1.背景
在集成KVM的虛擬化平臺上，爲實現host主機和guest VM之間數據通信需要創建共享內存，其驅動的添加方式可參考Add device driver (uio_ivshmem.ko) on guest。由於才接觸makefile，不懂的地方在此記錄，如果理解有誤，希望大佬指正。
2.編譯生成uio_ivshmem.ko的makefile文件
# obj-m is a list of what kernel modules to build. The .o and other
# objects will be automatically built from the corresponding .c file -
# no need to list the source files explicitly.
obj-m := uio_ivshmem.o
# KDIR is the location of the kernel source. The current standard is
# to link to the associated source tree from the directory containing
# the compiled modules.
KDIR := /lib/modules/$(shell uname -r)/build
# PWD is the current working directory and the location of our module
# source files.
PWD := $(shell pwd)
# default is the default make target. The rule here says to run make
# with a working directory of the directory containing the kernel
# source and compile only the modules in the PWD (local) directory.
default:
$(MAKE) -C $(KDIR) M=$(PWD) modules
install:
cp uio_ivshmem.ko /lib/modules/$(shell uname -r)/kernel/drivers/uio/
clean:
rm -f *.ko *.o uio_ivshmem.mod.c Module.symvers
3.代碼解釋
（1）命令行敲入make，讀取當前目錄下的makefile，執行的target爲：default
（2）$(MAKE) -C $(KDIR) M=$(PWD) modules 這行代碼裏，$(MAKE)相當於make，若去掉參數，整行代碼可先看做 make modules，說明這個命令用於編譯內核模塊。
（3）-C參數是選擇路徑，加上$(KDIR)，就會去執行該目錄下的makefile，M 參數是編譯外部模塊，生成的模塊不會直接加載在內核中，M="dir"則是編譯的路徑。
參考博文：
https://blog.csdn.net/importantg/article/details/51763551
https://blog.csdn.net/xiaowulang20082008/article/details/50586985
https://blog.csdn.net/lgjjeff/article/details/90489331