Monitor and emit events for XenStore changes to XenBus trees

of the devices we manage.  These changes can be due to writes
we make ourselves or due to changes made by the control domain.
The goal of these changes is to insure that all state transitions
can be detected regardless of their source and to allow common
device policies (e.g. "onlined" backend devices) to be centralized
in the XenBus bus code.

sys/xen/xenbus/xenbusvar.h:
sys/xen/xenbus/xenbus.c:
sys/xen/xenbus/xenbus_if.m:
	Add a new method for XenBus drivers "localend_changed".
	This method is invoked whenever a write is detected to
	a device's XenBus tree.  The default implementation of
	this method is a no-op.

sys/xen/xenbus/xenbus_if.m:
sys/dev/xen/netfront/netfront.c:
sys/dev/xen/blkfront/blkfront.c:
sys/dev/xen/blkback/blkback.c:
	Change the signature of the "otherend_changed" method.
	This notification cannot fail, so it should return void.

sys/xen/xenbus/xenbusb_back.c:
	Add "online" device handling to the XenBus Back Bus
	support code.  An online backend device remains active
	after a front-end detaches as a reconnect is expected
	to occur in the near future.

sys/xen/interface/io/xenbus.h:
	Add comment block further explaining the meaning and
	driver responsibilities associated with the XenBus
	Closed state.

sys/xen/xenbus/xenbusb.c:
sys/xen/xenbus/xenbusb.h:
sys/xen/xenbus/xenbusb_back.c:
sys/xen/xenbus/xenbusb_front.c:
sys/xen/xenbus/xenbusb_if.m:
	o Register a XenStore watch against the local XenBus tree
	  for all devices.
	o Cache the string length of the path to our local tree.
	o Allow the xenbus front and back drivers to hook/filter both
	  local and otherend watch processing.
	o Update the device ivar version of "state" when we detect
	  a XenStore update of that node.

sys/dev/xen/control/control.c:
sys/xen/xenbus/xenbus.c:
sys/xen/xenbus/xenbusb.c:
sys/xen/xenbus/xenbusb.h:
sys/xen/xenbus/xenbusvar.h:
sys/xen/xenstore/xenstorevar.h:
	Allow clients of the XenStore watch mechanism to attach
	a single uintptr_t worth of client data to the watch.
	This removes the need to carefully place client watch
	data within enclosing objects so that a cast or offsetof
	calculation can be used to convert from watch to enclosing
	object.

Sponsored by:	Spectra Logic Corporation
MFC after:	1 week

1 files changed, 6 insertions, 3 deletions

diff --git a/sys/xen/xenstore/xenstorevar.h b/sys/xen/xenstore/xenstorevar.h
index df41e31..4a1382d 100644
--- a/sys/xen/xenstore/xenstorevar.h
+++ b/sys/xen/xenstore/xenstorevar.h
@@ -56,8 +56,8 @@ struct xenstore_domain_interface;
 struct xs_watch;
 extern struct xenstore_domain_interface *xen_store;
 
-typedef	void (xs_watch_cb_t)(struct xs_watch *,
-				   const char **vec, unsigned int len);
+typedef	void (xs_watch_cb_t)(struct xs_watch *, const char **vec,
+    unsigned int len);
 
 /* Register callback to watch subtree (node) in the XenStore. */
 struct xs_watch
@@ -69,6 +69,9 @@ struct xs_watch
 
 	/* Callback (executed in a process context with no locks held). */
 	xs_watch_cb_t *callback;
+
+	/* Callback client data untouched by the XenStore watch mechanism. */
+	uintptr_t callback_data;
 };
 LIST_HEAD(xs_watch_list, xs_watch);
 
@@ -301,7 +304,7 @@ int xs_gather(struct xs_transaction t, const char *dir, ...);
  * XenStore watches allow a client to be notified via a callback (embedded
  * within the watch object) of changes to an object in the XenStore.
  *
- * \param watch  A xenbus_watch struct with it's node and callback fields
+ * \param watch  An xs_watch struct with it's node and callback fields
  *               properly initialized.
  *
  * \return  On success, 0. Otherwise an errno value indicating the