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The most common question around building the Linux kernel with clang is "does it work?" and the answer has always been "it depends on your architecture, configuration, and LLVM version" with no hard answers for users wanting to experiment. LLVM support has significantly improved over the past couple of years, resulting in more architectures and configurations supported, and continuous integration has made it easier to see what works and what does not. Add a section that goes over what architectures are supported in the current kernel version, how they should be built (with just clang or the LLVM utilities as well), and the level of support they receive. This will make it easier for people to try out building their kernel with LLVM and reporting issues that come about from it. Suggested-by: Miguel Ojeda <ojeda@kernel.org> Signed-off-by: Nathan Chancellor <natechancellor@gmail.com> Reviewed-by: Nick Desaulniers <ndesaulnier@google.com> Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
135 lines
4.3 KiB
ReStructuredText
135 lines
4.3 KiB
ReStructuredText
.. _kbuild_llvm:
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==============================
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Building Linux with Clang/LLVM
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==============================
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This document covers how to build the Linux kernel with Clang and LLVM
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utilities.
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About
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-----
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The Linux kernel has always traditionally been compiled with GNU toolchains
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such as GCC and binutils. Ongoing work has allowed for `Clang
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<https://clang.llvm.org/>`_ and `LLVM <https://llvm.org/>`_ utilities to be
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used as viable substitutes. Distributions such as `Android
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<https://www.android.com/>`_, `ChromeOS
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<https://www.chromium.org/chromium-os>`_, and `OpenMandriva
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<https://www.openmandriva.org/>`_ use Clang built kernels. `LLVM is a
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collection of toolchain components implemented in terms of C++ objects
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<https://www.aosabook.org/en/llvm.html>`_. Clang is a front-end to LLVM that
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supports C and the GNU C extensions required by the kernel, and is pronounced
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"klang," not "see-lang."
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Clang
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-----
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The compiler used can be swapped out via ``CC=`` command line argument to ``make``.
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``CC=`` should be set when selecting a config and during a build. ::
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make CC=clang defconfig
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make CC=clang
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Cross Compiling
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---------------
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A single Clang compiler binary will typically contain all supported backends,
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which can help simplify cross compiling. ::
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ARCH=arm64 CROSS_COMPILE=aarch64-linux-gnu- make CC=clang
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``CROSS_COMPILE`` is not used to prefix the Clang compiler binary, instead
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``CROSS_COMPILE`` is used to set a command line flag: ``--target=<triple>``. For
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example: ::
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clang --target=aarch64-linux-gnu foo.c
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LLVM Utilities
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--------------
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LLVM has substitutes for GNU binutils utilities. Kbuild supports ``LLVM=1``
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to enable them. ::
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make LLVM=1
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They can be enabled individually. The full list of the parameters: ::
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make CC=clang LD=ld.lld AR=llvm-ar NM=llvm-nm STRIP=llvm-strip \
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OBJCOPY=llvm-objcopy OBJDUMP=llvm-objdump READELF=llvm-readelf \
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HOSTCC=clang HOSTCXX=clang++ HOSTAR=llvm-ar HOSTLD=ld.lld
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Currently, the integrated assembler is disabled by default. You can pass
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``LLVM_IAS=1`` to enable it.
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Supported Architectures
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-----------------------
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LLVM does not target all of the architectures that Linux supports and
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just because a target is supported in LLVM does not mean that the kernel
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will build or work without any issues. Below is a general summary of
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architectures that currently work with ``CC=clang`` or ``LLVM=1``. Level
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of support corresponds to "S" values in the MAINTAINERS files. If an
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architecture is not present, it either means that LLVM does not target
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it or there are known issues. Using the latest stable version of LLVM or
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even the development tree will generally yield the best results.
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An architecture's ``defconfig`` is generally expected to work well,
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certain configurations may have problems that have not been uncovered
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yet. Bug reports are always welcome at the issue tracker below!
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.. list-table::
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:widths: 10 10 10
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:header-rows: 1
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* - Architecture
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- Level of support
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- ``make`` command
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* - arm
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- Supported
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- ``LLVM=1``
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* - arm64
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- Supported
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- ``LLVM=1``
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* - mips
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- Maintained
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- ``CC=clang``
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* - powerpc
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- Maintained
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- ``CC=clang``
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* - riscv
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- Maintained
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- ``CC=clang``
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* - s390
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- Maintained
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- ``CC=clang``
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* - x86
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- Supported
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- ``LLVM=1``
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Getting Help
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------------
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- `Website <https://clangbuiltlinux.github.io/>`_
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- `Mailing List <https://groups.google.com/forum/#!forum/clang-built-linux>`_: <clang-built-linux@googlegroups.com>
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- `Issue Tracker <https://github.com/ClangBuiltLinux/linux/issues>`_
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- IRC: #clangbuiltlinux on chat.freenode.net
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- `Telegram <https://t.me/ClangBuiltLinux>`_: @ClangBuiltLinux
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- `Wiki <https://github.com/ClangBuiltLinux/linux/wiki>`_
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- `Beginner Bugs <https://github.com/ClangBuiltLinux/linux/issues?q=is%3Aopen+is%3Aissue+label%3A%22good+first+issue%22>`_
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.. _getting_llvm:
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Getting LLVM
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-------------
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- https://releases.llvm.org/download.html
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- https://github.com/llvm/llvm-project
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- https://llvm.org/docs/GettingStarted.html
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- https://llvm.org/docs/CMake.html
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- https://apt.llvm.org/
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- https://www.archlinux.org/packages/extra/x86_64/llvm/
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- https://github.com/ClangBuiltLinux/tc-build
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- https://github.com/ClangBuiltLinux/linux/wiki/Building-Clang-from-source
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- https://android.googlesource.com/platform/prebuilts/clang/host/linux-x86/
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