XenBus是通过XenStore协商的一套协议，前端和后端通过XenBus进行协商最终建立起通信，前端和后端都是状态机，而状态转换的条件就是由XenBus来完成的，本篇将介绍XenBus. 源码可以从Github上得到。

XenBus

The XenBus, in the context of device drivers, is an informal protocol built on top of the XenStore, which provides a way of enumerating the (virtual) devices available to a given domain, and connecting to them. Implementing the XenBus interface is not required when porting a kernel to Xen. It is predominantly used in Linux to isolate the Xen-specific code behind a relatively abstract interface.(from chapter 6.4^[1])

XenBus由于是一个通信协议，因此不是必须要实现的(e.g.XenStore & Console). 使用XenBus能够和Dom0上的Backend进行协商，不过我们同样可以实现自己的协议。

XenStore与Console也属于设备的一种，而这两者就是直接从start_info获取的，其余的外设则是通过XenStore进行交流.

State Machine

xenbus.hlink

enum xenbus_state
{
	XenbusStateUnknown      = 0,
	XenbusStateInitialising = 1,
	XenbusStateInitWait     = 2,  /* Finished early initialisation, but waiting
					 for information from the peer or hotplug scripts. */
	XenbusStateInitialised  = 3,  /* Initialised and waiting for a
					 connection from the peer. */
	XenbusStateConnected    = 4,
	XenbusStateClosing      = 5,  /* The device is being closed
					 due to an error or an unplug event. */
	XenbusStateClosed       = 6,

	/*
	* Reconfiguring: The device is being reconfigured.
	*/
	XenbusStateReconfiguring = 7,
	XenbusStateReconfigured  = 8
};

使用XenBus协议的设备状态有以上这些，其中Reconfigure我们暂不讨论，一般设备不重新配置的话是不需要这个状态的，状态的转换一般如↓:

xenbus_client.clink

183: static int __xenbus_switch_state(struct xenbus_device *dev,
		      enum xenbus_state state, int depth)
{
	struct xenbus_transaction xbt;
	int current_state;
	int err, abort;

	if (state == dev->state)
		return 0;

	xenbus_transaction_start(&xbt);
	xenbus_scanf(xbt, dev->nodename, "state", "%d", &current_state);
	xenbus_printf(xbt, dev->nodename, "state", "%d", state);
	err = xenbus_transaction_end(xbt, abort);

  dev->state = state;
	return 0;
}

 * xenbus_watch_path - register a watch
 * @dev: xenbus device
 * @path: path to watch
 * @watch: watch to register
 * @callback: callback to register
115: int xenbus_watch_path(struct xenbus_device *dev, const char *path,
		      struct xenbus_watch *watch, void (*callback)(struct xenbus_watch *,
				       const char **, unsigned int))

在xenbus.h中有所有状态的声明, 由switch_state进行状态的迁移，switch_state会通过XenStore接口(Linux中即XenBus接口)向XenStore实时写入目前设备的状态由此另一端就能通过状态来判断是否是否应该读取某些值，通过这个协议前后端就可以进行协商(告知对方shared page的grant reference等)

而两端也都会在对方Xenstore的key上注册watch来监控变动，这样当一方状态改变时另一方就能迅速做出相应。

一般来说我们会把尽可能多的工作交给back来完成, 这样Guest的driver就可以尽可能简化, 而共享页等分配则交由Guest来做, 当然自己的Driver也可以不这么做..而前后端的状态会互相影响:

一般Back初始化后front才开始初始化, (front可能会从xenstore读一些关于back的信息)，back的初始化会用到front的一些数据(grant ref等), 因此在front初始化完成前back会进入Initwait等待front向xenstore中写入信息。之后当guest连接到bus后back也会连接到bus上，此时driver就可以正常工作了。

当任何一方断开时另一方也会断开。

Add State to Driver

我们现在给前一篇完成的Driver中加入XenBus的状态:

alice_dom0.clink

/* We set up the callback functions */
static struct xenbus_driver alice_back_driver = {
	.ids  = alice_back_ids,
	.probe = alice_back_probe,
	.otherend_changed = alice_back_otherend_changed,
};

/* The function is called on a state change of the frontend driver */
static void alice_back_otherend_changed(struct xenbus_device *dev, enum xenbus_state frontend_state)
{
	switch (frontend_state) {
		case XenbusStateInitialising:
			set_backend_state(dev, XenbusStateInitWait);
			break;

		case XenbusStateConnected:
			set_backend_state(dev, XenbusStateConnected);
			break;
    ...

		default:
			xenbus_dev_fatal(dev, -EINVAL, "saw state %s (%d) at frontend",
					xenbus_strstate(frontend_state), frontend_state);
			break;
	}
}

主要内容就是加入otherend_changed函数并在里面根据另一端的状态切换本端的状态。顺便提一下activate.sh需要给domU和dom0的Key加入读写权限:

Manual of config ---->link

1
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# Make sure the domU can read the dom0 data
xenstore-chmod $DOM0_KEY b0 r$DOMU_ID
xenstore-chmod $DOMU_KEY b$DOMU_ID r0

Demo

# DomU: 
@domU: sudo insmod alice_domU.ko; dmesg | tail -n 5
[   70.232619] DomU: Alice_front inited!
[  205.066293] DomU: Probe called.
[  205.069744] DomU: Connecting the frontend now
[  205.071032] DomU: Other side says it is connected as well.

# Dom0
@dom0: sudo insmod alice_dom0.ko; dmesg | tail -n 3
[  367.269307] Dom0: Alice_back inited!
[  414.873681] Dom0: Probe called.
[  414.875169] Dom0: Connect the backend

Xen Log 15-Connect to Xenbus

XenBus

State Machine

Add State to Driver

Demo

Reference