Xen Makefiles

Documentation for the build system of Xen, found in xen.git/xen/.

1 Makefile files

Description of the syntax that can be used in most Makefiles named 'Makefile'. ('xen/Makefile' isn't part of the description.)

'Makefile's are consumed by 'Rules.mk' when building.

1.1 Goal definitions

Goal definitions are the main part (heart) of the Makefile. These lines define the files to be built, any special compilation options, and any subdirectories to be entered recursively.

The most simple makefile contains one line:


obj-y += foo.o

This tells the build system that there is one object in that directory, named foo.o. foo.o will be built from foo.c or foo.S.

The following pattern is often used to have object selected depending on the configuration:


obj-$(CONFIG_FOO) += foo.o

$(CONFIG_FOO) can evaluates to y. If CONFIG_FOO is not y, then the file will not be compiled nor linked.

1.2 Descending down in directories

A Makefile is only responsible for building objects in its own directory. Files in subdirectories should be taken care of by Makefiles in these subdirs. The build system will automatically invoke make recursively in subdirectories, provided you let it know of them.

To do so, obj-y is used. acpi lives in a separate directory, and the Makefile present in drivers/ tells the build system to descend down using the following assignment.


obj-$(CONFIG_ACPI) += acpi/

If CONFIG_ACPI is set to 'y' the corresponding obj- variable will be set, and the build system will descend down in the apci directory. The build system only uses this information to decide that it needs to visit the directory, it is the Makefile in the subdirectory that specifies what is modular and what is built-in.

It is good practice to use a CONFIG_ variable when assigning directory names. This allows the build system to totally skip the directory if the corresponding CONFIG_ option is not set to 'y'.

1.3 Compilation flags

CFLAGS-y and AFLAGS-y These two flags apply only to the makefile in which they are assigned. They are used for all the normal cc, as and ld invocations happening during a recursive build.

$(CFLAGS-y) is necessary because the top Makefile owns the variable $(XEN_CFLAGS) and uses it for compilation flags for the entire tree. And the variable $(CFLAGS) is modified by Config.mk which evaluated in every subdirs.

CFLAGS-y specifies options for compiling with $(CC). AFLAGS-y specifies assembler options.

2 Build system infrastructure

This chapter describe some of the macro used when building Xen.

2.1 Macros

if_changed if_changed is the infrastructure used for the following commands.


target: source(s) FORCE
    $(call if_changed,ld/objcopy/...)

When the rule is evaluated, it is checked to see if any files need an update, or the command line has changed since the last invocation. The latter will force a rebuild if any options to the executable have changed. Any target that utilises if_changed must be listed in $(targets), otherwise the command line check will fail, and the target will always be built. if_changed may be used in conjunction with custom commands as defined in "Custom commands".

Note: It is a typical mistake to forget the FORCE prerequisite. Another common pitfall is that whitespace is sometimes significant; for instance, the below will fail (note the extra space after the comma):

target: source(s) FORCE

WRONG! $(call if_changed, ld/objcopy/...)


if_changed should not be used more than once per target. It stores the executed command in a corresponding .cmd file and multiple calls would result in overwrites and unwanted results when the target is up to date and only the tests on changed commands trigger execution of commands.

ld Link target.


targets += setup setup.o bootsect bootsect.o
$(obj)/setup $(obj)/bootsect: %: %.o FORCE
    $(call if_changed,ld)

$(targets) are assigned all potential targets, by which the build system knows the targets and will:

  1. check for commandline changes

The ": %: %.o" part of the prerequisite is a shorthand that frees us from listing the setup.o and bootsect.o files.


It is a common mistake to forget the "targets :=" assignment, resulting in the target file being recompiled for no obvious reason.

objcopy Copy binary. Uses OBJCOPYFLAGS usually specified in arch/$(ARCH)/Makefile.

2.2 Custom commands

When the build system is executing with V=0, then only a shorthand of a command is normally displayed. To enable this behaviour for custom commands, two variables are required to be set:

quiet_cmd_<command> - what shall be echoed
      cmd_<command> - the command to execute


# xsm/flask/Makefile
mkflask := policy/mkflask.sh
quiet_cmd_mkflask = MKFLASK $@
cmd_mkflask = $(CONFIG_SHELL) $(mkflask) $(AWK) include \

include/flask.h: $(FLASK_H_DEPEND) $(mkflask) FORCE
    $(call if_changed,mkflask)

When updating the include/flask.h target, the line:

MKFLASK include/flask.h

will be displayed with "make V=0". (V=0 is the default)