On Mon, Feb 24, 2020 at 09:41:28AM -0800, Nick Desaulniers wrote: > Added to kbuild documentation. Provides more official info on building > kernels with Clang and LLVM than our wiki. > > Suggested-by: Kees Cook <keescook@xxxxxxxxxxxx> > Reviewed-by: Nathan Chancellor <natechancellor@xxxxxxxxx> > Signed-off-by: Nick Desaulniers <ndesaulniers@xxxxxxxxxx> > --- > Documentation/kbuild/index.rst | 1 + > Documentation/kbuild/llvm.rst | 80 ++++++++++++++++++++++++++++++++++ > 2 files changed, 81 insertions(+) > create mode 100644 Documentation/kbuild/llvm.rst > > diff --git a/Documentation/kbuild/index.rst b/Documentation/kbuild/index.rst > index 0f144fad99a6..3882bd5f7728 100644 > --- a/Documentation/kbuild/index.rst > +++ b/Documentation/kbuild/index.rst > @@ -19,6 +19,7 @@ Kernel Build System > > issues > reproducible-builds > + llvm > > .. only:: subproject and html > > diff --git a/Documentation/kbuild/llvm.rst b/Documentation/kbuild/llvm.rst > new file mode 100644 > index 000000000000..68ae022aebc0 > --- /dev/null > +++ b/Documentation/kbuild/llvm.rst > @@ -0,0 +1,80 @@ > +============================== > +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. On going 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. > + > + ARCH=arm64 CROSS_COMPILE=aarch64-linux-gnu- make CC=clang > + > +`CROSS_COMPILE` is not used to suffix the Clang compiler binary, instead s/suffix/prefix/ > +`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. These can be invoked as > +additional parameters to `make`. > + > + make CC=clang AS=clang LD=ld.lld AR=llvm-ar NM=llvm-nm STRIP=llvm-strip \\ > + OBJCOPY=llvm-objcopy OBJDUMP=llvm-objdump OBJSIZE=llvm-objsize \\ > + READELF=llvm-readelf HOSTCC=clang HOSTCXX=clang++ HOSTAR=llvm-ar \\ > + HOSTLD=ld.lld > + > +Getting Help > +------------ > + > +- `Website <https://clangbuiltlinux.github.io/>`_ > +- `Mailing List <https://groups.google.com/forum/#!forum/clang-built-linux>`_: <clang-built-linux@xxxxxxxxxxxxxxxx> > +- `Issue Tracker <https://github.com/ClangBuiltLinux/linux/issues>`_ > +- IRC: #clangbuiltlinux on chat.freenode.net > +- `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 > +------------- > + > +- http://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/ Should this also include an update to Documentation/process/changes.rst with the minimum version required? (I would expect this to be "9" for Clang, and "11" for ld.lld.) Otherwise, yes, with Randy and Masahiro's suggestions, please consider it: Reviewed-by: Kees Cook <keescook@xxxxxxxxxxxx> -- Kees Cook
