xen_kconfig_memo.txt

all: dist

dist: DESTDIR=$(DISTDIR)/install
dist: $(TARGS_DIST) dist-misc

SUBSYSTEMS?=xen tools stubdom docs
TARGS_DIST=$(patsubst %, dist-%, $(SUBSYSTEMS))

dist-%: DESTDIR=$(DISTDIR)/install
dist-%: install-%
	@: # do nothing

install-xen:
	$(MAKE) -C xen install

# Now, build process descend into subdir: xen
# Note, double colon rule is used
install:: include/config/auto.conf

build install::
ifneq ($(XEN_TARGET_ARCH),x86_32)
	$(MAKE) -f Rules.mk _$@
else
	echo "*** Xen x86/32 target no longer supported!"
endif


include/config/%.conf: include/config/auto.conf.cmd $(KCONFIG_CONFIG)
	$(MAKE) -f $(BASEDIR)/tools/kconfig/Makefile.kconfig ARCH=$(ARCH) SRCARCH=$(SRCARCH) HOSTCC="$(HOSTCC)" HOSTCXX="$(HOSTCXX)" silentoldconfig

# Empty recipe，avoid implicit rule
include/config/auto.conf.cmd: ;

# Allow people to just run `make` as before and not force them to configure
$(KCONFIG_CONFIG):
	$(MAKE) -f $(BASEDIR)/tools/kconfig/Makefile.kconfig ARCH=$(ARCH) SRCARCH=$(SRCARCH) HOSTCC="$(HOSTCC)" HOSTCXX="$(HOSTCXX)" defconfig

So, 对于 Xen 来说，如果用户没有显式的 make *config 来生成 .config 文件，则会通过 defconfig 来生成。
$(BASEDIR)/tools/kconfig/Makefile.kconfig 包含了 tools/kconfig 下的 Makefile 和 Makefile.host,
Makefile.host 用于编译 kconfig 的 host program，Makefile 则包含各种 config target，defconfig 就定义在其中。

编译的重头落在这条 recipe:

	$(MAKE) -f Rules.mk _$@

而 xen/Rule.mk 中又 include xen/Makefile, _install 定义在这里：

_install: $(TARGET)$(CONFIG_XEN_INSTALL_SUFFIX)

Rules.mk 中 include 了
	Makefile(此时是 xen/Makefile)
	$(XEN_ROOT)/Config.mk
	$(XEN_ROOT)/config/$(XEN_TARGET_ARCH).mk

变量 TARGET 定义在 xen/Rule.mk 中:
	TARGET := $(BASEDIR)/xen

xen/Makefile 中有:
	export BASEDIR := $(CURDIR)

config/x86_64.mk 有:
	CONFIG_XEN_INSTALL_SUFFIX := .gz

所以将上面的 rule 扩展出来就是：

_install: $(TARGET)$(CONFIG_XEN_INSTALL_SUFFIX)
==》
_install: <完整路径>/xen/xen.gz

# In xen/Makefile
(TARGET).gz: $(TARGET)
	gzip -n -f -9 < $< > $@.new
	mv $@.new $@

$(TARGET): delete-unfresh-files
	...
	$(MAKE) -f $(BASEDIR)/Rules.mk -C arch/$(TARGET_ARCH) $@

xen/arch/x86/Makefile:

$(TARGET): $(TARGET)-syms $(efi-y) boot/mkelf32

$(TARGET)-syms: prelink.o xen.lds $(BASEDIR)/common/symbols-dummy.o

prelink.o: $(ALL_OBJS)
	$(LD) $(LDFLAGS) -r -o $@ $^

# In xen/Rules.mk
ALL_OBJS-y               += $(BASEDIR)/common/built_in.o
ALL_OBJS-y               += $(BASEDIR)/drivers/built_in.o
ALL_OBJS-y               += $(BASEDIR)/xsm/built_in.o
ALL_OBJS-y               += $(BASEDIR)/arch/$(TARGET_ARCH)/built_in.o
ALL_OBJS-$(CONFIG_CRYPTO)   += $(BASEDIR)/crypto/built_in.o
ALL_OBJS := $(ALL_OBJS-y)

# In xen/arch/x86/Makefile
ALL_OBJS := $(BASEDIR)/arch/x86/boot/built_in.o $(BASEDIR)/arch/x86/efi/built_in.o $(ALL_OBJS)

# In xen/Rules.mk
%/built_in.o: FORCE
	$(MAKE) -f $(BASEDIR)/Rules.mk -C $* built_in.o

built_in.o: $(obj-y)
	$(LD) $(LDFLAGS) -r -o $@ $^

因为有 -C $*，且 xen/Rules.mk 中有: include Makefile，以 $(BASEDIR)/common/built_in.o 为例，被 include 的 Makefile 是 $(BASEDIR)/common/Makefile，
这个 Makefile 中定义的是 obj-y，subdir-y 等， 跟 kernel 一样，每个目录下的所有 object file 被 link 为 built_in.o，子目录的 built_in.o 被 link 到父目录
的 built_in.o 中：

	subdir-y += $(filter %/,$(obj-y))
	obj-y += $(patsubst %/,%/built_in.o,$(subdir-y))