1 .. _kbuild_llvm: 1 .. _kbuild_llvm:
2 2
3 ============================== 3 ==============================
4 Building Linux with Clang/LLVM 4 Building Linux with Clang/LLVM
5 ============================== 5 ==============================
6 6
7 This document covers how to build the Linux ke 7 This document covers how to build the Linux kernel with Clang and LLVM
8 utilities. 8 utilities.
9 9
10 About 10 About
11 ----- 11 -----
12 12
13 The Linux kernel has always traditionally been 13 The Linux kernel has always traditionally been compiled with GNU toolchains
14 such as GCC and binutils. Ongoing work has all 14 such as GCC and binutils. Ongoing work has allowed for `Clang
15 <https://clang.llvm.org/>`_ and `LLVM <https:/ 15 <https://clang.llvm.org/>`_ and `LLVM <https://llvm.org/>`_ utilities to be
16 used as viable substitutes. Distributions such 16 used as viable substitutes. Distributions such as `Android
17 <https://www.android.com/>`_, `ChromeOS 17 <https://www.android.com/>`_, `ChromeOS
18 <https://www.chromium.org/chromium-os>`_, `Ope !! 18 <https://www.chromium.org/chromium-os>`_, and `OpenMandriva
19 <https://www.openmandriva.org/>`_, and `Chimer !! 19 <https://www.openmandriva.org/>`_ use Clang built kernels. `LLVM is a
20 <https://chimera-linux.org/>`_ use Clang built !! 20 collection of toolchain components implemented in terms of C++ objects
21 datacenter fleets also run kernels built with !! 21 <https://www.aosabook.org/en/llvm.html>`_. Clang is a front-end to LLVM that
22 !! 22 supports C and the GNU C extensions required by the kernel, and is pronounced
23 `LLVM is a collection of toolchain components !! 23 "klang," not "see-lang."
24 objects <https://www.aosabook.org/en/llvm.html !! 24
25 that supports C and the GNU C extensions requi !! 25 Clang
26 pronounced "klang," not "see-lang." !! 26 -----
27 27
28 Building with LLVM !! 28 The compiler used can be swapped out via ``CC=`` command line argument to ``make``.
29 ------------------ !! 29 ``CC=`` should be set when selecting a config and during a build. ::
30 30
31 Invoke ``make`` via:: !! 31 make CC=clang defconfig
32 32
33 make LLVM=1 !! 33 make CC=clang
>> 34
>> 35 Cross Compiling
>> 36 ---------------
>> 37
>> 38 A single Clang compiler binary will typically contain all supported backends,
>> 39 which can help simplify cross compiling. ::
34 40
35 to compile for the host target. For cross comp !! 41 make ARCH=arm64 CC=clang CROSS_COMPILE=aarch64-linux-gnu-
36 42
37 make LLVM=1 ARCH=arm64 !! 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
>> 45 example: ::
38 46
39 The LLVM= argument !! 47 clang --target=aarch64-linux-gnu foo.c
40 ------------------ <<
41 48
42 LLVM has substitutes for GNU binutils utilitie !! 49 LLVM Utilities
43 individually. The full list of supported make !! 50 --------------
>> 51
>> 52 LLVM has substitutes for GNU binutils utilities. They can be enabled individually.
>> 53 The full list of supported make variables::
44 54
45 make CC=clang LD=ld.lld AR=llvm-ar NM= 55 make CC=clang LD=ld.lld AR=llvm-ar NM=llvm-nm STRIP=llvm-strip \
46 OBJCOPY=llvm-objcopy OBJDUMP=llvm-ob 56 OBJCOPY=llvm-objcopy OBJDUMP=llvm-objdump READELF=llvm-readelf \
47 HOSTCC=clang HOSTCXX=clang++ HOSTAR= 57 HOSTCC=clang HOSTCXX=clang++ HOSTAR=llvm-ar HOSTLD=ld.lld
48 58
49 ``LLVM=1`` expands to the above. !! 59 To simplify the above command, Kbuild supports the ``LLVM`` variable::
>> 60
>> 61 make LLVM=1
50 62
51 If your LLVM tools are not available in your P 63 If your LLVM tools are not available in your PATH, you can supply their
52 location using the LLVM variable with a traili 64 location using the LLVM variable with a trailing slash::
53 65
54 make LLVM=/path/to/llvm/ 66 make LLVM=/path/to/llvm/
55 67
56 which will use ``/path/to/llvm/clang``, ``/pat !! 68 which will use ``/path/to/llvm/clang``, ``/path/to/llvm/ld.lld``, etc.
57 following may also be used:: <<
58 <<
59 PATH=/path/to/llvm:$PATH make LLVM=1 <<
60 69
61 If your LLVM tools have a version suffix and y 70 If your LLVM tools have a version suffix and you want to test with that
62 explicit version rather than the unsuffixed ex 71 explicit version rather than the unsuffixed executables like ``LLVM=1``, you
63 can pass the suffix using the ``LLVM`` variabl 72 can pass the suffix using the ``LLVM`` variable::
64 73
65 make LLVM=-14 74 make LLVM=-14
66 75
67 which will use ``clang-14``, ``ld.lld-14``, et 76 which will use ``clang-14``, ``ld.lld-14``, etc.
68 77
69 To support combinations of out of tree paths w <<
70 recommend:: <<
71 <<
72 PATH=/path/to/llvm/:$PATH make LLVM=-1 <<
73 <<
74 ``LLVM=0`` is not the same as omitting ``LLVM` 78 ``LLVM=0`` is not the same as omitting ``LLVM`` altogether, it will behave like
75 ``LLVM=1``. If you only wish to use certain LL !! 79 ``LLVM=1``. If you only wish to use certain LLVM utilities, use their respective
76 respective make variables. !! 80 make variables.
77 <<
78 The same value used for ``LLVM=`` should be se <<
79 if configuring and building via distinct comma <<
80 as an environment variable when running script <<
81 ``make``. <<
82 <<
83 Cross Compiling <<
84 --------------- <<
85 <<
86 A single Clang compiler binary (and correspond <<
87 typically contain all supported back ends, whi <<
88 compiling especially when ``LLVM=1`` is used. <<
89 ``CROSS_COMPILE`` or target-triple-prefixes be <<
90 <<
91 make LLVM=1 ARCH=arm64 <<
92 <<
93 As an example of mixing LLVM and GNU utilities <<
94 which does not yet have ``ld.lld`` or ``llvm-o <<
95 invoke ``make`` via:: <<
96 81
97 make LLVM=1 ARCH=s390 LD=s390x-linux-g !! 82 The integrated assembler is enabled by default. You can pass ``LLVM_IAS=0`` to
98 OBJCOPY=s390x-linux-gnu-objcopy !! 83 disable it.
99 84
100 This example will invoke ``s390x-linux-gnu-ld. !! 85 Omitting CROSS_COMPILE
101 ``s390x-linux-gnu-objcopy``, so ensure those a <<
102 <<
103 ``CROSS_COMPILE`` is not used to prefix the Cl <<
104 corresponding LLVM utilities) as is the case f <<
105 is not set. <<
106 <<
107 The LLVM_IAS= argument <<
108 ---------------------- 86 ----------------------
109 87
110 Clang can assemble assembler code. You can pas !! 88 As explained above, ``CROSS_COMPILE`` is used to set ``--target=<triple>``.
111 behavior and have Clang invoke the correspondi <<
112 instead. Example:: <<
113 <<
114 make LLVM=1 LLVM_IAS=0 <<
115 89
116 ``CROSS_COMPILE`` is necessary when cross comp !! 90 If ``CROSS_COMPILE`` is not specified, the ``--target=<triple>`` is inferred
117 is used in order to set ``--prefix=`` for the !! 91 from ``ARCH``.
118 corresponding non-integrated assembler (typica <<
119 system assembler when targeting another archit <<
120 92
121 make LLVM=1 ARCH=arm LLVM_IAS=0 CROSS_ !! 93 That means if you use only LLVM tools, ``CROSS_COMPILE`` becomes unnecessary.
122 94
>> 95 For example, to cross-compile the arm64 kernel::
123 96
124 Ccache !! 97 make ARCH=arm64 LLVM=1
125 ------ <<
126 98
127 ``ccache`` can be used with ``clang`` to impro !! 99 If ``LLVM_IAS=0`` is specified, ``CROSS_COMPILE`` is also used to derive
128 KBUILD_BUILD_TIMESTAMP_ should be set to a det !! 100 ``--prefix=<path>`` to search for the GNU assembler and linker. ::
129 in order to avoid 100% cache misses, see Repro <<
130 101
131 KBUILD_BUILD_TIMESTAMP='' make LLVM=1 !! 102 make ARCH=arm64 LLVM=1 LLVM_IAS=0 CROSS_COMPILE=aarch64-linux-gnu-
132 <<
133 .. _KBUILD_BUILD_TIMESTAMP: kbuild.html#kbuild <<
134 .. _Reproducible_builds: reproducible-builds.h <<
135 103
136 Supported Architectures 104 Supported Architectures
137 ----------------------- 105 -----------------------
138 106
139 LLVM does not target all of the architectures 107 LLVM does not target all of the architectures that Linux supports and
140 just because a target is supported in LLVM doe 108 just because a target is supported in LLVM does not mean that the kernel
141 will build or work without any issues. Below i 109 will build or work without any issues. Below is a general summary of
142 architectures that currently work with ``CC=cl 110 architectures that currently work with ``CC=clang`` or ``LLVM=1``. Level
143 of support corresponds to "S" values in the MA 111 of support corresponds to "S" values in the MAINTAINERS files. If an
144 architecture is not present, it either means t 112 architecture is not present, it either means that LLVM does not target
145 it or there are known issues. Using the latest 113 it or there are known issues. Using the latest stable version of LLVM or
146 even the development tree will generally yield 114 even the development tree will generally yield the best results.
147 An architecture's ``defconfig`` is generally e 115 An architecture's ``defconfig`` is generally expected to work well,
148 certain configurations may have problems that 116 certain configurations may have problems that have not been uncovered
149 yet. Bug reports are always welcome at the iss 117 yet. Bug reports are always welcome at the issue tracker below!
150 118
151 .. list-table:: 119 .. list-table::
152 :widths: 10 10 10 120 :widths: 10 10 10
153 :header-rows: 1 121 :header-rows: 1
154 122
155 * - Architecture 123 * - Architecture
156 - Level of support 124 - Level of support
157 - ``make`` command 125 - ``make`` command
158 * - arm 126 * - arm
159 - Supported 127 - Supported
160 - ``LLVM=1`` 128 - ``LLVM=1``
161 * - arm64 129 * - arm64
162 - Supported 130 - Supported
163 - ``LLVM=1`` 131 - ``LLVM=1``
164 * - hexagon <<
165 - Maintained <<
166 - ``LLVM=1`` <<
167 * - loongarch <<
168 - Maintained <<
169 - ``LLVM=1`` <<
170 * - mips 132 * - mips
171 - Maintained 133 - Maintained
172 - ``LLVM=1`` !! 134 - ``CC=clang``
173 * - powerpc 135 * - powerpc
174 - Maintained 136 - Maintained
175 - ``LLVM=1`` !! 137 - ``CC=clang``
176 * - riscv 138 * - riscv
177 - Supported <<
178 - ``LLVM=1`` <<
179 * - s390 <<
180 - Maintained 139 - Maintained
181 - ``LLVM=1`` (LLVM >= 18.1.0), ``CC=clang !! 140 - ``CC=clang``
182 * - um (User Mode) !! 141 * - s390
183 - Maintained 142 - Maintained
184 - ``LLVM=1`` !! 143 - ``CC=clang``
185 * - x86 144 * - x86
186 - Supported 145 - Supported
187 - ``LLVM=1`` 146 - ``LLVM=1``
188 147
189 Getting Help 148 Getting Help
190 ------------ 149 ------------
191 150
192 - `Website <https://clangbuiltlinux.github.io/ 151 - `Website <https://clangbuiltlinux.github.io/>`_
193 - `Mailing List <https://lore.kernel.org/llvm/> 152 - `Mailing List <https://lore.kernel.org/llvm/>`_: <llvm@lists.linux.dev">https://lore.kernel.org/llvm/>`_: <llvm@lists.linux.dev>
194 - `Old Mailing List Archives <https://groups.g 153 - `Old Mailing List Archives <https://groups.google.com/g/clang-built-linux>`_
195 - `Issue Tracker <https://github.com/ClangBuil 154 - `Issue Tracker <https://github.com/ClangBuiltLinux/linux/issues>`_
196 - IRC: #clangbuiltlinux on irc.libera.chat 155 - IRC: #clangbuiltlinux on irc.libera.chat
197 - `Telegram <https://t.me/ClangBuiltLinux>`_: 156 - `Telegram <https://t.me/ClangBuiltLinux>`_: @ClangBuiltLinux
198 - `Wiki <https://github.com/ClangBuiltLinux/li 157 - `Wiki <https://github.com/ClangBuiltLinux/linux/wiki>`_
199 - `Beginner Bugs <https://github.com/ClangBuil 158 - `Beginner Bugs <https://github.com/ClangBuiltLinux/linux/issues?q=is%3Aopen+is%3Aissue+label%3A%22good+first+issue%22>`_
200 159
201 .. _getting_llvm: 160 .. _getting_llvm:
202 161
203 Getting LLVM 162 Getting LLVM
204 ------------- 163 -------------
205 <<
206 We provide prebuilt stable versions of LLVM on <<
207 <https://kernel.org/pub/tools/llvm/>`_. These <<
208 data for building Linux kernels, which should <<
209 relative to other distributions of LLVM. <<
210 <<
211 Below are links that may be useful for buildin <<
212 it through a distribution's package manager. <<
213 164
214 - https://releases.llvm.org/download.html 165 - https://releases.llvm.org/download.html
215 - https://github.com/llvm/llvm-project 166 - https://github.com/llvm/llvm-project
216 - https://llvm.org/docs/GettingStarted.html 167 - https://llvm.org/docs/GettingStarted.html
217 - https://llvm.org/docs/CMake.html 168 - https://llvm.org/docs/CMake.html
218 - https://apt.llvm.org/ 169 - https://apt.llvm.org/
219 - https://www.archlinux.org/packages/extra/x86 170 - https://www.archlinux.org/packages/extra/x86_64/llvm/
220 - https://github.com/ClangBuiltLinux/tc-build 171 - https://github.com/ClangBuiltLinux/tc-build
221 - https://github.com/ClangBuiltLinux/linux/wik 172 - https://github.com/ClangBuiltLinux/linux/wiki/Building-Clang-from-source
222 - https://android.googlesource.com/platform/pr 173 - https://android.googlesource.com/platform/prebuilts/clang/host/linux-x86/
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