summaryrefslogtreecommitdiffstats
path: root/Documentation/kbuild/llvm.rst
diff options
context:
space:
mode:
Diffstat (limited to 'Documentation/kbuild/llvm.rst')
-rw-r--r--Documentation/kbuild/llvm.rst222
1 files changed, 222 insertions, 0 deletions
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/