3519c4d6e0
Based on a vote at the LLVM BoF at Plumbers 2020, we decided to start small, supporting just one formal upstream release of LLVM for now. We can probably widen the support window of supported versions over time. Also, note that LLVM's release process is different than GCC's. GCC tends to have 1 major release per year while releasing minor updates to the past 3 major versions. LLVM tends to support one major release and one minor release every six months. Signed-off-by: Nick Desaulniers <ndesaulniers@google.com> Tested-by: Gustavo A. R. Silva <gustavoars@kernel.org> Tested-by: Nathan Chancellor <natechancellor@gmail.com> Reviewed-by: Kees Cook <keescook@chromium.org> Reviewed-by: Nathan Chancellor <natechancellor@gmail.com> Reviewed-by: Masahiro Yamada <masahiroy@kernel.org> Acked-by: Will Deacon <will@kernel.org> Link: https://lore.kernel.org/r/20200826191555.3350406-1-ndesaulniers@google.com Signed-off-by: Jonathan Corbet <corbet@lwn.net>
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92 lines
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.. _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 OBJSIZE=llvm-size \\
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READELF=llvm-readelf HOSTCC=clang HOSTCXX=clang++ HOSTAR=llvm-ar \\
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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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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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