Features/Meson

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

Summary of changes

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.

Projects

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)

Links to Meson issues and pull requests

Meson changes that are pending or have not been put to use yet.

Pull requests

python: add automatic dependency tracking for scripts

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

Merged

allow custom targets in extract_objects and test (0.61.0)

fix custom_target whose input is extract_objects of generated sources (0.63.0)

Removes dependency of dbus-display on --enable-modules and allows modinfo to receive object files instead of source files. Needs new ninja though.

Add declare_dependency(objects: ...) (1.1.0)

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