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[Xen-changelog] [linux-2.6.18-xen] pv-on-hvm/kexec: add xs_reset_watches to shutdown watches from old kernel

# HG changeset patch
# User Olaf Hering <olaf@xxxxxxxxx>
# Date 1317886181 -7200
# Node ID fd01f089048e4d9ea35fdb417b4f54d28a9bf16c
# Parent  5bda145fc6dcda60cd125699d7ece4bbade0660f
pv-on-hvm/kexec: add xs_reset_watches to shutdown watches from old kernel

commit ddacf5ef684a655abe2bb50c4b2a5b72ae0d5e05

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 client (see changeset
23839:42a45baf037d in xen-unstable.hg). Without the patch for xenstored
the registration of watches will fail and some features of a PVonHVM
guest are not available. The guest is still able to boot, but repeated
kexec boots will fail.

  - use xs_single instead of passing a dummy string to xs_talkv

  - ignore -EEXIST in xs_reset_watches


  - 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@xxxxxxxxx>
Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>

diff -r 5bda145fc6dc -r fd01f089048e drivers/xen/xenbus/xenbus_xs.c
--- a/drivers/xen/xenbus/xenbus_xs.c    Wed Oct 05 14:44:49 2011 +0200
+++ b/drivers/xen/xenbus/xenbus_xs.c    Thu Oct 06 09:29:41 2011 +0200
@@ -627,6 +627,17 @@
        return NULL;
+static void xs_reset_watches(void)
+#ifndef CONFIG_XEN
+       int err;
+       err = xs_error(xs_single(XBT_NIL, XS_RESET_WATCHES, "", NULL));
+       if (err && err != -EEXIST)
+               printk(KERN_WARNING "xs_reset_watches failed: %d\n", err);
 /* Register callback to watch this node. */
 int register_xenbus_watch(struct xenbus_watch *watch)
@@ -928,5 +939,8 @@
        if (IS_ERR(task))
                return PTR_ERR(task);
+       /* shutdown watches for kexec boot */
+       xs_reset_watches();
        return 0;

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