project +- https://llvm.org/docs/GettingStarted.html +- https://llvm.org/docs/CMake.html +- https://apt.llvm.org/ +- https://www.archlinux.org/packages/extra/x86_64/llvm/ +- https://github.com/ClangBuiltLinux/tc-build +- https://github.com/ClangBuiltLinux/linux/wiki/Building-Clang-from-source +- https://android.googlesource.com/platform/prebuilts/clang/host/linux-x86/ |