# The QEMU build system architecture

This document aims to help developers understand the architecture of the QEMU build system. As with projects using GNU autotools, the QEMU build system has two stages, first the developer runs the “configure” script to determine the local build environment characteristics, then they run “make” to build the project. There is about where the similarities with GNU autotools end, so try to forget what you know about them.

## Stage 1: configure

The QEMU configure script is written directly in shell, and should be compatible with any POSIX shell, hence it uses #!/bin/sh. An important implication of this is that it is important to avoid using bash-isms on development platforms where bash is the primary host.

In contrast to autoconf scripts, QEMU’s configure is expected to be silent while it is checking for features. It will only display output when an error occurs, or to show the final feature enablement summary on completion.

Because QEMU uses the Meson build system under the hood, only VPATH builds are supported. There are two general ways to invoke configure & perform a build:

> - VPATH, build artifacts outside of QEMU source tree entirely:

>       cd ../
>       mkdir build
>       cd build
>       ../qemu/configure
>       make

> - VPATH, build artifacts in a subdir of QEMU source tree:

>       mkdir build
>       cd build
>       ../configure
>       make

The configure script automatically recognizes command line options for which a same-named Meson option exists; dashes in the command line are replaced with underscores.

Many checks on the compilation environment are still found in configure rather than `meson.build`, but new checks should be added directly to `meson.build`.

Patches are also welcome to move existing checks from the configure phase to `meson.build`. When doing so, ensure that `meson.build` does not use anymore the keys that you have removed from `config-host.mak`. Typically these will be replaced in `meson.build` by boolean variables, `get_option('optname')` invocations, or `dep.found()` expressions. In general, the remaining checks have little or no interdependencies, so they can be moved one by one.

### Helper functions

The configure script provides a variety of helper functions to assist developers in checking for system features:

`do_cc $ARGS...`

  Attempt to run the system C compiler passing it $ARGS…

`do_cxx $ARGS...`

  Attempt to run the system C++ compiler passing it $ARGS…

`compile_object $CFLAGS`

  Attempt to compile a test program with the system C compiler using $CFLAGS. The test program must have been previously written to a file called $TMPC. The replacement in Meson is the compiler object `cc`, which has methods such as `cc.compiles()`, `cc.check_header()`, `cc.has_function()`.

`compile_prog $CFLAGS $LDFLAGS`

  Attempt to compile a test program with the system C compiler using $CFLAGS and link it with the system linker using $LDFLAGS. The test program must have been previously written to a file called $TMPC. The replacement in Meson is `cc.find_library()` and `cc.links()`.

`has $COMMAND`

  Determine if $COMMAND exists in the current environment, either as a shell builtin, or executable binary, returning 0 on success. The replacement in Meson is `find_program()`.

`check_define $NAME`

  Determine if the macro $NAME is defined by the system C compiler

`check_include $NAME`

  Determine if the include $NAME file is available to the system C compiler. The replacement in Meson is `cc.has_header()`.

`write_c_skeleton`

  Write a minimal C program main() function to the temporary file indicated by $TMPC

`error_exit $MESSAGE $MORE...`

  Print $MESSAGE to stderr, followed by $MORE… and then exit from the configure script with non-zero status

`query_pkg_config $ARGS...`

  Run pkg-config passing it $ARGS. If QEMU is doing a static build, then –static will be automatically added to $ARGS

## Stage 2: Meson

The Meson build system is currently used to describe the build process for:

1. executables, which include:

   - Tools - `qemu-img`, `qemu-nbd`, `qga` (guest agent), etc

   - System emulators - `qemu-system-$ARCH`

   - Userspace emulators - `qemu-$ARCH`

   - Unit tests

2. documentation

3. ROMs, which can be either installed as binary blobs or compiled

4. other data files, such as icons or desktop files

All executables are built by default, except for some `contrib/` binaries that are known to fail to build on some platforms (for example 32-bit or big-endian platforms). Tests are also built by default, though that might change in the future.

The source code is highly modularized, split across many files to facilitate building of all of these components with as little duplicated compilation as possible. Using the Meson “sourceset” functionality, `meson.build` files group the source files in rules that are enabled according to the available system libraries and to various configuration symbols. Sourcesets belong to one of four groups:

Subsystem sourcesets:

  Various subsystems that are common to both tools and emulators have their own sourceset, for example `block_ss` for the block device subsystem, `chardev_ss` for the character device subsystem, etc. These sourcesets are then turned into static libraries as follows:

      libchardev = static_library('chardev', chardev_ss.sources(),
                                  name_suffix: 'fa',
                                  build_by_default: false)

      chardev = declare_dependency(link_whole: libchardev)

  As of Meson 0.55.1, the special `.fa` suffix should be used for everything that is used with `link_whole`, to ensure that the link flags are placed correctly in the command line.

Target-independent emulator sourcesets:

  Various general purpose helper code is compiled only once and the .o files are linked into all output binaries that need it. This includes error handling infrastructure, standard data structures, platform portability wrapper functions, etc.

  Target-independent code lives in the `common_ss`, `softmmu_ss` and `user_ss` sourcesets. `common_ss` is linked into all emulators, `softmmu_ss` only in system emulators, `user_ss` only in user-mode emulators.

  Target-independent sourcesets must exercise particular care when using `if_false` rules. The `if_false` rule will be used correctly when linking emulator binaries; however, when compiling target-independent files into .o files, Meson may need to pick both the `if_true` and `if_false` sides to cater for targets that want either side. To achieve that, you can add a special rule using the `CONFIG_ALL` symbol:

      # Some targets have CONFIG_ACPI, some don't, so this is not enough
      softmmu_ss.add(when: 'CONFIG_ACPI', if_true: files('acpi.c'),
                                          if_false: files('acpi-stub.c'))

      # This is required as well:
      softmmu_ss.add(when: 'CONFIG_ALL', if_true: files('acpi-stub.c'))

Target-dependent emulator sourcesets:

  In the target-dependent set lives CPU emulation, some device emulation and much glue code. This sometimes also has to be compiled multiple times, once for each target being built. Target-dependent files are included in the `specific_ss` sourceset.

  Each emulator also includes sources for files in the `hw/` and `target/` subdirectories. The subdirectory used for each emulator comes from the target’s definition of `TARGET_BASE_ARCH` or (if missing) `TARGET_ARCH`, as found in `default-configs/targets/*.mak`.

  Each subdirectory in `hw/` adds one sourceset to the `hw_arch` dictionary, for example:

      arm_ss = ss.source_set()
      arm_ss.add(files('boot.c'), fdt)
      ...
      hw_arch += {'arm': arm_ss}

  The sourceset is only used for system emulators.

  Each subdirectory in `target/` instead should add one sourceset to each of the `target_arch` and `target_softmmu_arch`, which are used respectively for all emulators and for system emulators only. For example:

      arm_ss = ss.source_set()
      arm_softmmu_ss = ss.source_set()
      ...
      target_arch += {'arm': arm_ss}
      target_softmmu_arch += {'arm': arm_softmmu_ss}

Module sourcesets:

  There are two dictionaries for modules: `modules` is used for target-independent modules and `target_modules` is used for target-dependent modules. When modules are disabled the `module` source sets are added to `softmmu_ss` and the `target_modules` source sets are added to `specific_ss`.

  Both dictionaries are nested. One dictionary is created per subdirectory, and these per-subdirectory dictionaries are added to the toplevel dictionaries. For example:

      hw_display_modules = {}
      qxl_ss = ss.source_set()
      ...
      hw_display_modules += { 'qxl': qxl_ss }
      modules += { 'hw-display': hw_display_modules }

Utility sourcesets:

  All binaries link with a static library `libqemuutil.a`. This library is built from several sourcesets; most of them however host generated code, and the only two of general interest are `util_ss` and `stub_ss`.

  The separation between these two is purely for documentation purposes. `util_ss` contains generic utility files. Even though this code is only linked in some binaries, sometimes it requires hooks only in some of these and depend on other functions that are not fully implemented by all QEMU binaries. `stub_ss` links dummy stubs that will only be linked into the binary if the real implementation is not present. In a way, the stubs can be thought of as a portable implementation of the weak symbols concept.

The following files concur in the definition of which files are linked into each emulator:

`default-configs/devices/*.mak`

  The files under `default-configs/devices/` control the boards and devices that are built into each QEMU system emulation targets. They merely contain a list of config variable definitions such as:

      include arm-softmmu.mak
      CONFIG_XLNX_ZYNQMP_ARM=y
      CONFIG_XLNX_VERSAL=y

`*/Kconfig`

  These files are processed together with `default-configs/devices/*.mak` and describe the dependencies between various features, subsystems and device models. They are described in QEMU and Kconfig

`default-configs/targets/*.mak`

  These files mostly define symbols that appear in the `*-config-target.h` file for each emulator [1]. However, the `TARGET_ARCH` and `TARGET_BASE_ARCH` will also be used to select the `hw/` and `target/` subdirectories that are compiled into each target.

[1]

This header is included by `qemu/osdep.h` when compiling files from the target-specific sourcesets.

These files rarely need changing unless you are adding a completely new target, or enabling new devices or hardware for a particular system/userspace emulation target

### Adding checks

New checks should be added to Meson. Compiler checks can be as simple as the following:

    config_host_data.set('HAVE_BTRFS_H', cc.has_header('linux/btrfs.h'))

A more complex task such as adding a new dependency usually comprises the following tasks:

> - Add a Meson build option to meson_options.txt.

> - Add code to perform the actual feature check.

> - Add code to include the feature status in `config-host.h`

> - Add code to print out the feature status in the configure summary upon completion.

Taking the probe for SDL2_Image as an example, we have the following in `meson_options.txt`:

    option('sdl_image', type : 'feature', value : 'auto',
           description: 'SDL Image support for icons')

Unless the option was given a non-`auto` value (on the configure command line), the detection code must be performed only if the dependency will be used:

    sdl_image = not_found
    if not get_option('sdl_image').auto() or have_system
      sdl_image = dependency('SDL2_image', required: get_option('sdl_image'),
                             method: 'pkg-config',
                             static: enable_static)
    endif

This avoids warnings on static builds of user-mode emulators, for example. Most of the libraries used by system-mode emulators are not available for static linking.

The other supporting code is generally simple:

    # Create config-host.h (if applicable)
    config_host_data.set('CONFIG_SDL_IMAGE', sdl_image.found())

    # Summary
    summary_info += {'SDL image support': sdl_image.found()}

For the configure script to parse the new option, the `scripts/meson-buildoptions.sh` file must be up-to-date; `make update-buildoptions` (or just `make`) will take care of updating it.

### Support scripts

Meson has a special convention for invoking Python scripts: if their first line is `#! /usr/bin/env python3` and the file is not executable, find_program() arranges to invoke the script under the same Python interpreter that was used to invoke Meson. This is the most common and preferred way to invoke support scripts from Meson build files, because it automatically uses the value of configure’s –python= option.

In case the script is not written in Python, use a `#! /usr/bin/env ...` line and make the script executable.

Scripts written in Python, where it is desirable to make the script executable (for example for test scripts that developers may want to invoke from the command line, such as tests/qapi-schema/test-qapi.py), should be invoked through the `python` variable in meson.build. For example:

    test('QAPI schema regression tests', python,
         args: files('test-qapi.py'),
         env: test_env, suite: ['qapi-schema', 'qapi-frontend'])

This is needed to obey the –python= option passed to the configure script, which may point to something other than the first python3 binary on the path.

## Stage 3: makefiles

The use of GNU make is required with the QEMU build system.

The output of Meson is a build.ninja file, which is used with the Ninja build system. QEMU uses a different approach, where Makefile rules are synthesized from the build.ninja file. The main Makefile includes these rules and wraps them so that e.g. submodules are built before QEMU. The resulting build system is largely non-recursive in nature, in contrast to common practices seen with automake.

Tests are also ran by the Makefile with the traditional `make check` phony target, while benchmarks are run with `make bench`. Meson test suites such as `unit` can be ran with `make check-unit` too. It is also possible to run tests defined in meson.build with `meson test`.

### Useful make targets

`help`

  Print a help message for the most common build targets.

`print-VAR`

  Print the value of the variable VAR. Useful for debugging the build system.

## Important files for the build system

### Statically defined files

The following key files are statically defined in the source tree, with the rules needed to build QEMU. Their behaviour is influenced by a number of dynamically created files listed later.

`Makefile`

  The main entry point used when invoking make to build all the components of QEMU. The default ‘all’ target will naturally result in the build of every component. Makefile takes care of recursively building submodules directly via a non-recursive set of rules.

`*/meson.build`

  The meson.build file in the root directory is the main entry point for the Meson build system, and it coordinates the configuration and build of all executables. Build rules for various subdirectories are included in other meson.build files spread throughout the QEMU source tree.

`tests/Makefile.include`

  Rules for external test harnesses. These include the TCG tests, `qemu-iotests` and the Avocado-based integration tests.

`tests/docker/Makefile.include`

  Rules for Docker tests. Like tests/Makefile, this file is included directly by the top level Makefile, anything defined in this file will influence the entire build system.

`tests/vm/Makefile.include`

  Rules for VM-based tests. Like tests/Makefile, this file is included directly by the top level Makefile, anything defined in this file will influence the entire build system.

### Dynamically created files

The following files are generated dynamically by configure in order to control the behaviour of the statically defined makefiles. This avoids the need for QEMU makefiles to go through any pre-processing as seen with autotools, where Makefile.am generates Makefile.in which generates Makefile.

Built by configure:

`config-host.mak`

  When configure has determined the characteristics of the build host it will write a long list of variables to config-host.mak file. This provides the various install directories, compiler / linker flags and a variety of `CONFIG_*` variables related to optionally enabled features. This is imported by the top level Makefile and meson.build in order to tailor the build output.

  config-host.mak is also used as a dependency checking mechanism. If make sees that the modification timestamp on configure is newer than that on config-host.mak, then configure will be re-run.

  The variables defined here are those which are applicable to all QEMU build outputs. Variables which are potentially different for each emulator target are defined by the next file…

Built by Meson:

`${TARGET-NAME}-config-devices.mak`

  TARGET-NAME is again the name of a system or userspace emulator. The config-devices.mak file is automatically generated by make using the scripts/make_device_config.sh program, feeding it the default-configs/$TARGET-NAME file as input.

`config-host.h`, `$TARGET_NAME-config-target.h`, `$TARGET_NAME-config-devices.h`

  These files are used by source code to determine what features are enabled. They are generated from the contents of the corresponding `*.mak` files using Meson’s `configure_file()` function.

`build.ninja`

  The build rules.

Built by Makefile:

`Makefile.ninja`

  A Makefile include that bridges to ninja for the actual build. The Makefile is mostly a list of targets that Meson included in build.ninja.

`Makefile.mtest`

  The Makefile definitions that let “make check” run tests defined in meson.build. The rules are produced from Meson’s JSON description of tests (obtained with “meson introspect –tests”) through the script scripts/mtest2make.py.