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diff --git a/docs/build-system.txt b/docs/build-system.txt new file mode 100644 index 0000000000..5ddddeaafb --- /dev/null +++ b/docs/build-system.txt @@ -0,0 +1,507 @@ + 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. + +Adding new checks to the configure script usually comprises the +following tasks: + + - Initialize one or more variables with the default feature state. + + Ideally features should auto-detect whether they are present, + so try to avoid hardcoding the initial state to either enabled + or disabled, as that forces the user to pass a --enable-XXX + / --disable-XXX flag on every invocation of configure. + + - Add support to the command line arg parser to handle any new + --enable-XXX / --disable-XXX flags required by the feature XXX. + + - Add information to the help output message to report on the new + feature flag. + + - Add code to perform the actual feature check. As noted above, try to + be fully dynamic in checking enablement/disablement. + + - Add code to print out the feature status in the configure summary + upon completion. + + - Add any new makefile variables to $config_host_mak on completion. + + +Taking (a simplified version of) the probe for gnutls from configure, +we have the following pieces: + + # Initial variable state + gnutls="" + + ..snip.. + + # Configure flag processing + --disable-gnutls) gnutls="no" + ;; + --enable-gnutls) gnutls="yes" + ;; + + ..snip.. + + # Help output feature message + gnutls GNUTLS cryptography support + + ..snip.. + + # Test for gnutls + if test "$gnutls" != "no"; then + if ! $pkg_config --exists "gnutls"; then + gnutls_cflags=`$pkg_config --cflags gnutls` + gnutls_libs=`$pkg_config --libs gnutls` + libs_softmmu="$gnutls_libs $libs_softmmu" + libs_tools="$gnutls_libs $libs_tools" + QEMU_CFLAGS="$QEMU_CFLAGS $gnutls_cflags" + gnutls="yes" + elif test "$gnutls" = "yes"; then + feature_not_found "gnutls" "Install gnutls devel" + else + gnutls="no" + fi + fi + + ..snip.. + + # Completion feature summary + echo "GNUTLS support $gnutls" + + ..snip.. + + # Define make variables + if test "$gnutls" = "yes" ; then + echo "CONFIG_GNUTLS=y" >> $config_host_mak + fi + + +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. + + - 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. + + - has $COMMAND + + Determine if $COMMAND exists in the current environment, either as a + shell builtin, or executable binary, returning 0 on success. + + - path_of $COMMAND + + Return the fully qualified path of $COMMAND, printing it to stdout, + and returning 0 on success. + + - 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 + + - write_c_skeleton + + Write a minimal C program main() function to the temporary file + indicated by $TMPC + + - feature_not_found $NAME $REMEDY + + Print a message to stderr that the feature $NAME was not available + on the system, suggesting the user try $REMEDY to address the + problem. + + - 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 $ARGS + + +Stage 2: makefiles +================== + +The use of GNU make is required with the QEMU build system. + +Although the source code is spread across multiple subdirectories, the +build system should be considered largely non-recursive in nature, in +contrast to common practices seen with automake. There is some recursive +invocation of make, but this is related to the things being built, +rather than the source directory structure. + +QEMU currently supports both VPATH and non-VPATH builds, so there are +three general ways to invoke configure & perform a build. + + - VPATH, build artifacts outside of QEMU source tree entirely + + cd ../ + mkdir build + cd build + ../qemu/configure + make + + - VPATH, build artifacts in a subdir of QEMU source tree + + mkdir build + cd build + ../configure + make + + - non-VPATH, build artifacts everywhere + + ./configure + make + +The QEMU maintainers generally recommend that a VPATH build is used by +developers. Patches to QEMU are expected to ensure VPATH build still +works. + + +Module structure +---------------- + +There are a number of key outputs of the QEMU build system: + + - Tools - qemu-img, qemu-nbd, qga (guest agent), etc + - System emulators - qemu-system-$ARCH + - Userspace emulators - qemu-$ARCH + - Unit tests + +The source code is highly modularized, split across many files to +facilitate building of all of these components with as little duplicated +compilation as possible. There can be considered to be two distinct +groups of files, those which are independent of the QEMU emulation +target and those which are dependent on the QEMU emulation target. + +In the target-independent set lives various general purpose helper code, +such as error handling infrastructure, standard data structures, +platform portability wrapper functions, etc. This code can be compiled +once only and the .o files linked into all output binaries. + +In the target-dependent set lives CPU emulation, device emulation and +much glue code. This sometimes also has to be compiled multiple times, +once for each target being built. + +The utility code that is used by all binaries is built into a +static archive called libqemuutil.a, which is then linked to all the +binaries. In order to provide hooks that are only needed by some of the +binaries, code in libqemuutil.a may depend on other functions that are +not fully implemented by all QEMU binaries. To deal with this there is a +second library called libqemustub.a which provides dummy stubs for all +these functions. These will get lazy linked into the binary if the real +implementation is not present. In this way, the libqemustub.a static +library can be thought of as a portable implementation of the weak +symbols concept. All binaries should link to both libqemuutil.a and +libqemustub.a. e.g. + + qemu-img$(EXESUF): qemu-img.o ..snip.. libqemuutil.a libqemustub.a + + +Windows platform portability +---------------------------- + +On Windows, all binaries have the suffix '.exe', so all Makefile rules +which create binaries must include the $(EXESUF) variable on the binary +name. e.g. + + qemu-img$(EXESUF): qemu-img.o ..snip.. + +This expands to '.exe' on Windows, or '' on other platforms. + +A further complication for the system emulator binaries is that +two separate binaries need to be generated. + +The main binary (e.g. qemu-system-x86_64.exe) is linked against the +Windows console runtime subsystem. These are expected to be run from a +command prompt window, and so will print stderr to the console that +launched them. + +The second binary generated has a 'w' on the end of its name (e.g. +qemu-system-x86_64w.exe) and is linked against the Windows graphical +runtime subsystem. These are expected to be run directly from the +desktop and will open up a dedicated console window for stderr output. + +The Makefile.target will generate the binary for the graphical subsystem +first, and then use objcopy to relink it against the console subsystem +to generate the second binary. + + +Object variable naming +---------------------- + +The QEMU convention is to define variables to list different groups of +object files. These are named with the convention $PREFIX-obj-y. For +example the libqemuutil.a file will be linked with all objects listed +in a variable 'util-obj-y'. So, for example, util/Makefile.obj will +contain a set of definitions looking like + + util-obj-y += bitmap.o bitops.o hbitmap.o + util-obj-y += fifo8.o + util-obj-y += acl.o + util-obj-y += error.o qemu-error.o + +When there is an object file which needs to be conditionally built based +on some characteristic of the host system, the configure script will +define a variable for the conditional. For example, on Windows it will +define $(CONFIG_POSIX) with a value of 'n' and $(CONFIG_WIN32) with a +value of 'y'. It is now possible to use the config variables when +listing object files. For example, + + util-obj-$(CONFIG_WIN32) += oslib-win32.o qemu-thread-win32.o + util-obj-$(CONFIG_POSIX) += oslib-posix.o qemu-thread-posix.o + +On Windows this expands to + + util-obj-y += oslib-win32.o qemu-thread-win32.o + util-obj-n += oslib-posix.o qemu-thread-posix.o + +Since libqemutil.a links in $(util-obj-y), the POSIX specific files +listed against $(util-obj-n) are ignored on the Windows platform builds. + + +CFLAGS / LDFLAGS / LIBS handling +-------------------------------- + +There are many different binaries being built with differing purposes, +and some of them might even be 3rd party libraries pulled in via git +submodules. As such the use of the global CFLAGS variable is generally +avoided in QEMU, since it would apply to too many build targets. + +Flags that are needed by any QEMU code (i.e. everything *except* GIT +submodule projects) are put in $(QEMU_CFLAGS) variable. For linker +flags the $(LIBS) variable is sometimes used, but a couple of more +targeted variables are preferred. $(libs_softmmu) is used for +libraries that must be linked to system emulator targets, $(LIBS_TOOLS) +is used for tools like qemu-img, qemu-nbd, etc and $(LIBS_QGA) is used +for the QEMU guest agent. There is currently no specific variable for +the userspace emulator targets as the global $(LIBS), or more targeted +variables shown below, are sufficient. + +In addition to these variables, it is possible to provide cflags and +libs against individual source code files, by defining variables of the +form $FILENAME-cflags and $FILENAME-libs. For example, the curl block +driver needs to link to the libcurl library, so block/Makefile defines +some variables: + + curl.o-cflags := $(CURL_CFLAGS) + curl.o-libs := $(CURL_LIBS) + +The scope is a little different between the two variables. The libs get +used when linking any target binary that includes the curl.o object +file, while the cflags get used when compiling the curl.c file only. + + +Statically defined files +------------------------ + +The following key files are statically defined in the source tree, with +the rules needed to build QEMU. Their behaviour is influenced by a +number of dynamically created files listed later. + +- Makefile + +The main entry point used when invoking make to build all the components +of QEMU. The default 'all' target will naturally result in the build of +every component. The various tools and helper binaries are built +directly via a non-recursive set of rules. + +Each system/userspace emulation target needs to have a slightly +different set of make rules / variables. Thus, make will be recursively +invoked for each of the emulation targets. + +The recursive invocation will end up processing the toplevel +Makefile.target file (more on that later). + + +- */Makefile.objs + +Since the source code is spread across multiple directories, the rules +for each file are similarly modularized. Thus each subdirectory +containing .c files will usually also contain a Makefile.objs file. +These files are not directly invoked by a recursive make, but instead +they are imported by the top level Makefile and/or Makefile.target + +Each Makefile.objs usually just declares a set of variables listing the +.o files that need building from the source files in the directory. They +will also define any custom linker or compiler flags. For example in +block/Makefile.objs + + block-obj-$(CONFIG_LIBISCSI) += iscsi.o + block-obj-$(CONFIG_CURL) += curl.o + + ..snip... + + iscsi.o-cflags := $(LIBISCSI_CFLAGS) + iscsi.o-libs := $(LIBISCSI_LIBS) + curl.o-cflags := $(CURL_CFLAGS) + curl.o-libs := $(CURL_LIBS) + +If there are any rules defined in the Makefile.objs file, they should +all use $(obj) as a prefix to the target, e.g. + + $(obj)/generated-tcg-tracers.h: $(obj)/generated-tcg-tracers.h-timestamp + + +- Makefile.target + +This file provides the entry point used to build each individual system +or userspace emulator target. Each enabled target has its own +subdirectory. For example if configure is run with the argument +'--target-list=x86_64-softmmu', then a sub-directory 'x86_64-softmu' +will be created, containing a 'Makefile' which symlinks back to +Makefile.target + +So when the recursive '$(MAKE) -C x86_64-softmmu' is invoked, it ends up +using Makefile.target for the build rules. + + +- rules.mak + +This file provides the generic helper rules for invoking build tools, in +particular the compiler and linker. This also contains the magic (hairy) +'unnest-vars' function which is used to merge the variable definitions +from all Makefile.objs in the source tree down into the main Makefile +context. + + +- default-configs/*.mak + +The files under default-configs/ control what emulated hardware is built +into each QEMU system and userspace emulator targets. They merely +contain a long list of config variable definitions. For example, +default-configs/x86_64-softmmu.mak has: + + include pci.mak + include sound.mak + include usb.mak + CONFIG_QXL=$(CONFIG_SPICE) + CONFIG_VGA_ISA=y + CONFIG_VGA_CIRRUS=y + CONFIG_VMWARE_VGA=y + CONFIG_VIRTIO_VGA=y + ...snip... + +These files rarely need changing unless new devices / hardware need to +be enabled for a particular system/userspace emulation target + + +- tests/Makefile + +Rules for building the unit tests. This file is included directly by the +top level Makefile, so anything defined in this file will influence the +entire build system. Care needs to be taken when writing rules for tests +to ensure they only apply to the unit test execution / build. + + +- po/Makefile + +Rules for building and installing the binary message catalogs from the +text .po file sources. This almost never needs changing for any reason. + + +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. + + +- 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 in order to tailor the build +output. + +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... + +It 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.h + +The config-host.h file is used by source code to determine what features +are enabled. It is generated from the contents of config-host.mak using +the scripts/create_config program. This extracts all the CONFIG_* variables, +most of the HOST_* variables and a few other misc variables from +config-host.mak, formatting them as C preprocessor macros. + + +- $TARGET-NAME/config-target.mak + +TARGET-NAME is the name of a system or userspace emulator, for example, +x86_64-softmmu denotes the system emulator for the x86_64 architecture. +This file contains the variables which need to vary on a per-target +basis. For example, it will indicate whether KVM or Xen are enabled for +the target and any other potential custom libraries needed for linking +the target. + + +- $TARGET-NAME/config-devices.mak + +TARGET-NAME is again the name of a system or userspace emulator. The +config-devices.mak file is automatically generated by make using the +scripts/make_device_config.sh program, feeding it the +default-configs/$TARGET-NAME file as input. + + +- $TARGET-NAME/Makefile + +This is the entrypoint used when make recurses to build a single system +or userspace emulator target. It is merely a symlink back to the +Makefile.target in the top level. |