Certain page table manipulation operations in Xen 4.1.x, 4.2.x, and earlier are not preemptible, which allows local PV kernels to cause a denial of service via vectors related to "deep page table traversal."
http://lists.fedoraproject.org/pipermail/package-announce/2013-May/105533.html
http://lists.opensuse.org/opensuse-security-announce/2014-03/msg00021.html
http://secunia.com/advisories/53187
http://secunia.com/advisories/55082
http://security.gentoo.org/glsa/glsa-201309-24.xml