在kernel代码中，有时会看到函数声明中有的参数带有__user修饰符：
ssize_t (*write) (struct file *, const char __user *, size_t, loff_t *);
LDD给出的解释：
This annotation is a form of documentation, noting that a pointer is a user-space address that cannot be directly dereferenced. For normal compilation, __user has no effect, but it can be used by external checking software to find misuse of user-space addresses.
__user表明参数是一个用户空间的指针，不能在kernel代码中直接访问。也方便其它工具对代码进行检查。
这两天在调试开源项目的一个Makefile，发现了remake这个工具（项目主页：http://bashdb.sourceforge.net/remake/），真的很好用。稍微大型点的开源项目，Makefile都很复杂，一旦出了错，很令人抓狂。而这个remake工具产生的输出能把整个编译过程的来龙去脉打印的很清楚。举例如下，一个简单的编译Linux模块的Makefile：
ifneq ($(KERNELRELEASE),)
obj-m := hello.o
else
KDIR ?= /lib/modules/`uname -r`/build
default:
$(MAKE) -C $(KDIR) M=$$PWD
endif
执行remake -x命令：
# remake -x
Reading makefiles...
Updating goal targets....
File 'default' does not exist.
Must remake target 'default'.
Makefile:8: target 'default' does not exist
##>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>
remake -C /lib/modules/`uname -r`/build M=$PWD
##<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<
Reading makefiles...
Updating goal targets....
File 'all' does not exist.
Must remake target 'all'.
remake[1]: Entering directory '/usr/src/linux-3.12.49-6-obj/x86_64/default'
Makefile:26: target 'all' does not exist
##>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>
remake -C ../../../linux-3.12.49-6 O=/usr/src/linux-3.12.49-6-obj/x86_64/default/.
##<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<
Reading makefiles...
Updating goal targets....
File '_all' does not exist.
File 'sub-make' does not exist.
File 'FORCE' does not exist.
Must remake target 'FORCE'.
Successfully remade target file 'FORCE'.
Must remake target 'sub-make'.
Makefile:195: update target 'sub-make' due to: FORCE
##>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>
echo "make[1]: Entering directory \`/usr/src/linux-3.12.49-6-obj/x86_64/default'"
##<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<
make[1]: Entering directory `/usr/src/linux-3.12.49-6-obj/x86_64/default'
##>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>
remake -C /usr/src/linux-3.12.49-6-obj/x86_64/default \
KBUILD_SRC=/usr/src/linux-3.12.49-6 \
KBUILD_EXTMOD="/root/Documents/test" -f /usr/src/linux-3.12.49-6/Makefile \
##<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<
......
可以看到，输出了非常详细的日志信息，很方便debug。
Hypervisor connection是libvirt一个核心概念（以下内容摘自这里）：
A connection is the primary or top level object in the libvirt API and Python libvirt module. An instance of this object is required before attempting to use almost any of the classes or methods. A connection is associated with a particular hypervisor, which may be running locally on the same machine as the libvirt client application, or on a remote machine over the network. In all cases, the connection is represented by an instance of the virConnect class and identified by a URI. The URI scheme and path defines the hypervisor to connect to, while the host part of the URI determines where it is located.
An application is permitted to open multiple connections at the same time, even when using more than one type of hypervisor on a single machine. For example, a host may provide both KVM full machine virtualization and LXC container virtualization. A connection object may be used concurrently across multiple threads. Once a connection has been established, it is possible to obtain handles to other managed objects or create new managed objects.
以下代码测试Xen连接：
#!/usr/bin/python
from __future__ import print_function
import sys
import libvirt
conn = libvirt.open('xen:///')
if conn == None:
print('Failed to open connection to xen:///', file=sys.stderr)
exit(1)
else:
print('Open connection success', file=sys.stdout)
conn.close()
exit(0)
以下内容摘自Libvert terminology and goals：
a node is a single physical machine
an hypervisor is a layer of software allowing to virtualize a node in a set of virtual machines with possibly different configurations than the node itself
a domain is an instance of an operating system (or subsystem in the case of container virtualization) running on a virtualized machine provided by the hypervisor Hypervisor and domains running on a node
Now we can define the goal of libvirt: to provide a common and stable layer sufficient to securely manage domains on a node, possibly remote.
在libvirt中，node即指物理机器，domain可以理解为虚拟机。