498 lines
18 KiB
ReStructuredText
498 lines
18 KiB
ReStructuredText
==================================
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The QEMU build system architecture
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==================================
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This document aims to help developers understand the architecture of the
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QEMU build system. As with projects using GNU autotools, the QEMU build
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system has two stages, first the developer runs the "configure" script
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to determine the local build environment characteristics, then they run
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"make" to build the project. There is about where the similarities with
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GNU autotools end, so try to forget what you know about them.
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Stage 1: configure
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==================
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The QEMU configure script is written directly in shell, and should be
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compatible with any POSIX shell, hence it uses #!/bin/sh. An important
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implication of this is that it is important to avoid using bash-isms on
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development platforms where bash is the primary host.
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In contrast to autoconf scripts, QEMU's configure is expected to be
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silent while it is checking for features. It will only display output
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when an error occurs, or to show the final feature enablement summary
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on completion.
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Because QEMU uses the Meson build system under the hood, only VPATH
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builds are supported. There are two general ways to invoke configure &
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perform a build:
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- VPATH, build artifacts outside of QEMU source tree entirely::
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cd ../
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mkdir build
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cd build
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../qemu/configure
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make
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- VPATH, build artifacts in a subdir of QEMU source tree::
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mkdir build
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cd build
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../configure
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make
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For now, checks on the compilation environment are found in configure
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rather than meson.build, though this is expected to change. The command
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line is parsed in the configure script and, whenever needed, converted
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into the appropriate options to Meson.
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New checks should be added to Meson, which usually comprises the
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following tasks:
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- Add a Meson build option to meson_options.txt.
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- Add support to the command line arg parser to handle any new
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`--enable-XXX`/`--disable-XXX` flags required by the feature.
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- Add information to the help output message to report on the new
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feature flag.
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- Add code to perform the actual feature check.
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- Add code to include the feature status in `config-host.h`
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- Add code to print out the feature status in the configure summary
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upon completion.
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Taking the probe for SDL2_Image as an example, we have the following pieces
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in configure::
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# Initial variable state
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sdl_image=auto
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..snip..
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# Configure flag processing
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--disable-sdl-image) sdl_image=disabled
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;;
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--enable-sdl-image) sdl_image=enabled
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;;
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..snip..
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# Help output feature message
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sdl-image SDL Image support for icons
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..snip..
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# Meson invocation
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-Dsdl_image=$sdl_image
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In meson_options.txt::
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option('sdl', type : 'feature', value : 'auto',
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description: 'SDL Image support for icons')
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In meson.build::
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# Detect dependency
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sdl_image = dependency('SDL2_image', required: get_option('sdl_image'),
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method: 'pkg-config',
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static: enable_static)
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# Create config-host.h (if applicable)
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config_host_data.set('CONFIG_SDL_IMAGE', sdl_image.found())
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# Summary
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summary_info += {'SDL image support': sdl_image.found()}
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Helper functions
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----------------
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The configure script provides a variety of helper functions to assist
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developers in checking for system features:
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`do_cc $ARGS...`
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Attempt to run the system C compiler passing it $ARGS...
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`do_cxx $ARGS...`
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Attempt to run the system C++ compiler passing it $ARGS...
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`compile_object $CFLAGS`
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Attempt to compile a test program with the system C compiler using
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$CFLAGS. The test program must have been previously written to a file
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called $TMPC.
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`compile_prog $CFLAGS $LDFLAGS`
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Attempt to compile a test program with the system C compiler using
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$CFLAGS and link it with the system linker using $LDFLAGS. The test
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program must have been previously written to a file called $TMPC.
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`has $COMMAND`
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Determine if $COMMAND exists in the current environment, either as a
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shell builtin, or executable binary, returning 0 on success.
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`path_of $COMMAND`
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Return the fully qualified path of $COMMAND, printing it to stdout,
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and returning 0 on success.
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`check_define $NAME`
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Determine if the macro $NAME is defined by the system C compiler
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`check_include $NAME`
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Determine if the include $NAME file is available to the system C
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compiler
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`write_c_skeleton`
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Write a minimal C program main() function to the temporary file
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indicated by $TMPC
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`feature_not_found $NAME $REMEDY`
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Print a message to stderr that the feature $NAME was not available
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on the system, suggesting the user try $REMEDY to address the
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problem.
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`error_exit $MESSAGE $MORE...`
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Print $MESSAGE to stderr, followed by $MORE... and then exit from the
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configure script with non-zero status
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`query_pkg_config $ARGS...`
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Run pkg-config passing it $ARGS. If QEMU is doing a static build,
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then --static will be automatically added to $ARGS
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Stage 2: Meson
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==============
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The Meson build system is currently used to describe the build
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process for:
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1) executables, which include:
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- Tools - qemu-img, qemu-nbd, qga (guest agent), etc
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- System emulators - qemu-system-$ARCH
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- Userspace emulators - qemu-$ARCH
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- Some (but not all) unit tests
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2) documentation
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3) ROMs, which can be either installed as binary blobs or compiled
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4) other data files, such as icons or desktop files
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The source code is highly modularized, split across many files to
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facilitate building of all of these components with as little duplicated
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compilation as possible. The Meson "sourceset" functionality is used
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to list the files and their dependency on various configuration
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symbols.
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Various subsystems that are common to both tools and emulators have
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their own sourceset, for example `block_ss` for the block device subsystem,
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`chardev_ss` for the character device subsystem, etc. These sourcesets
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are then turned into static libraries as follows::
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libchardev = static_library('chardev', chardev_ss.sources(),
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name_suffix: 'fa',
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build_by_default: false)
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chardev = declare_dependency(link_whole: libchardev)
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The special `.fa` suffix is needed as long as unit tests are built with
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the older Makefile infrastructure, and will go away later.
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Files linked into emulator targets there can be split into two distinct groups
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of files, those which are independent of the QEMU emulation target and
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those which are dependent on the QEMU emulation target.
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In the target-independent set lives various general purpose helper code,
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such as error handling infrastructure, standard data structures,
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platform portability wrapper functions, etc. This code can be compiled
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once only and the .o files linked into all output binaries.
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Target-independent code lives in the `common_ss`, `softmmu_ss` and
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`user_ss` sourcesets. `common_ss` is linked into all emulators, `softmmu_ss`
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only in system emulators, `user_ss` only in user-mode emulators.
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In the target-dependent set lives CPU emulation, device emulation and
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much glue code. This sometimes also has to be compiled multiple times,
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once for each target being built.
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All binaries link with a static library `libqemuutil.a`, which is then
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linked to all the binaries. `libqemuutil.a` is built from several
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sourcesets; most of them however host generated code, and the only two
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of general interest are `util_ss` and `stub_ss`.
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The separation between these two is purely for documentation purposes.
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`util_ss` contains generic utility files. Even though this code is only
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linked in some binaries, sometimes it requires hooks only in some of
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these and depend on other functions that are not fully implemented by
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all QEMU binaries. `stub_ss` links dummy stubs that will only be linked
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into the binary if the real implementation is not present. In a way,
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the stubs can be thought of as a portable implementation of the weak
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symbols concept.
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The following files concur in the definition of which files are linked
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into each emulator:
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`default-configs/*.mak`
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The files under default-configs/ control what emulated hardware is built
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into each QEMU system and userspace emulator targets. They merely contain
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a list of config variable definitions like the machines that should be
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included. For example, default-configs/aarch64-softmmu.mak has::
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include arm-softmmu.mak
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CONFIG_XLNX_ZYNQMP_ARM=y
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CONFIG_XLNX_VERSAL=y
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`*/Kconfig`
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These files are processed together with `default-configs/*.mak` and
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describe the dependencies between various features, subsystems and
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device models. They are described in kconfig.rst.
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These files rarely need changing unless new devices / hardware need to
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be enabled for a particular system/userspace emulation target
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Support scripts
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---------------
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Meson has a special convention for invoking Python scripts: if their
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first line is `#! /usr/bin/env python3` and the file is *not* executable,
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find_program() arranges to invoke the script under the same Python
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interpreter that was used to invoke Meson. This is the most common
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and preferred way to invoke support scripts from Meson build files,
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because it automatically uses the value of configure's --python= option.
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In case the script is not written in Python, use a `#! /usr/bin/env ...`
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line and make the script executable.
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Scripts written in Python, where it is desirable to make the script
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executable (for example for test scripts that developers may want to
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invoke from the command line, such as tests/qapi-schema/test-qapi.py),
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should be invoked through the `python` variable in meson.build. For
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example::
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test('QAPI schema regression tests', python,
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args: files('test-qapi.py'),
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env: test_env, suite: ['qapi-schema', 'qapi-frontend'])
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This is needed to obey the --python= option passed to the configure
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script, which may point to something other than the first python3
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binary on the path.
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Stage 3: makefiles
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==================
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The use of GNU make is required with the QEMU build system.
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The output of Meson is a build.ninja file, which is used with the Ninja
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build system. QEMU uses a different approach, where Makefile rules are
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synthesized from the build.ninja file. The main Makefile includes these
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rules and wraps them so that e.g. submodules are built before QEMU.
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The resulting build system is largely non-recursive in nature, in
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contrast to common practices seen with automake.
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Tests are also ran by the Makefile with the traditional `make check`
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phony target. Meson test suites such as `unit` can be ran with `make
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check-unit` too. It is also possible to run tests defined in meson.build
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with `meson test`.
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The following text is only relevant for unit tests which still have to
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be converted to Meson.
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All binaries should link to `libqemuutil.a`, e.g.:
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qemu-img$(EXESUF): qemu-img.o ..snip.. libqemuutil.a
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On Windows, all binaries have the suffix `.exe`, so all Makefile rules
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which create binaries must include the $(EXESUF) variable on the binary
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name. e.g.
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qemu-img$(EXESUF): qemu-img.o ..snip..
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This expands to `.exe` on Windows, or an empty string on other platforms.
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Variable naming
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---------------
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The QEMU convention is to define variables to list different groups of
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object files. These are named with the convention $PREFIX-obj-y. The
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Meson `chardev` variable in the previous example corresponds to a
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variable 'chardev-obj-y'.
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Likewise, tests that are executed by `make check-unit` are grouped into
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a variable check-unit-y, like this:
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check-unit-y += tests/test-visitor-serialization$(EXESUF)
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check-unit-y += tests/test-iov$(EXESUF)
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check-unit-y += tests/test-bitmap$(EXESUF)
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When a test or object file which needs to be conditionally built based
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on some characteristic of the host system, the configure script will
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define a variable for the conditional. For example, on Windows it will
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define $(CONFIG_POSIX) with a value of 'n' and $(CONFIG_WIN32) with a
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value of 'y'. It is now possible to use the config variables when
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listing object files. For example,
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check-unit-$(CONFIG_POSIX) += tests/test-vmstate$(EXESUF)
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On Windows this expands to
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check-unit-n += tests/vmstate.exe
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Since the `check-unit` target only runs tests included in `$(check-unit-y)`,
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POSIX specific tests listed in `$(util-obj-n)` are ignored on the Windows
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platform builds.
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CFLAGS / LDFLAGS / LIBS handling
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--------------------------------
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There are many different binaries being built with differing purposes,
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and some of them might even be 3rd party libraries pulled in via git
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submodules. As such the use of the global CFLAGS variable is generally
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avoided in QEMU, since it would apply to too many build targets.
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Flags that are needed by any QEMU code (i.e. everything *except* GIT
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submodule projects) are put in $(QEMU_CFLAGS) variable. For linker
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flags the $(LIBS) variable is sometimes used, but a couple of more
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targeted variables are preferred.
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In addition to these variables, it is possible to provide cflags and
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libs against individual source code files, by defining variables of the
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form $FILENAME-cflags and $FILENAME-libs. For example, the test
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test-crypto-tlscredsx509 needs to link to the libtasn1 library,
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so tests/Makefile.include defines some variables:
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tests/crypto-tls-x509-helpers.o-cflags := $(TASN1_CFLAGS)
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tests/crypto-tls-x509-helpers.o-libs := $(TASN1_LIBS)
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The scope is a little different between the two variables. The libs get
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used when linking any target binary that includes the curl.o object
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file, while the cflags get used when compiling the curl.c file only.
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Important files for the build system
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====================================
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Statically defined files
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------------------------
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The following key files are statically defined in the source tree, with
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the rules needed to build QEMU. Their behaviour is influenced by a
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number of dynamically created files listed later.
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`Makefile`
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The main entry point used when invoking make to build all the components
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of QEMU. The default 'all' target will naturally result in the build of
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every component. Makefile takes care of recursively building submodules
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directly via a non-recursive set of rules.
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`*/meson.build`
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The meson.build file in the root directory is the main entry point for the
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Meson build system, and it coordinates the configuration and build of all
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executables. Build rules for various subdirectories are included in
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other meson.build files spread throughout the QEMU source tree.
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`tests/Makefile.include`
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Rules for building the unit tests. This file is included directly by the
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top level Makefile, so anything defined in this file will influence the
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entire build system. Care needs to be taken when writing rules for tests
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to ensure they only apply to the unit test execution / build.
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`tests/docker/Makefile.include`
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Rules for Docker tests. Like tests/Makefile, this file is included
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directly by the top level Makefile, anything defined in this file will
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influence the entire build system.
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`tests/vm/Makefile.include`
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Rules for VM-based tests. Like tests/Makefile, this file is included
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directly by the top level Makefile, anything defined in this file will
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influence the entire build system.
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Dynamically created files
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-------------------------
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The following files are generated dynamically by configure in order to
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control the behaviour of the statically defined makefiles. This avoids
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the need for QEMU makefiles to go through any pre-processing as seen
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with autotools, where Makefile.am generates Makefile.in which generates
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Makefile.
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Built by configure:
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`config-host.mak`
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When configure has determined the characteristics of the build host it
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will write a long list of variables to config-host.mak file. This
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provides the various install directories, compiler / linker flags and a
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variety of `CONFIG_*` variables related to optionally enabled features.
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This is imported by the top level Makefile and meson.build in order to
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tailor the build output.
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config-host.mak is also used as a dependency checking mechanism. If make
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sees that the modification timestamp on configure is newer than that on
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config-host.mak, then configure will be re-run.
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The variables defined here are those which are applicable to all QEMU
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build outputs. Variables which are potentially different for each
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emulator target are defined by the next file...
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`$TARGET-NAME/config-target.mak`
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TARGET-NAME is the name of a system or userspace emulator, for example,
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x86_64-softmmu denotes the system emulator for the x86_64 architecture.
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This file contains the variables which need to vary on a per-target
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basis. For example, it will indicate whether KVM or Xen are enabled for
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the target and any other potential custom libraries needed for linking
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the target.
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Built by Meson:
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`${TARGET-NAME}-config-devices.mak`
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TARGET-NAME is again the name of a system or userspace emulator. The
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config-devices.mak file is automatically generated by make using the
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scripts/make_device_config.sh program, feeding it the
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default-configs/$TARGET-NAME file as input.
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`config-host.h`, `$TARGET-NAME/config-target.h`, `$TARGET-NAME/config-devices.h`
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These files are used by source code to determine what features
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are enabled. They are generated from the contents of the corresponding
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`*.h` files using the scripts/create_config program. This extracts
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relevant variables and formats them as C preprocessor macros.
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`build.ninja`
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The build rules.
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Built by Makefile:
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`Makefile.ninja`
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A Makefile conversion of the build rules in build.ninja. The conversion
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is straightforward and, were it necessary to debug the rules produced
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by Meson, it should be enough to look at build.ninja. The conversion
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is performed by scripts/ninjatool.py.
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`Makefile.mtest`
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The Makefile definitions that let "make check" run tests defined in
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meson.build. The rules are produced from Meson's JSON description of
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tests (obtained with "meson introspect --tests") through the script
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scripts/mtest2make.py.
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Useful make targets
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-------------------
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`help`
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Print a help message for the most common build targets.
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`print-VAR`
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Print the value of the variable VAR. Useful for debugging the build
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system.
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