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diff --git a/Documentation/kbuild/llvm.rst b/Documentation/kbuild/llvm.rst new file mode 100644 index 000000000..6b2bac8e9 --- /dev/null +++ b/Documentation/kbuild/llvm.rst @@ -0,0 +1,179 @@ +.. _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>`_, and `OpenMandriva +<https://www.openmandriva.org/>`_ use Clang built kernels. `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." + +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 + +Cross Compiling +--------------- + +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- + +``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: :: + + clang --target=aarch64-linux-gnu foo.c + +LLVM Utilities +-------------- + +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 + +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. + +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. + +``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 integrated assembler is enabled by default. You can pass ``LLVM_IAS=0`` to +disable it. + +Omitting CROSS_COMPILE +---------------------- + +As explained above, ``CROSS_COMPILE`` is used to set ``--target=<triple>``. + +If ``CROSS_COMPILE`` is not specified, the ``--target=<triple>`` is inferred +from ``ARCH``. + +That means if you use only LLVM tools, ``CROSS_COMPILE`` becomes unnecessary. + +For example, to cross-compile the arm64 kernel:: + + make ARCH=arm64 LLVM=1 + +If ``LLVM_IAS=0`` is specified, ``CROSS_COMPILE`` is also used to derive +``--prefix=<path>`` to search for the GNU assembler and linker. :: + + make ARCH=arm64 LLVM=1 LLVM_IAS=0 CROSS_COMPILE=aarch64-linux-gnu- + +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`` + * - mips + - Maintained + - ``LLVM=1`` + * - powerpc + - Maintained + - ``CC=clang`` + * - riscv + - Maintained + - ``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 +------------- + +- 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/ |