--- /dev/null
+Introduction to build-root/Makefile
+===================================
+
+The vpp build system consists of a top-level Makefile, a data-driven
+build-root/Makefile, and a set of makefile fragments. The various parts
+come together as the result of a set of well-thought-out conventions.
+
+This section describes build-root/Makefile in some detail.
+
+Repository Groups and Source Paths
+----------------------------------
+
+Current vpp workspaces comprise a single repository group. The file
+.../build-root/build-config.mk defines a key variable called
+SOURCE\_PATH. The SOURCE\_PATH variable names the set of repository
+groups. At the moment, there is only one repository group.
+
+Single pass build system, dependencies and components
+-----------------------------------------------------
+
+The vpp build system caters to components built with GNU autoconf /
+automake. Adding such components is a simple process. Dealing with
+components which use BSD-style raw Makefiles is a more difficult.
+Dealing with toolchain components such as gcc, glibc, and binutils can
+be considerably more complicated.
+
+The vpp build system is a **single-pass** build system. A partial order
+must exist for any set of components: the set of (a before b) tuples
+must resolve to an ordered list. If you create a circular dependency of
+the form; (a,b) (b,c) (c,a), gmake will try to build the target list,
+but there’s a 0.0% chance that the results will be pleasant. Cut-n-paste
+mistakes in .../build-data/packages/.mk can produce confusing failures.
+
+In a single-pass build system, it’s best to separate libraries and
+applications which instantiate them. For example, if vpp depends on
+libfoo.a, and myapp depends on both vpp and libfoo.a, it's best to place
+libfoo.a and myapp in separate components. The build system will build
+libfoo.a, vpp, and then (as a separate component) myapp. If you try to
+build libfoo.a and myapp from the same component, it won’t work.
+
+If you absolutely, positively insist on having myapp and libfoo.a in the
+same source tree, you can create a pseudo-component in a separate .mk
+file in the .../build-data/packages/ directory. Define the code
+phoneycomponent\_source = realcomponent, and provide manual
+configure/build/install targets.
+
+Separate components for myapp, libfoo.a, and vpp is the best and easiest
+solution. However, the “mumble\_source = realsource” degree of freedom
+exists to solve intractable circular dependencies, such as: to build
+gcc-bootstrap, followed by glibc, followed by “real” gcc/g++ [which
+depends on glibc too].
+
+.../build-root
+--------------
+
+The .../build-root directory contains the repository group specification
+build-config.mk, the main Makefile, and the system-wide set of
+autoconf/automake variable overrides in config.site. We'll describe
+these files in some detail. To be clear about expectations: the main
+Makefile and config.site file are subtle and complex. It's unlikely that
+you'll need or want to modify them. Poorly planned changes in either
+place typically cause bugs that are difficult to solve.
+
+.../build-root/build-config.mk
+------------------------------
+
+As described above, the build-config.mk file is straightforward: it sets
+the make variable SOURCE\_PATH to a list of repository group absolute
+paths.
+
+The SOURCE\_PATH variable If you choose to move a workspace, make sure
+to modify the paths defined by the SOURCE\_PATH variable. Those paths
+need to match changes you make in the workspace paths. For example, if
+you place the vpp directory in the workspace of a user named jsmith, you
+might change the SOURCE\_PATH to:
+
+SOURCE\_PATH = /home/jsmithuser/workspace/vpp
+
+The "out of the box" setting should work 99.5% of the time:
+
+::
+
+ SOURCE_PATH = $(CURDIR)/..
+
+.../vpp/build-root/Makefile
+---------------------------
+
+The main Makefile is complex in a number of dimensions. If you think you
+need to modify it, it's a good idea to do some research, or ask for
+advice before you change it.
+
+The main Makefile was organized and designed to provide the following
+characteristics: excellent performance, accurate dependency processing,
+cache enablement, timestamp optimizations, git integration,
+extensibility, builds with cross-compilation tool chains, and builds
+with embedded Linux distributions.
+
+If you really need to do so, you can build double-cross tools with it,
+with a minimum amount of fuss. For example, you could: compile gdb on
+x86\_64, to run on PowerPC, to debug the Xtensa instruction set.
+
+The PLATFORM variable
+---------------------
+
+The PLATFORM make/environment variable controls a number of important
+characteristics, primarily:
+
+- CPU architecture
+- The list of images to build.
+
+With respect to .../build-root/Makefile, the list of images to build is
+specified by the target. For example:
+
+::
+
+ make PLATFORM=vpp TAG=vpp_debug install-deb
+
+builds vpp debug Debian packages.
+
+The main Makefile interprets $PLATFORM by attempting to "-include" the
+file /build-data/platforms.mk:
+
+::
+
+ $(foreach d,$(FULL_SOURCE_PATH), \
+ $(eval -include $(d)/platforms.mk))
+
+By convention, we don't define **platforms** in the
+...//build-data/platforms.mk file.
+
+In the vpp case, we search for platform definition makefile fragments in
+.../vpp/build-data/platforms.mk, as follows:
+
+::
+
+ $(foreach d,$(SOURCE_PATH_BUILD_DATA_DIRS), \
+ $(eval -include $(d)/platforms/*.mk))
+
+With vpp, which uses the "vpp" platform as discussed above, we end up
+"-include"-ing .../vpp/build-data/platforms/vpp.mk.
+
+The platform-specific .mk fragment
+----------------------------------
+
+Here are the contents of .../build-data/platforms/vpp.mk:
+
+::
+
+ MACHINE=$(shell uname -m)
+
+ vpp_arch = native
+ ifeq ($(TARGET_PLATFORM),thunderx)
+ vpp_dpdk_target = arm64-thunderx-linuxapp-gcc
+ endif
+ vpp_native_tools = vppapigen
+
+ vpp_uses_dpdk = yes
+
+ # Uncoment to enable building unit tests
+ # vpp_enable_tests = yes
+
+ vpp_root_packages = vpp vom japi
+
+ # DPDK configuration parameters
+ # vpp_uses_dpdk_mlx4_pmd = yes
+ # vpp_uses_dpdk_mlx5_pmd = yes
+ # vpp_uses_external_dpdk = yes
+ # vpp_dpdk_inc_dir = /usr/include/dpdk
+ # vpp_dpdk_lib_dir = /usr/lib
+ # vpp_dpdk_shared_lib = yes
+
+ # Use '--without-libnuma' for non-numa aware architecture
+ # Use '--enable-dlmalloc' to use dlmalloc instead of mheap
+ vpp_configure_args_vpp = --enable-dlmalloc
+ sample-plugin_configure_args_vpp = --enable-dlmalloc
+
+ # load balancer plugin is not portable on 32 bit platform
+ ifeq ($(MACHINE),i686)
+ vpp_configure_args_vpp += --disable-lb-plugin
+ endif
+
+ vpp_debug_TAG_CFLAGS = -g -O0 -DCLIB_DEBUG -DFORTIFY_SOURCE=2 \
+ -fstack-protector-all -fPIC -Werror
+ vpp_debug_TAG_CXXFLAGS = -g -O0 -DCLIB_DEBUG -DFORTIFY_SOURCE=2 \
+ -fstack-protector-all -fPIC -Werror
+ vpp_debug_TAG_LDFLAGS = -g -O0 -DCLIB_DEBUG -DFORTIFY_SOURCE=2 \
+ -fstack-protector-all -fPIC -Werror
+
+ vpp_TAG_CFLAGS = -g -O2 -DFORTIFY_SOURCE=2 -fstack-protector -fPIC -Werror
+ vpp_TAG_CXXFLAGS = -g -O2 -DFORTIFY_SOURCE=2 -fstack-protector -fPIC -Werror
+ vpp_TAG_LDFLAGS = -g -O2 -DFORTIFY_SOURCE=2 -fstack-protector -fPIC -Werror -pie -Wl,-z,now
+
+ vpp_clang_TAG_CFLAGS = -g -O2 -DFORTIFY_SOURCE=2 -fstack-protector -fPIC -Werror
+ vpp_clang_TAG_LDFLAGS = -g -O2 -DFORTIFY_SOURCE=2 -fstack-protector -fPIC -Werror
+
+ vpp_gcov_TAG_CFLAGS = -g -O0 -DCLIB_DEBUG -fPIC -Werror -fprofile-arcs -ftest-coverage
+ vpp_gcov_TAG_LDFLAGS = -g -O0 -DCLIB_DEBUG -fPIC -Werror -coverage
+
+ vpp_coverity_TAG_CFLAGS = -g -O2 -fPIC -Werror -D__COVERITY__
+ vpp_coverity_TAG_LDFLAGS = -g -O2 -fPIC -Werror -D__COVERITY__
+
+Note the following variable settings:
+
+- The variable \_arch sets the CPU architecture used to build the
+ per-platform cross-compilation toolchain. With the exception of the
+ "native" architecture - used in our example - the vpp build system
+ produces cross-compiled binaries.
+
+- The variable \_native\_tools lists the required set of self-compiled
+ build tools.
+
+- The variable \_root\_packages lists the set of images to build when
+ specifying the target: make PLATFORM= TAG= [install-deb \|
+ install-rpm].
+
+The TAG variable
+----------------
+
+The TAG variable indirectly sets CFLAGS and LDFLAGS, as well as the
+build and install directory names in the .../vpp/build-root directory.
+See definitions above.
+
+Important targets build-root/Makefile
+-------------------------------------
+
+The main Makefile and the various makefile fragments implement the
+following user-visible targets:
+
++------------------+----------------------+--------------------------------------------------------------------------------------+
+| Target | ENV Variable Settings| Notes |
+| | | |
++==================+======================+======================================================================================+
+| foo | bar | mumble |
++------------------+----------------------+--------------------------------------------------------------------------------------+
+| bootstrap-tools | none | Builds the set of native tools needed by the vpp build system to |
+| | | build images. Example: vppapigen. In a full cross compilation case might include |
+| | | include "make", "git", "find", and "tar |
++------------------+----------------------+--------------------------------------------------------------------------------------+
+| install-tools | PLATFORM | Builds the tool chain for the indicated <platform>. Not used in vpp builds |
++------------------+----------------------+--------------------------------------------------------------------------------------+
+| distclean | none | Roto-rooters everything in sight: toolchains, images, and so forth. |
++------------------+----------------------+--------------------------------------------------------------------------------------+
+| install-deb | PLATFORM and TAG | Build Debian packages comprising components listed in <platform>_root_packages, |
+| | | using compile / link options defined by TAG. |
++------------------+----------------------+--------------------------------------------------------------------------------------+
+| install-rpm | PLATFORM and TAG | Build RPMs comprising components listed in <platform>_root_packages, |
+| | | using compile / link options defined by TAG. |
++------------------+----------------------+--------------------------------------------------------------------------------------+
+
+Additional build-root/Makefile environment variable settings
+------------------------------------------------------------
+
+These variable settings may be of use:
+
++----------------------+------------------------------------------------------------------------------------------------------------+
+| ENV Variable | Notes |
++======================+======================+=====================================================================================+
+| BUILD_DEBUG=vx | Directs Makefile et al. to make a good-faith effort to show what's going on in excruciating detail. |
+| | Use it as follows: "make ... BUILD_DEBUG=vx". Fairly effective in Makefile debug situations. |
++----------------------+------------------------------------------------------------------------------------------------------------+
+| V=1 | print detailed cc / ld command lines. Useful for discovering if -DFOO=11 is in the command line or not |
++----------------------+------------------------------------------------------------------------------------------------------------+
+| CC=mygcc | Override the configured C-compiler |
++----------------------+------------------------------------------------------------------------------------------------------------+
+
+.../build-root/config.site
+--------------------------
+
+The contents of .../build-root/config.site override individual autoconf /
+automake default variable settings. Here are a few sample settings related to
+building a full toolchain:
+
+::
+
+ # glibc needs these setting for cross compiling
+ libc_cv_forced_unwind=yes
+ libc_cv_c_cleanup=yes
+ libc_cv_ssp=no
+
+Determining the set of variables which need to be overridden, and the
+override values is a matter of trial and error. It should be
+unnecessary to modify this file for use with fd.io vpp.
+
+.../build-data/platforms.mk
+---------------------------
+
+Each repo group includes the platforms.mk file, which is included by
+the main Makefile. The vpp/build-data/platforms.mk file is not terribly
+complex. As of this writing, .../build-data/platforms.mk file accomplishes two
+tasks.
+
+First, it includes vpp/build-data/platforms/\*.mk:
+
+::
+
+ # Pick up per-platform makefile fragments
+ $(foreach d,$(SOURCE_PATH_BUILD_DATA_DIRS), \
+ $(eval -include $(d)/platforms/*.mk))
+
+This collects the set of platform definition makefile fragments, as discussed above.
+
+Second, platforms.mk implements the user-visible "install-deb" target.
+
+.../build-data/packages/\*.mk
+-----------------------------
+
+Each component needs a makefile fragment in order for the build system
+to recognize it. The per-component makefile fragments vary
+considerably in complexity. For a component built with GNU autoconf /
+automake which does not depend on other components, the make fragment
+can be empty. See .../build-data/packages/vpp.mk for an uncomplicated
+but fully realistic example.
+
+Here are some of the important variable settings in per-component makefile fragments:
+
++----------------------+------------------------------------------------------------------------------------------------------------+
+| Variable | Notes |
++======================+======================+=====================================================================================+
+| xxx_configure_depend | Lists the set of component build dependencies for the xxx component. In plain English: don't try to |
+| | configure this component until you've successfully built the indicated targets. Almost always, |
+| | xxx_configure_depend will list a set of "yyy-install" targets. Note the pattern: |
+| | "variable names contain underscores, make target names contain hyphens" |
++----------------------+------------------------------------------------------------------------------------------------------------+
+| xxx_configure_args | (optional) Lists any additional arguments to pass to the xxx component "configure" script. |
+| | The main Makefile %-configure rule adds the required settings for --libdir, --prefix, and |
+| | --host (when cross-compiling) |
++----------------------+------------------------------------------------------------------------------------------------------------+
+| xxx_CPPFLAGS | Adds -I stanzas to CPPFLAGS for components upon which xxx depends. |
+| | Almost invariably "xxx_CPPFLAGS = $(call installed_includes_fn, dep1 dep2 dep3)", where dep1, dep2, and |
+| | dep3 are listed in xxx_configure_depend. It is bad practice to set "-g -O3" here. Those settings |
+| | belong in a TAG. |
++----------------------+------------------------------------------------------------------------------------------------------------+
+| xxx_LDFLAGS | Adds -Wl,-rpath -Wl,depN stanzas to LDFLAGS for components upon which xxx depends. |
+| | Almost invariably "xxx_LDFLAGS = $(call installed_lib_fn, dep1 dep2 dep3)", where dep1, dep2, and |
+| | dep3 are listed in xxx_configure_depend. It is bad manners to set "-liberty-or-death" here. |
+| | Those settings belong in Makefile.am. |
++----------------------+------------------------------------------------------------------------------------------------------------+
+
+When dealing with "irritating" components built with raw Makefiles
+which only work when building in the source tree, we use a specific
+strategy in the xxx.mk file.
+
+The strategy is simple for those components: We copy the source tree
+into .../vpp/build-root/build-xxx. This works, but completely defeats
+dependency processing. This strategy is acceptable only for 3rd party
+software which won't need extensive (or preferably any) modifications.
+
+Take a look at .../vpp/build-data/packages/dpdk.mk. When invoked, the
+dpdk_configure variable copies source code into $(PACKAGE_BUILD_DIR),
+and performs the BSD equivalent of "autoreconf -i -f" to configure the
+build area. The rest of the file is similar: a bunch of hand-rolled
+glue code which manages to make the dpdk act like a good vpp build
+citizen even though it is not.
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