cachepc-linux

Fork of AMDESE/linux with modifications for CachePC side-channel attack
git clone https://git.sinitax.com/sinitax/cachepc-linux
Log | Files | Refs | README | LICENSE | sfeed.txt

llvm.rst (5787B)


      1.. _kbuild_llvm:
      2
      3==============================
      4Building Linux with Clang/LLVM
      5==============================
      6
      7This document covers how to build the Linux kernel with Clang and LLVM
      8utilities.
      9
     10About
     11-----
     12
     13The Linux kernel has always traditionally been compiled with GNU toolchains
     14such as GCC and binutils. Ongoing work has allowed for `Clang
     15<https://clang.llvm.org/>`_ and `LLVM <https://llvm.org/>`_ utilities to be
     16used as viable substitutes. Distributions such as `Android
     17<https://www.android.com/>`_, `ChromeOS
     18<https://www.chromium.org/chromium-os>`_, and `OpenMandriva
     19<https://www.openmandriva.org/>`_ use Clang built kernels.  `LLVM is a
     20collection of toolchain components implemented in terms of C++ objects
     21<https://www.aosabook.org/en/llvm.html>`_. Clang is a front-end to LLVM that
     22supports C and the GNU C extensions required by the kernel, and is pronounced
     23"klang," not "see-lang."
     24
     25Clang
     26-----
     27
     28The compiler used can be swapped out via ``CC=`` command line argument to ``make``.
     29``CC=`` should be set when selecting a config and during a build. ::
     30
     31	make CC=clang defconfig
     32
     33	make CC=clang
     34
     35Cross Compiling
     36---------------
     37
     38A single Clang compiler binary will typically contain all supported backends,
     39which can help simplify cross compiling. ::
     40
     41	make ARCH=arm64 CC=clang CROSS_COMPILE=aarch64-linux-gnu-
     42
     43``CROSS_COMPILE`` is not used to prefix the Clang compiler binary, instead
     44``CROSS_COMPILE`` is used to set a command line flag: ``--target=<triple>``. For
     45example: ::
     46
     47	clang --target=aarch64-linux-gnu foo.c
     48
     49LLVM Utilities
     50--------------
     51
     52LLVM has substitutes for GNU binutils utilities. They can be enabled individually.
     53The full list of supported make variables::
     54
     55	make CC=clang LD=ld.lld AR=llvm-ar NM=llvm-nm STRIP=llvm-strip \
     56	  OBJCOPY=llvm-objcopy OBJDUMP=llvm-objdump READELF=llvm-readelf \
     57	  HOSTCC=clang HOSTCXX=clang++ HOSTAR=llvm-ar HOSTLD=ld.lld
     58
     59To simplify the above command, Kbuild supports the ``LLVM`` variable::
     60
     61	make LLVM=1
     62
     63If your LLVM tools are not available in your PATH, you can supply their
     64location using the LLVM variable with a trailing slash::
     65
     66	make LLVM=/path/to/llvm/
     67
     68which will use ``/path/to/llvm/clang``, ``/path/to/llvm/ld.lld``, etc.
     69
     70If your LLVM tools have a version suffix and you want to test with that
     71explicit version rather than the unsuffixed executables like ``LLVM=1``, you
     72can pass the suffix using the ``LLVM`` variable::
     73
     74	make LLVM=-14
     75
     76which will use ``clang-14``, ``ld.lld-14``, etc.
     77
     78``LLVM=0`` is not the same as omitting ``LLVM`` altogether, it will behave like
     79``LLVM=1``. If you only wish to use certain LLVM utilities, use their respective
     80make variables.
     81
     82The integrated assembler is enabled by default. You can pass ``LLVM_IAS=0`` to
     83disable it.
     84
     85Omitting CROSS_COMPILE
     86----------------------
     87
     88As explained above, ``CROSS_COMPILE`` is used to set ``--target=<triple>``.
     89
     90If ``CROSS_COMPILE`` is not specified, the ``--target=<triple>`` is inferred
     91from ``ARCH``.
     92
     93That means if you use only LLVM tools, ``CROSS_COMPILE`` becomes unnecessary.
     94
     95For example, to cross-compile the arm64 kernel::
     96
     97	make ARCH=arm64 LLVM=1
     98
     99If ``LLVM_IAS=0`` is specified, ``CROSS_COMPILE`` is also used to derive
    100``--prefix=<path>`` to search for the GNU assembler and linker. ::
    101
    102	make ARCH=arm64 LLVM=1 LLVM_IAS=0 CROSS_COMPILE=aarch64-linux-gnu-
    103
    104Supported Architectures
    105-----------------------
    106
    107LLVM does not target all of the architectures that Linux supports and
    108just because a target is supported in LLVM does not mean that the kernel
    109will build or work without any issues. Below is a general summary of
    110architectures that currently work with ``CC=clang`` or ``LLVM=1``. Level
    111of support corresponds to "S" values in the MAINTAINERS files. If an
    112architecture is not present, it either means that LLVM does not target
    113it or there are known issues. Using the latest stable version of LLVM or
    114even the development tree will generally yield the best results.
    115An architecture's ``defconfig`` is generally expected to work well,
    116certain configurations may have problems that have not been uncovered
    117yet. Bug reports are always welcome at the issue tracker below!
    118
    119.. list-table::
    120   :widths: 10 10 10
    121   :header-rows: 1
    122
    123   * - Architecture
    124     - Level of support
    125     - ``make`` command
    126   * - arm
    127     - Supported
    128     - ``LLVM=1``
    129   * - arm64
    130     - Supported
    131     - ``LLVM=1``
    132   * - hexagon
    133     - Maintained
    134     - ``LLVM=1``
    135   * - mips
    136     - Maintained
    137     - ``LLVM=1``
    138   * - powerpc
    139     - Maintained
    140     - ``CC=clang``
    141   * - riscv
    142     - Maintained
    143     - ``LLVM=1``
    144   * - s390
    145     - Maintained
    146     - ``CC=clang``
    147   * - um (User Mode)
    148     - Maintained
    149     - ``LLVM=1``
    150   * - x86
    151     - Supported
    152     - ``LLVM=1``
    153
    154Getting Help
    155------------
    156
    157- `Website <https://clangbuiltlinux.github.io/>`_
    158- `Mailing List <https://lore.kernel.org/llvm/>`_: <llvm@lists.linux.dev>
    159- `Old Mailing List Archives <https://groups.google.com/g/clang-built-linux>`_
    160- `Issue Tracker <https://github.com/ClangBuiltLinux/linux/issues>`_
    161- IRC: #clangbuiltlinux on irc.libera.chat
    162- `Telegram <https://t.me/ClangBuiltLinux>`_: @ClangBuiltLinux
    163- `Wiki <https://github.com/ClangBuiltLinux/linux/wiki>`_
    164- `Beginner Bugs <https://github.com/ClangBuiltLinux/linux/issues?q=is%3Aopen+is%3Aissue+label%3A%22good+first+issue%22>`_
    165
    166.. _getting_llvm:
    167
    168Getting LLVM
    169-------------
    170
    171- https://releases.llvm.org/download.html
    172- https://github.com/llvm/llvm-project
    173- https://llvm.org/docs/GettingStarted.html
    174- https://llvm.org/docs/CMake.html
    175- https://apt.llvm.org/
    176- https://www.archlinux.org/packages/extra/x86_64/llvm/
    177- https://github.com/ClangBuiltLinux/tc-build
    178- https://github.com/ClangBuiltLinux/linux/wiki/Building-Clang-from-source
    179- https://android.googlesource.com/platform/prebuilts/clang/host/linux-x86/