xen: add xenstore watcher infrastructure

A Xen PV frontend communicates its state to the PV backend by writing to
the 'state' key in the frontend area in xenstore. It is therefore
necessary for a XenDevice implementation to be notified whenever the
value of this key changes.

This patch adds code to do this as follows:

- an 'fd handler' is registered on the libxenstore handle which will be
  triggered whenever a 'watch' event occurs
- primitives are added to xen-bus-helper to add or remove watch events
- a list of Notifier objects is added to XenBus to provide a mechanism
  to call the appropriate 'watch handler' when its associated event
  occurs

The xen-block implementation is extended with a 'frontend_changed' method,
which calls as-yet stub 'connect' and 'disconnect' functions when the
relevant frontend state transitions occur. A subsequent patch will supply
a full implementation for these functions.

Signed-off-by: Paul Durrant <paul.durrant@citrix.com>
Reviewed-by: Anthony Perard <anthony.perard@citrix.com>
Signed-off-by: Anthony PERARD <anthony.perard@citrix.com>

include/hw/xen/xen-bus-helper.h

@@ -36,4 +36,10 @@ int xs_node_scanf(struct xs_handle *xsh,  xs_transaction_t tid,
                   const char *node, const char *key, Error **errp,
                   const char *fmt, ...);
 
+/* Watch node/key unless node is empty, in which case watch key */
+void xs_node_watch(struct xs_handle *xsh, const char *node, const char *key,
+                   char *token, Error **errp);
+void xs_node_unwatch(struct xs_handle *xsh, const char *node, const char *key,
+                     const char *token, Error **errp);
+
 #endif /* HW_XEN_BUS_HELPER_H */

include/hw/xen/xen-bus.h

@@ -10,6 +10,11 @@
 
 #include "hw/xen/xen_common.h"
 #include "hw/sysbus.h"
+#include "qemu/notify.h"
+
+typedef void (*XenWatchHandler)(void *opaque);
+
+typedef struct XenWatch XenWatch;
 
 typedef struct XenDevice {
     DeviceState qdev;
@@ -18,10 +23,14 @@ typedef struct XenDevice {
     char *backend_path, *frontend_path;
     enum xenbus_state backend_state, frontend_state;
     Notifier exit;
+    XenWatch *frontend_state_watch;
 } XenDevice;
 
 typedef char *(*XenDeviceGetName)(XenDevice *xendev, Error **errp);
 typedef void (*XenDeviceRealize)(XenDevice *xendev, Error **errp);
+typedef void (*XenDeviceFrontendChanged)(XenDevice *xendev,
+                                         enum xenbus_state frontend_state,
+                                         Error **errp);
 typedef void (*XenDeviceUnrealize)(XenDevice *xendev, Error **errp);
 
 typedef struct XenDeviceClass {
@@ -32,6 +41,7 @@ typedef struct XenDeviceClass {
     const char *device;
     XenDeviceGetName get_name;
     XenDeviceRealize realize;
+    XenDeviceFrontendChanged frontend_changed;
     XenDeviceUnrealize unrealize;
 } XenDeviceClass;
 
@@ -47,6 +57,7 @@ typedef struct XenBus {
     BusState qbus;
     domid_t backend_id;
     struct xs_handle *xsh;
+    NotifierList watch_notifiers;
 } XenBus;
 
 typedef struct XenBusClass {
@@ -64,4 +75,8 @@ typedef struct XenBusClass {
 
 void xen_bus_init(void);
 
+void xen_device_backend_set_state(XenDevice *xendev,
+                                  enum xenbus_state state);
+enum xenbus_state xen_device_backend_get_state(XenDevice *xendev);
+
 #endif /* HW_XEN_BUS_H */