Features/Meson

For the rationale behind the conversion and the development notes, see Features/Meson/Design.

Summary of changes

New system requirements

• Ninja 1.7
• Meson 0.55.3 (in order to use the system-wide install).

Source code changes

• You cannot #include "config-target.h" and "config-devices.h". Instead, you #include CONFIG_TARGET and CONFIG_DEVICES (i.e. filenames are specified via macros; meson.build takes care of defining the macros).
• .c files (including .inc.c files) cannot be #included. They have been renamed to .c.inc, for consistency with existing .rst.inc files.
• The name of files produced by decodetree is fixed (the C code for XYZ.decode ends up in decode-XYZ.c.inc).
• Subdirectories with trace events require a forwarding header file; see hw/scsi/trace.h for an example.

Build layout changes

• The Makefile is not recursive anymore. You don't make aarch64-softmmu/all or make arm-linux-user/all, you make qemu-system-aarch64 or make qemu-arm.
• Emulator binaries have moved to the root of the build directory. For now, paths such as cris-softmmu/qemu-system-cris are preserved for backwards compatibility through symlinks (created by configure). They are just a user convenience however. You cannot "rm" them to force a rebuild, for example.
• Other binaries have moved to the directory where they reside in the source tree. For example, "virtiofsd" has moved to "tools/virtiofsd/virtiofsd".
• Most binaries are built by default by make all, including several in contrib/ that weren't built before. Currently, vhost-user-blk and rdmacm-mux are not because they fail on 32-bit and big-endian platforms respectively.
• Sphinx manuals are only built if their constituent files are changed, therefore "make sphinxdocs" will be plenty fast when you've only modified a file or two.
• Dependencies added to libqemuutil.a will propagate to all programs that link to it. If util/ code has dependencies, it should be conditional on have_system or have_block if applicable (this was a bug in the Makefiles).
• For bisection: incremental builds across the conversion work fine in the forwards direction. They probably don't work at all backwards across the conversion, so you'll need to throw away your build tree when bisection moves you backwards across the conversion. This is mitigated by the fact that bisection usually starts at a release, and the patches have been applied on top of the 5.1.0 tag.

New conventions

• New configure options should also be added to meson_options.txt so that dependencies are tested in meson.build rather than configure. How to do so is documented in docs/devel.

Impact to developers

• If you were already using out-of-tree (VPATH) builds, nothing changes; an incremental pull of these changes should still build.
• If you are used to in-tree builds, you'll want to do make distclean prior to merging in these changes (if you forget, git will remind you about various trace.h files that still exist as built files in your in-tree build vs. checked-in files post-patch). After that one-time clean, you can then proceed to do ./configure && make as before, but things will now automatically create a subdirectory build/ on your behalf where the actual build is performed.

CI holes

The following issues were _not_ found by GitLab CI:

• SDL 2.0.8 requires -Wno-undef
• s390x-softmmu is special for cross-compilation, because it is the only target to use the host C compiler.
• There is special code in configure to handle a C++ compiler that does not play well with the C compiler.
• Not all linux-user TCG tests are covered.
• Big endian platforms are not covered.
• ARM host platforms are not covered.
• 32-bit POSIX platforms are not covered.
• SystemTap is not covered.
• daxctl is not covered.
• --disable-tools --enable-system builds are not covered.
• Peter uses Ubuntu+MXE instead of Fedora-mingw to build cross compilers. docker-test-mingw only works for Fedora and should be made more generic. Also container images for Fedora-mingw should be prepared in the same way as the existing container images for Debian+MXE.

Reported and unfixed bugs

• meson.build files are missing copyright/license header comments

Known issues

• Meson does not like outputs of custom_target (e.g. tracetool and qapi-gen.py) to be in multiple directories. Right now this is only a problem for the QAPI tests.

Next steps

Move pkg-config tests to meson.build

Replace declare_dependency statements with dependency statements. Note that Meson warns for dependencies available only as shared libraries, so if a dependency is only used by emulators or tools it should be guarded by "if have_system" or "if have_block".

Benefit: allows removing *_CFLAGS and *_LIBS variables from config-host.mak. Allows using dependency objects instead of CONFIG_* symbols when declaring sourcesets.

Change simple config-host.mak symbols to Meson options

Some symbols (e.g. CONFIG_PARALLELS) are not anymore used in the Makefiles, and are only placed in config-host.mak to be read from Meson. Instead pass them through -D... command-line options

Benefit: allows removing CONFIG_* variables from config-host.mak.

Move other compile tests from configure to meson.build

Write a linter for meson.build

Visit the output of meson introspect --ast meson.build (including subdirectories) and warn for:

• dependencies searched with a method other than "pkg-config" or "system"
• dependencies lacking "static: enable_static"
• variables not defined on all paths (Meson accepts undefined variables on the RHS of short-circuiting boolean operators)

TBD

Create configure help and command line parsing code from meson_options.txt.

Use the output of "meson introspect --buildoptions" to initialize variables, parse command line options, print the help and create -D command line options for meson. PoC here (requires developers to install "jq").

Benefit: remove code duplication, centralize command line handling

Decide what to do about remaining build rules

Here you can find old patches to convert the build rules in roms/ and pc-bios/. However, they should probably use their own build system with full support for cross-compilation, a la tests/tcg.

Pending Meson changes

Issues

add_global_dependency or dependency::get_compile_args

Needed to move glib detection to Meson.

Add "c_winlibs" und "cpp_winlibs" to target functions

Should be useful for qemu-ga and friends.

Pull requests

python: add automatic dependency tracking for scripts

Avoids listing sources for qapi-gen.py and tracetool.py

add fs.expand method

Could be useful to compute TARGET_DIRS

transitively pass compile flags for internal dependency

Enables a better fix for https://bugs.launchpad.net/qemu/+bug/1909256

meson test

Improve `meson test` (0.57)

Get rid of scripts/tap-driver.pl

introduce add_test_setup(exclude suites: ...) keyword argument (0.57)

Can be useful to simplify invocation of "slow" vs. "quick" tests without a Makefile wrapper

mtest: limit "magic" CTRL+C behavior to process group leaders (0.59.3)

Needed to call "meson test" from make

extract_objects

extract_objects: fixes, tests and documentation for using the result in a custom_target (0.58.2)

Useful to parse compile_commands.json in scripts/modinfo.sh

allow custom targets in extract_objects and test (0.61.0)

Useful for modinfo to receive object files instead of source files

Others

Utility methods for feature objects (0.59.0)

Simplifies use of get_option()

"links" test with file argument should use the appropriate compiler for the source file (0.60.0)

Needed to move C++ compiler detection