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docs/devel: update build system docs

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configure is only doing compiler and host setup now, so adjust the
relevant documentation.  It is also possible to build emulators with
ninja directly if one is so inclined, so mention that as well.

The Python virtual environment set up is a new major task of configure
as well.  Mention it in the list of produced files, while leaving it
for a future patch to document how it works and how ``mkvenv ensure``
is used.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
This commit is contained in:
Paolo Bonzini 2022-10-18 09:56:43 +02:00
parent d71ccfa1b8
commit fe3ab4eb2d
1 changed files with 177 additions and 110 deletions
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docs/devel/build-system.rst
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@ -4,30 +4,14 @@ 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
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
"make" to build the project. This is about where the similarities with
GNU autotools end, so try to forget what you know about them.
The two general ways to perform a build are as follows:
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::
- build artifacts outside of QEMU source tree entirely::
cd ../
mkdir build
@ -35,80 +19,122 @@ perform a build:
../qemu/configure
make
- VPATH, build artifacts in a subdir of QEMU source tree::
- 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.
Most of the actual build process uses Meson under the hood, therefore
build artifacts cannot be placed in the source tree itself.
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.
Stage 1: configure
==================
Helper functions
----------------
The configure script has five tasks:
The configure script provides a variety of helper functions to assist
developers in checking for system features:
- detect the host architecture
``do_cc $ARGS...``
Attempt to run the system C compiler passing it $ARGS...
- list the targets for which to build emulators; the list of
targets also affects which firmware binaries and tests to build
``do_cxx $ARGS...``
Attempt to run the system C++ compiler passing it $ARGS...
- find the compilers (native and cross) used to build executables,
firmware and tests. The results are written as either Makefile
fragments (``config-host.mak``) or a Meson machine file
(``config-meson.cross``)
``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()``.
- create a virtual environment in which all Python code runs during
the build, and possibly install packages into it from PyPI
``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()``.
- invoke Meson in the virtual environment, to perform the actual
configuration step for the emulator build
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.
Almost all QEMU developers that need to modify the build system will
only be concerned with Meson, and therefore can skip the rest of this
section.
Modifying ``configure``
-----------------------
``configure`` is a shell script; it uses ``#!/bin/sh`` and therefore
should be compatible with any POSIX shell. It is important to avoid
using bash-isms to avoid breaking development platforms where bash is
the primary host.
The configure script provides a variety of functions to help writing
portable shell code and providing consistent behavior across architectures
and operating systems:
``error_exit $MESSAGE $MORE...``
Print $MESSAGE to stderr, followed by $MORE... and then exit from the
configure script with non-zero status.
``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()``.
``probe_target_compiler $TARGET``
Detect a cross compiler and cross tools for the QEMU target $TARGET (e.g.,
``$CPU-softmmu``, ``$CPU-linux-user``, ``$CPU-bsd-user``). If a working
compiler is present, return success and set variables ``$target_cc``,
``$target_ar``, etc. to non-empty values.
``write_target_makefile``
Write a Makefile fragment to stdout, exposing the result of the most
``probe_target_compiler`` call as the usual Make variables (``CC``,
``AR``, ``LD``, etc.).
Configure does not generally perform tests for compiler options beyond
basic checks to detect the host platform and ensure the compiler is
functioning. These are performed using a few more helper functions:
``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.
``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.
``check_define $NAME``
Determine if the macro $NAME is defined by the system C compiler
Determine if the macro $NAME is defined by the system C compiler.
``do_compiler $CC $ARGS...``
Attempt to run the C compiler $CC, passing it $ARGS... This function
does not use flags passed via options such as ``--extra-cflags``, and
therefore can be used to check for cross compilers. However, most
such checks are done at ``make`` time instead (see for example the
``cc-option`` macro in ``pc-bios/option-rom/Makefile``).
``write_c_skeleton``
Write a minimal C program main() function to the temporary file
indicated by $TMPC
indicated by $TMPC.
``error_exit $MESSAGE $MORE...``
Print $MESSAGE to stderr, followed by $MORE... and then exit from the
configure script with non-zero status
Python virtual environments and the QEMU build system
-----------------------------------------------------
TBD
Stage 2: Meson
==============
The Meson build system is currently used to describe the build
process for:
The Meson build system describes the build and install process for:
1) executables, which include:
- Tools - ``qemu-img``, ``qemu-nbd``, ``qga`` (guest agent), etc
- Tools - ``qemu-img``, ``qemu-nbd``, ``qemu-ga`` (guest agent), etc
- System emulators - ``qemu-system-$ARCH``
@ -118,7 +144,8 @@ process for:
2) documentation
3) ROMs, which can be either installed as binary blobs or compiled
3) ROMs, whether provided as binary blobs in the QEMU distributions
or cross compiled under the direction of the configure script
4) other data files, such as icons or desktop files
@ -273,8 +300,7 @@ system/userspace emulation target
Adding checks
-------------
New checks should be added to Meson. Compiler checks can be as simple as
the following::
Compiler checks can be as simple as the following::
config_host_data.set('HAVE_BTRFS_H', cc.has_header('linux/btrfs.h'))
@ -351,22 +377,30 @@ script, which may point to something other than the first python3
binary on the path.
Stage 3: makefiles
==================
Stage 3: Make
=============
The use of GNU make is required with the QEMU build system.
The next step in building QEMU is to invoke make. GNU Make is required
to build QEMU, and may be installed as ``gmake`` on some hosts.
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.
The output of Meson is a ``build.ninja`` file, which is used with the
Ninja build tool. However, QEMU's build comprises other components than
just the emulators (namely firmware and the tests in ``tests/tcg``) which
need different cross compilers. The QEMU Makefile wraps both Ninja and
the smaller build systems for firmware and tests; it also takes care of
running ``configure`` again when the script changes. Apart from invoking
these sub-Makefiles, the resulting build is largely non-recursive.
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``.
Tests, whether defined in ``meson.build`` or not, 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``, and ``make check-tcg`` builds and runs "non-Meson"
tests for all targets.
If desired, it is also possible to use ``ninja`` and ``meson test``,
respectively to build emulators and run tests defined in meson.build.
The main difference is that ``make`` needs the ``-jN`` flag in order to
enable parallel builds or tests.
Useful make targets
-------------------
@ -378,6 +412,7 @@ Useful make targets
Print the value of the variable VAR. Useful for debugging the build
system.
Important files for the build system
====================================
@ -391,8 +426,7 @@ 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.
every component.
``*/meson.build``
The meson.build file in the root directory is the main entry point for the
@ -401,59 +435,92 @@ number of dynamically created files listed later.
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.
Rules for external test harnesses. These include the TCG tests
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.
Rules for Docker tests. Like ``tests/Makefile.include``, 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.
Rules for VM-based tests. Like ``tests/Makefile.include``, 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.
The following files are generated at run-time in order to control the
behaviour of the Makefiles. This avoids the need for QEMU makefiles to
go through any pre-processing as seen with autotools, where configure
generates ``Makefile`` from ``Makefile.in``.
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
will write them to this file for use in ``Makefile`` and to a smaller
extent ``meson.build``. These include the paths to various tools 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
``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.
``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...
The variables defined here apply to all QEMU
build outputs.
``config-meson.cross``
A Meson "cross file" (or native file) used to communicate the paths to
the toolchain and other configuration options.
``config.status``
A small shell script that will invoke configure again with the same
environment variables that were set during the first run. It's used to
rerun configure after changes to the source code, but it can also be
inspected manually to check the contents of the environment.
``Makefile.prereqs``
A set of Makefile dependencies that order the build and execution of
firmware and tests after the container images and emulators that they
need.
``pc-bios/*/config.mak``, ``tests/tcg/config-host.mak``, ``tests/tcg/*/config-target.mak``
Configuration variables used to build the firmware and TCG tests,
including paths to cross compilation toolchains.
``pyvenv``
A Python virtual environment that is used for all Python code running
during the build. Using a virtual environment ensures that even code
that is run via ``sphinx-build``, ``meson`` etc. uses the same interpreter
and packages.
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``
Used by C code to determine the properties of the build environment
and the set of enabled features for the entire build.
``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.
``${TARGET-NAME}-config-devices.mak``
TARGET-NAME is the name of a system emulator. The file is
generated by Meson using files under ``configs/devices`` as input.
``${TARGET-NAME}-config-target.mak``
TARGET-NAME is the name of a system or usermode emulator. The file is
generated by Meson using files under ``configs/targets`` as input.
``$TARGET_NAME-config-target.h``, ``$TARGET_NAME-config-devices.h``
Used by C code to determine the properties and enabled
features for each target. enabled. They are generated from
the contents of the corresponding ``*.mak`` files using Meson's
``configure_file()`` function; each target can include them using
the ``CONFIG_TARGET`` and ``CONFIG_DEVICES`` macro respectively.
``build.ninja``
The build rules.