On older 32-bit systems with only 1 CPU and an IOAPIC, the 32-bit Xen dom0 kernel may fail to boot with message similar to:
(XEN) irq.c:794: dom0: pirq 2 or vector 144 already mapped
(XEN) ioapic_guest_write: apic=0, pin=2, old_irq=-1, new_irq=4
(XEN) ioapic_guest_write: old_entry=00010000, new_entry=000009e4
(XEN) ioapic_guest_write: Attempt to add IO-APIC pin for in-use IRQ!
To workaround this problem, pass the option "noapic" to the hypervisor while booting the kernel. This bug exists in the RHEL-5.4 beta kernel, but should be addressed before RHEL-5.4 Final.