Domains with physical device access (i.e. driver domains) receive
limited access to certain PCI devices (bus address space and
interrupts). However many guest operating systems attempt to
determine the PCI configuration by directly access the PCI BIOS,
which cannot be allowed for safety.
Instead, Xen provides the following hypercall:
Set and query IRQ configuration details, set the system IOPL, set the
TSS IO bitmap.
For examples of using physdev_op(), see the
Xen-specific PCI code in the linux sparse tree.