xenbus_read_driver_state
if (xenbus_read_driver_state(oename) != XenbusStateConnected)
state = xenbus_read_driver_state(dev->otherend);
if ((xenbus_read_driver_state(xsname) >= XenbusStateClosing)) {
(xenbus_read_driver_state(xsname) >= XenbusStateClosing))
extern XenbusState xenbus_read_driver_state(const char *path);