The MITRE CVE dictionary describes this issue as:
Xen in the Linux kernel, when running a guest on a host without hardware assisted paging (HAP), allows guest users to cause a denial of service (invalid pointer dereference and hypervisor crash) via the SAHF instruction.
Find out more about CVE-2011-2519 from the
MITRE CVE dictionary dictionary and
This issue only affects Red Hat Enterprise Linux 5. The versions of the Linux
kernel-xen as shipped with Red Hat Enterprise Linux 4, 6, and Red Hat Enterprise MRG are not affected. This has been addressed in Red Hat Enterprise Linux 5 via https://rhn.redhat.com/errata/RHSA-2011-1212.html.