1 ==================================
2 The QEMU build system architecture
3 ==================================
5 This document aims to help developers understand the architecture of the
6 QEMU build system. As with projects using GNU autotools, the QEMU build
7 system has two stages, first the developer runs the "configure" script
8 to determine the local build environment characteristics, then they run
9 "make" to build the project. There is about where the similarities with
10 GNU autotools end, so try to forget what you know about them.
16 The QEMU configure script is written directly in shell, and should be
17 compatible with any POSIX shell, hence it uses #!/bin/sh. An important
18 implication of this is that it is important to avoid using bash-isms on
19 development platforms where bash is the primary host.
21 In contrast to autoconf scripts, QEMU's configure is expected to be
22 silent while it is checking for features. It will only display output
23 when an error occurs, or to show the final feature enablement summary
26 Because QEMU uses the Meson build system under the hood, only VPATH
27 builds are supported. There are two general ways to invoke configure &
30 - VPATH, build artifacts outside of QEMU source tree entirely::
38 - VPATH, build artifacts in a subdir of QEMU source tree::
45 For now, checks on the compilation environment are found in configure
46 rather than meson.build, though this is expected to change. The command
47 line is parsed in the configure script and, whenever needed, converted
48 into the appropriate options to Meson.
50 New checks should be added to Meson, which usually comprises the
53 - Add a Meson build option to meson_options.txt.
55 - Add support to the command line arg parser to handle any new
56 `--enable-XXX`/`--disable-XXX` flags required by the feature.
58 - Add information to the help output message to report on the new
61 - Add code to perform the actual feature check.
63 - Add code to include the feature status in `config-host.h`
65 - Add code to print out the feature status in the configure summary
69 Taking the probe for SDL2_Image as an example, we have the following pieces
72 # Initial variable state
77 # Configure flag processing
78 --disable-sdl-image) sdl_image=disabled
80 --enable-sdl-image) sdl_image=enabled
85 # Help output feature message
86 sdl-image SDL Image support for icons
91 -Dsdl_image=$sdl_image
93 In meson_options.txt::
95 option('sdl', type : 'feature', value : 'auto',
96 description: 'SDL Image support for icons')
101 sdl_image = dependency('SDL2_image', required: get_option('sdl_image'),
102 method: 'pkg-config',
103 static: enable_static)
105 # Create config-host.h (if applicable)
106 config_host_data.set('CONFIG_SDL_IMAGE', sdl_image.found())
109 summary_info += {'SDL image support': sdl_image.found()}
116 The configure script provides a variety of helper functions to assist
117 developers in checking for system features:
120 Attempt to run the system C compiler passing it $ARGS...
123 Attempt to run the system C++ compiler passing it $ARGS...
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()`.
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()`.
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()`.
144 Determine if the macro $NAME is defined by the system C compiler
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()`.
151 Write a minimal C program main() function to the temporary file
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
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
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 $ARGS
171 The Meson build system is currently used to describe the build
174 1) executables, which include:
176 - Tools - qemu-img, qemu-nbd, qga (guest agent), etc
178 - System emulators - qemu-system-$ARCH
180 - Userspace emulators - qemu-$ARCH
186 3) ROMs, which can be either installed as binary blobs or compiled
188 4) other data files, such as icons or desktop files
190 The source code is highly modularized, split across many files to
191 facilitate building of all of these components with as little duplicated
192 compilation as possible. The Meson "sourceset" functionality is used
193 to list the files and their dependency on various configuration
196 All executables are built by default, except for some `contrib/`
197 binaries that are known to fail to build on some platforms (for example
198 32-bit or big-endian platforms). Tests are also built by default,
199 though that might change in the future.
201 Various subsystems that are common to both tools and emulators have
202 their own sourceset, for example `block_ss` for the block device subsystem,
203 `chardev_ss` for the character device subsystem, etc. These sourcesets
204 are then turned into static libraries as follows::
206 libchardev = static_library('chardev', chardev_ss.sources(),
208 build_by_default: false)
210 chardev = declare_dependency(link_whole: libchardev)
212 As of Meson 0.55.1, the special `.fa` suffix should be used for everything
213 that is used with `link_whole`, to ensure that the link flags are placed
214 correctly in the command line.
216 Files linked into emulator targets there can be split into two distinct groups
217 of files, those which are independent of the QEMU emulation target and
218 those which are dependent on the QEMU emulation target.
220 In the target-independent set lives various general purpose helper code,
221 such as error handling infrastructure, standard data structures,
222 platform portability wrapper functions, etc. This code can be compiled
223 once only and the .o files linked into all output binaries.
224 Target-independent code lives in the `common_ss`, `softmmu_ss` and
225 `user_ss` sourcesets. `common_ss` is linked into all emulators, `softmmu_ss`
226 only in system emulators, `user_ss` only in user-mode emulators.
228 In the target-dependent set lives CPU emulation, device emulation and
229 much glue code. This sometimes also has to be compiled multiple times,
230 once for each target being built. Target-dependent files are included
231 in the `specific_ss` sourceset.
233 All binaries link with a static library `libqemuutil.a`, which is then
234 linked to all the binaries. `libqemuutil.a` is built from several
235 sourcesets; most of them however host generated code, and the only two
236 of general interest are `util_ss` and `stub_ss`.
238 The separation between these two is purely for documentation purposes.
239 `util_ss` contains generic utility files. Even though this code is only
240 linked in some binaries, sometimes it requires hooks only in some of
241 these and depend on other functions that are not fully implemented by
242 all QEMU binaries. `stub_ss` links dummy stubs that will only be linked
243 into the binary if the real implementation is not present. In a way,
244 the stubs can be thought of as a portable implementation of the weak
247 The following files concur in the definition of which files are linked
250 `default-configs/*.mak`
251 The files under default-configs/ control what emulated hardware is built
252 into each QEMU system and userspace emulator targets. They merely contain
253 a list of config variable definitions like the machines that should be
254 included. For example, default-configs/aarch64-softmmu.mak has::
256 include arm-softmmu.mak
257 CONFIG_XLNX_ZYNQMP_ARM=y
261 These files are processed together with `default-configs/*.mak` and
262 describe the dependencies between various features, subsystems and
263 device models. They are described in kconfig.rst.
265 These files rarely need changing unless new devices / hardware need to
266 be enabled for a particular system/userspace emulation target
272 Meson has a special convention for invoking Python scripts: if their
273 first line is `#! /usr/bin/env python3` and the file is *not* executable,
274 find_program() arranges to invoke the script under the same Python
275 interpreter that was used to invoke Meson. This is the most common
276 and preferred way to invoke support scripts from Meson build files,
277 because it automatically uses the value of configure's --python= option.
279 In case the script is not written in Python, use a `#! /usr/bin/env ...`
280 line and make the script executable.
282 Scripts written in Python, where it is desirable to make the script
283 executable (for example for test scripts that developers may want to
284 invoke from the command line, such as tests/qapi-schema/test-qapi.py),
285 should be invoked through the `python` variable in meson.build. For
288 test('QAPI schema regression tests', python,
289 args: files('test-qapi.py'),
290 env: test_env, suite: ['qapi-schema', 'qapi-frontend'])
292 This is needed to obey the --python= option passed to the configure
293 script, which may point to something other than the first python3
300 The use of GNU make is required with the QEMU build system.
302 The output of Meson is a build.ninja file, which is used with the Ninja
303 build system. QEMU uses a different approach, where Makefile rules are
304 synthesized from the build.ninja file. The main Makefile includes these
305 rules and wraps them so that e.g. submodules are built before QEMU.
306 The resulting build system is largely non-recursive in nature, in
307 contrast to common practices seen with automake.
309 Tests are also ran by the Makefile with the traditional `make check`
310 phony target, while benchmarks are run with `make bench`. Meson test
311 suites such as `unit` can be ran with `make check-unit` too. It is also
312 possible to run tests defined in meson.build with `meson test`.
314 Important files for the build system
315 ====================================
317 Statically defined files
318 ------------------------
320 The following key files are statically defined in the source tree, with
321 the rules needed to build QEMU. Their behaviour is influenced by a
322 number of dynamically created files listed later.
325 The main entry point used when invoking make to build all the components
326 of QEMU. The default 'all' target will naturally result in the build of
327 every component. Makefile takes care of recursively building submodules
328 directly via a non-recursive set of rules.
331 The meson.build file in the root directory is the main entry point for the
332 Meson build system, and it coordinates the configuration and build of all
333 executables. Build rules for various subdirectories are included in
334 other meson.build files spread throughout the QEMU source tree.
336 `tests/Makefile.include`
337 Rules for external test harnesses. These include the TCG tests,
338 `qemu-iotests` and the Avocado-based acceptance tests.
340 `tests/docker/Makefile.include`
341 Rules for Docker tests. Like tests/Makefile, this file is included
342 directly by the top level Makefile, anything defined in this file will
343 influence the entire build system.
345 `tests/vm/Makefile.include`
346 Rules for VM-based tests. Like tests/Makefile, this file is included
347 directly by the top level Makefile, anything defined in this file will
348 influence the entire build system.
350 Dynamically created files
351 -------------------------
353 The following files are generated dynamically by configure in order to
354 control the behaviour of the statically defined makefiles. This avoids
355 the need for QEMU makefiles to go through any pre-processing as seen
356 with autotools, where Makefile.am generates Makefile.in which generates
362 When configure has determined the characteristics of the build host it
363 will write a long list of variables to config-host.mak file. This
364 provides the various install directories, compiler / linker flags and a
365 variety of `CONFIG_*` variables related to optionally enabled features.
366 This is imported by the top level Makefile and meson.build in order to
367 tailor the build output.
369 config-host.mak is also used as a dependency checking mechanism. If make
370 sees that the modification timestamp on configure is newer than that on
371 config-host.mak, then configure will be re-run.
373 The variables defined here are those which are applicable to all QEMU
374 build outputs. Variables which are potentially different for each
375 emulator target are defined by the next file...
377 `$TARGET-NAME/config-target.mak`
378 TARGET-NAME is the name of a system or userspace emulator, for example,
379 x86_64-softmmu denotes the system emulator for the x86_64 architecture.
380 This file contains the variables which need to vary on a per-target
381 basis. For example, it will indicate whether KVM or Xen are enabled for
382 the target and any other potential custom libraries needed for linking
388 `${TARGET-NAME}-config-devices.mak`
389 TARGET-NAME is again the name of a system or userspace emulator. The
390 config-devices.mak file is automatically generated by make using the
391 scripts/make_device_config.sh program, feeding it the
392 default-configs/$TARGET-NAME file as input.
394 `config-host.h`, `$TARGET-NAME/config-target.h`, `$TARGET-NAME/config-devices.h`
395 These files are used by source code to determine what features
396 are enabled. They are generated from the contents of the corresponding
397 `*.h` files using the scripts/create_config program. This extracts
398 relevant variables and formats them as C preprocessor macros.
407 A Makefile conversion of the build rules in build.ninja. The conversion
408 is straightforward and, were it necessary to debug the rules produced
409 by Meson, it should be enough to look at build.ninja. The conversion
410 is performed by scripts/ninjatool.py.
413 The Makefile definitions that let "make check" run tests defined in
414 meson.build. The rules are produced from Meson's JSON description of
415 tests (obtained with "meson introspect --tests") through the script
416 scripts/mtest2make.py.
423 Print a help message for the most common build targets.
426 Print the value of the variable VAR. Useful for debugging the build