xref: /openbmc/qemu/docs/devel/build-system.rst (revision 19f4ed36)
1==================================
2The QEMU build system architecture
3==================================
4
5This document aims to help developers understand the architecture of the
6QEMU build system. As with projects using GNU autotools, the QEMU build
7system has two stages, first the developer runs the "configure" script
8to determine the local build environment characteristics, then they run
9"make" to build the project. There is about where the similarities with
10GNU autotools end, so try to forget what you know about them.
11
12
13Stage 1: configure
14==================
15
16The QEMU configure script is written directly in shell, and should be
17compatible with any POSIX shell, hence it uses #!/bin/sh. An important
18implication of this is that it is important to avoid using bash-isms on
19development platforms where bash is the primary host.
20
21In contrast to autoconf scripts, QEMU's configure is expected to be
22silent while it is checking for features. It will only display output
23when an error occurs, or to show the final feature enablement summary
24on completion.
25
26Because QEMU uses the Meson build system under the hood, only VPATH
27builds are supported.  There are two general ways to invoke configure &
28perform a build:
29
30 - VPATH, build artifacts outside of QEMU source tree entirely::
31
32     cd ../
33     mkdir build
34     cd build
35     ../qemu/configure
36     make
37
38 - VPATH, build artifacts in a subdir of QEMU source tree::
39
40     mkdir build
41     cd build
42     ../configure
43     make
44
45For now, checks on the compilation environment are found in configure
46rather than meson.build, though this is expected to change.  The command
47line is parsed in the configure script and, whenever needed, converted
48into the appropriate options to Meson.
49
50New checks should be added to Meson, which usually comprises the
51following tasks:
52
53 - Add a Meson build option to meson_options.txt.
54
55 - Add support to the command line arg parser to handle any new
56   `--enable-XXX`/`--disable-XXX` flags required by the feature.
57
58 - Add information to the help output message to report on the new
59   feature flag.
60
61 - Add code to perform the actual feature check.
62
63 - Add code to include the feature status in `config-host.h`
64
65 - Add code to print out the feature status in the configure summary
66   upon completion.
67
68
69Taking the probe for SDL2_Image as an example, we have the following pieces
70in configure::
71
72  # Initial variable state
73  sdl_image=auto
74
75  ..snip..
76
77  # Configure flag processing
78  --disable-sdl-image) sdl_image=disabled
79  ;;
80  --enable-sdl-image) sdl_image=enabled
81  ;;
82
83  ..snip..
84
85  # Help output feature message
86  sdl-image         SDL Image support for icons
87
88  ..snip..
89
90  # Meson invocation
91  -Dsdl_image=$sdl_image
92
93In meson_options.txt::
94
95  option('sdl', type : 'feature', value : 'auto',
96         description: 'SDL Image support for icons')
97
98In meson.build::
99
100  # Detect dependency
101  sdl_image = dependency('SDL2_image', required: get_option('sdl_image'),
102                         method: 'pkg-config',
103                         kwargs: static_kwargs)
104
105  # Create config-host.h (if applicable)
106  config_host_data.set('CONFIG_SDL_IMAGE', sdl_image.found())
107
108  # Summary
109  summary_info += {'SDL image support': sdl_image.found()}
110
111
112
113Helper functions
114----------------
115
116The configure script provides a variety of helper functions to assist
117developers in checking for system features:
118
119`do_cc $ARGS...`
120   Attempt to run the system C compiler passing it $ARGS...
121
122`do_cxx $ARGS...`
123   Attempt to run the system C++ compiler passing it $ARGS...
124
125`compile_object $CFLAGS`
126   Attempt to compile a test program with the system C compiler using
127   $CFLAGS. The test program must have been previously written to a file
128   called $TMPC.  The replacement in Meson is the compiler object `cc`,
129   which has methods such as `cc.compiles()`,
130   `cc.check_header()`, `cc.has_function()`.
131
132`compile_prog $CFLAGS $LDFLAGS`
133   Attempt to compile a test program with the system C compiler using
134   $CFLAGS and link it with the system linker using $LDFLAGS. The test
135   program must have been previously written to a file called $TMPC.
136   The replacement in Meson is `cc.find_library()` and `cc.links()`.
137
138`has $COMMAND`
139   Determine if $COMMAND exists in the current environment, either as a
140   shell builtin, or executable binary, returning 0 on success.  The
141   replacement in Meson is `find_program()`.
142
143`check_define $NAME`
144   Determine if the macro $NAME is defined by the system C compiler
145
146`check_include $NAME`
147   Determine if the include $NAME file is available to the system C
148   compiler.  The replacement in Meson is `cc.has_header()`.
149
150`write_c_skeleton`
151   Write a minimal C program main() function to the temporary file
152   indicated by $TMPC
153
154`feature_not_found $NAME $REMEDY`
155   Print a message to stderr that the feature $NAME was not available
156   on the system, suggesting the user try $REMEDY to address the
157   problem.
158
159`error_exit $MESSAGE $MORE...`
160   Print $MESSAGE to stderr, followed by $MORE... and then exit from the
161   configure script with non-zero status
162
163`query_pkg_config $ARGS...`
164   Run pkg-config passing it $ARGS. If QEMU is doing a static build,
165   then --static will be automatically added to $ARGS
166
167
168Stage 2: Meson
169==============
170
171The Meson build system is currently used to describe the build
172process for:
173
1741) executables, which include:
175
176   - Tools - qemu-img, qemu-nbd, qga (guest agent), etc
177
178   - System emulators - qemu-system-$ARCH
179
180   - Userspace emulators - qemu-$ARCH
181
182   - Unit tests
183
1842) documentation
185
1863) ROMs, which can be either installed as binary blobs or compiled
187
1884) other data files, such as icons or desktop files
189
190All executables are built by default, except for some `contrib/`
191binaries that are known to fail to build on some platforms (for example
19232-bit or big-endian platforms).  Tests are also built by default,
193though that might change in the future.
194
195The source code is highly modularized, split across many files to
196facilitate building of all of these components with as little duplicated
197compilation as possible. Using the Meson "sourceset" functionality,
198`meson.build` files group the source files in rules that are
199enabled according to the available system libraries and to various
200configuration symbols.  Sourcesets belong to one of four groups:
201
202Subsystem sourcesets:
203  Various subsystems that are common to both tools and emulators have
204  their own sourceset, for example `block_ss` for the block device subsystem,
205  `chardev_ss` for the character device subsystem, etc.  These sourcesets
206  are then turned into static libraries as follows::
207
208    libchardev = static_library('chardev', chardev_ss.sources(),
209                                name_suffix: 'fa',
210                                build_by_default: false)
211
212    chardev = declare_dependency(link_whole: libchardev)
213
214  As of Meson 0.55.1, the special `.fa` suffix should be used for everything
215  that is used with `link_whole`, to ensure that the link flags are placed
216  correctly in the command line.
217
218Target-independent emulator sourcesets:
219  Various general purpose helper code is compiled only once and
220  the .o files are linked into all output binaries that need it.
221  This includes error handling infrastructure, standard data structures,
222  platform portability wrapper functions, etc.
223
224  Target-independent code lives in the `common_ss`, `softmmu_ss` and
225  `user_ss` sourcesets.  `common_ss` is linked into all emulators,
226  `softmmu_ss` only in system emulators, `user_ss` only in user-mode
227  emulators.
228
229  Target-independent sourcesets must exercise particular care when using
230  `if_false` rules.  The `if_false` rule will be used correctly when linking
231  emulator binaries; however, when *compiling* target-independent files
232  into .o files, Meson may need to pick *both* the `if_true` and
233  `if_false` sides to cater for targets that want either side.  To
234  achieve that, you can add a special rule using the ``CONFIG_ALL``
235  symbol::
236
237    # Some targets have CONFIG_ACPI, some don't, so this is not enough
238    softmmu_ss.add(when: 'CONFIG_ACPI`, if_true: files('acpi.c'),
239                                        if_false: files('acpi-stub.c'))
240
241    # This is required as well:
242    softmmu_ss.add(when: 'CONFIG_ALL`, if_true: files('acpi-stub.c'))
243
244Target-dependent emulator sourcesets:
245  In the target-dependent set lives CPU emulation, some device emulation and
246  much glue code. This sometimes also has to be compiled multiple times,
247  once for each target being built.  Target-dependent files are included
248  in the `specific_ss` sourceset.
249
250  Each emulator also includes sources for files in the `hw/` and `target/`
251  subdirectories.  The subdirectory used for each emulator comes
252  from the target's definition of ``TARGET_BASE_ARCH`` or (if missing)
253  ``TARGET_ARCH``, as found in `default-configs/targets/*.mak`.
254
255  Each subdirectory in `hw/` adds one sourceset to the `hw_arch` dictionary,
256  for example::
257
258    arm_ss = ss.source_set()
259    arm_ss.add(files('boot.c'), fdt)
260    ...
261    hw_arch += {'arm': arm_ss}
262
263  The sourceset is only used for system emulators.
264
265  Each subdirectory in `target/` instead should add one sourceset to each
266  of the `target_arch` and `target_softmmu_arch`, which are used respectively
267  for all emulators and for system emulators only.  For example::
268
269    arm_ss = ss.source_set()
270    arm_softmmu_ss = ss.source_set()
271    ...
272    target_arch += {'arm': arm_ss}
273    target_softmmu_arch += {'arm': arm_softmmu_ss}
274
275Utility sourcesets:
276  All binaries link with a static library `libqemuutil.a`.  This library
277  is built from several sourcesets; most of them however host generated
278  code, and the only two of general interest are `util_ss` and `stub_ss`.
279
280  The separation between these two is purely for documentation purposes.
281  `util_ss` contains generic utility files.  Even though this code is only
282  linked in some binaries, sometimes it requires hooks only in some of
283  these and depend on other functions that are not fully implemented by
284  all QEMU binaries.  `stub_ss` links dummy stubs that will only be linked
285  into the binary if the real implementation is not present.  In a way,
286  the stubs can be thought of as a portable implementation of the weak
287  symbols concept.
288
289
290The following files concur in the definition of which files are linked
291into each emulator:
292
293`default-configs/devices/*.mak`
294  The files under `default-configs/devices/` control the boards and devices
295  that are built into each QEMU system emulation targets. They merely contain
296  a list of config variable definitions such as::
297
298    include arm-softmmu.mak
299    CONFIG_XLNX_ZYNQMP_ARM=y
300    CONFIG_XLNX_VERSAL=y
301
302`*/Kconfig`
303  These files are processed together with `default-configs/devices/*.mak` and
304  describe the dependencies between various features, subsystems and
305  device models.  They are described in :ref:`kconfig`
306
307`default-configs/targets/*.mak`
308  These files mostly define symbols that appear in the `*-config-target.h`
309  file for each emulator [#cfgtarget]_.  However, the ``TARGET_ARCH``
310  and ``TARGET_BASE_ARCH`` will also be used to select the `hw/` and
311  `target/` subdirectories that are compiled into each target.
312
313.. [#cfgtarget] This header is included by `qemu/osdep.h` when
314                compiling files from the target-specific sourcesets.
315
316These files rarely need changing unless you are adding a completely
317new target, or enabling new devices or hardware for a particular
318system/userspace emulation target
319
320
321Support scripts
322---------------
323
324Meson has a special convention for invoking Python scripts: if their
325first line is `#! /usr/bin/env python3` and the file is *not* executable,
326find_program() arranges to invoke the script under the same Python
327interpreter that was used to invoke Meson.  This is the most common
328and preferred way to invoke support scripts from Meson build files,
329because it automatically uses the value of configure's --python= option.
330
331In case the script is not written in Python, use a `#! /usr/bin/env ...`
332line and make the script executable.
333
334Scripts written in Python, where it is desirable to make the script
335executable (for example for test scripts that developers may want to
336invoke from the command line, such as tests/qapi-schema/test-qapi.py),
337should be invoked through the `python` variable in meson.build. For
338example::
339
340  test('QAPI schema regression tests', python,
341       args: files('test-qapi.py'),
342       env: test_env, suite: ['qapi-schema', 'qapi-frontend'])
343
344This is needed to obey the --python= option passed to the configure
345script, which may point to something other than the first python3
346binary on the path.
347
348
349Stage 3: makefiles
350==================
351
352The use of GNU make is required with the QEMU build system.
353
354The output of Meson is a build.ninja file, which is used with the Ninja
355build system.  QEMU uses a different approach, where Makefile rules are
356synthesized from the build.ninja file.  The main Makefile includes these
357rules and wraps them so that e.g. submodules are built before QEMU.
358The resulting build system is largely non-recursive in nature, in
359contrast to common practices seen with automake.
360
361Tests are also ran by the Makefile with the traditional `make check`
362phony target, while benchmarks are run with `make bench`.  Meson test
363suites such as `unit` can be ran with `make check-unit` too.  It is also
364possible to run tests defined in meson.build with `meson test`.
365
366Important files for the build system
367====================================
368
369Statically defined files
370------------------------
371
372The following key files are statically defined in the source tree, with
373the rules needed to build QEMU. Their behaviour is influenced by a
374number of dynamically created files listed later.
375
376`Makefile`
377  The main entry point used when invoking make to build all the components
378  of QEMU. The default 'all' target will naturally result in the build of
379  every component. Makefile takes care of recursively building submodules
380  directly via a non-recursive set of rules.
381
382`*/meson.build`
383  The meson.build file in the root directory is the main entry point for the
384  Meson build system, and it coordinates the configuration and build of all
385  executables.  Build rules for various subdirectories are included in
386  other meson.build files spread throughout the QEMU source tree.
387
388`tests/Makefile.include`
389  Rules for external test harnesses. These include the TCG tests,
390  `qemu-iotests` and the Avocado-based acceptance tests.
391
392`tests/docker/Makefile.include`
393  Rules for Docker tests. Like tests/Makefile, this file is included
394  directly by the top level Makefile, anything defined in this file will
395  influence the entire build system.
396
397`tests/vm/Makefile.include`
398  Rules for VM-based tests. Like tests/Makefile, this file is included
399  directly by the top level Makefile, anything defined in this file will
400  influence the entire build system.
401
402Dynamically created files
403-------------------------
404
405The following files are generated dynamically by configure in order to
406control the behaviour of the statically defined makefiles. This avoids
407the need for QEMU makefiles to go through any pre-processing as seen
408with autotools, where Makefile.am generates Makefile.in which generates
409Makefile.
410
411Built by configure:
412
413`config-host.mak`
414  When configure has determined the characteristics of the build host it
415  will write a long list of variables to config-host.mak file. This
416  provides the various install directories, compiler / linker flags and a
417  variety of `CONFIG_*` variables related to optionally enabled features.
418  This is imported by the top level Makefile and meson.build in order to
419  tailor the build output.
420
421  config-host.mak is also used as a dependency checking mechanism. If make
422  sees that the modification timestamp on configure is newer than that on
423  config-host.mak, then configure will be re-run.
424
425  The variables defined here are those which are applicable to all QEMU
426  build outputs. Variables which are potentially different for each
427  emulator target are defined by the next file...
428
429
430Built by Meson:
431
432`${TARGET-NAME}-config-devices.mak`
433  TARGET-NAME is again the name of a system or userspace emulator. The
434  config-devices.mak file is automatically generated by make using the
435  scripts/make_device_config.sh program, feeding it the
436  default-configs/$TARGET-NAME file as input.
437
438`config-host.h`, `$TARGET-NAME/config-target.h`, `$TARGET-NAME/config-devices.h`
439  These files are used by source code to determine what features
440  are enabled.  They are generated from the contents of the corresponding
441  `*.h` files using the scripts/create_config program. This extracts
442  relevant variables and formats them as C preprocessor macros.
443
444`build.ninja`
445  The build rules.
446
447
448Built by Makefile:
449
450`Makefile.ninja`
451  A Makefile include that bridges to ninja for the actual build.  The
452  Makefile is mostly a list of targets that Meson included in build.ninja.
453
454`Makefile.mtest`
455  The Makefile definitions that let "make check" run tests defined in
456  meson.build.  The rules are produced from Meson's JSON description of
457  tests (obtained with "meson introspect --tests") through the script
458  scripts/mtest2make.py.
459
460
461Useful make targets
462-------------------
463
464`help`
465  Print a help message for the most common build targets.
466
467`print-VAR`
468  Print the value of the variable VAR. Useful for debugging the build
469  system.
470