On systems with RHEL 5.6 or Oracle Linux 5.6, Xen might hang during bootup with the following messages:
(XEN) mtrr: your CPUs had inconsistent MTRRdefType settings (XEN) mtrr: probably your BIOS does not setup all CPUs. (XEN) mtrr: corrected configuration.
It is also possible that the system will complete Xen but hang on dom1.
If you do this, the server will only be able to use 80 CPUs.