[PATCH] xen/pv-on-hvm kexec: add xs_reset_watches to shutdown watches from old kernel

[Olaf Hering <olaf@aepfle.de>]

Add new xs_reset_watches function to shutdown watches from old kernel after
kexec boot.  The old kernel does not unregister all watches in the
shutdown path.  They are still active, the double registration can not
be detected by the new kernel.  When the watches fire, unexpected events
will arrive and the xenwatch thread will crash (jumps to NULL).  An
orderly reboot of a hvm guest will destroy the entire guest with all its
resources (including the watches) before it is rebuilt from scratch, so
the missing unregister is not an issue in that case.

With this change the xenstored is instructed to wipe all active watches
for the guest.  However, a patch for xenstored is required so that it
accepts the XS_RESET_WATCHES request from a guest.

v3:
  - use XS_RESET_WATCHES instead of XS_INTRODUCE

v2:
  - move all code which deals with XS_INTRODUCE into xs_introduce()
    (based on feedback from Ian Campbell)
  - remove casts from kvec assignment

Signed-off-by: Olaf Hering <olaf@aepfle.de>

---
 drivers/xen/xenbus/xenbus_xs.c     |   20 ++++++++++++++++++++
 include/xen/interface/io/xs_wire.h |    6 +++++-
 2 files changed, 25 insertions(+), 1 deletion(-)

Index: linux-3.0/drivers/xen/xenbus/xenbus_xs.c
===================================================================
--- linux-3.0.orig/drivers/xen/xenbus/xenbus_xs.c
+++ linux-3.0/drivers/xen/xenbus/xenbus_xs.c
@@ -620,6 +620,22 @@ static struct xenbus_watch *find_watch(c
 	return NULL;
 }
 
+static void xs_reset_watches(void)
+{
+	/* dummy, empty iov not handled by xenstored */
+	struct kvec iov[1];
+	int err;
+
+
+	iov[0].iov_base = "dummy";
+	iov[0].iov_len = strlen(iov[0].iov_base) + 1;
+
+	err = xs_error(xs_talkv(XBT_NIL, XS_RESET_WATCHES, iov,
+				 ARRAY_SIZE(iov), NULL));
+	if (err)
+		printk(KERN_WARNING "xs_reset_watches failed: %d\n", err);
+}
+
 /* Register callback to watch this node. */
 int register_xenbus_watch(struct xenbus_watch *watch)
 {
@@ -897,5 +913,9 @@ int xs_init(void)
 	if (IS_ERR(task))
 		return PTR_ERR(task);
 
+	/* shutdown watches for kexec boot */
+	if (xen_hvm_domain())
+		xs_reset_watches();
+
 	return 0;
 }
Index: linux-3.0/include/xen/interface/io/xs_wire.h
===================================================================
--- linux-3.0.orig/include/xen/interface/io/xs_wire.h
+++ linux-3.0/include/xen/interface/io/xs_wire.h
@@ -26,7 +26,11 @@ enum xsd_sockmsg_type
     XS_SET_PERMS,
     XS_WATCH_EVENT,
     XS_ERROR,
-    XS_IS_DOMAIN_INTRODUCED
+    XS_IS_DOMAIN_INTRODUCED,
+    XS_RESUME,
+    XS_SET_TARGET,
+    XS_RESTRICT,
+    XS_RESET_WATCHES
 };
 
 #define XS_WRITE_NONE "NONE"