Cannot specify user mode execution without specifying user-mode pagetables.
The problem is that a 64-bit guest can get one of its vcpus into non-kernel mode without first providing a valid non-kernel pagetable. The iret-into-userspace path has the right checks, but just setting the context on a fresh vcpu doesn't. :( The observed failure mode is usually a hard lockup of the host. This affects 64-bit version of kernel-xen.
This issue has been addressed in following products:
Red Hat Enterprise Linux 5
Via RHSA-2011:0833 https://rhn.redhat.com/errata/RHSA-2011-0833.html