When running SUSE Linux Enterprise Server (SLES) 11 SP1, if the number of I/O devices exceeds XEN's number of interrupts, the I/O devices might not work, and the OS might display corresponding error messages. For example:
Unable to allocate IRQ
Workaround
Add the following to the /boot/grub/menu.lst file:
extra_guest_irqs=64,2048 nr_irqs=2048