The instruction emulation in Xen 3.0.3 allows local SMP guest users to cause a denial of service (host crash) by replacing the instruction that causes the VM to exit in one thread with a different instruction in a different thread.
http://rhn.redhat.com/errata/RHSA-2011-1065.html
http://rhn.redhat.com/errata/RHSA-2011-1163.html