Roughly described, a data processing system comprises a memory addressable by a range of physical memory addresses; a plurality of non-privileged software domains each having a virtual memory address space; a privileged software domain; a memory management unit operable to perform virtual address translation of a virtual memory address into a physical memory address; and an I/O device supporting virtualised interfaces each associated with a respective non-privileged software domain, the I/O device comprising an operation management unit operable to perform virtual address translation in one or more of the virtual memory address spaces; wherein, for I/O operations requested by a virtualised interface, the I/O device invokes the operation management unit to perform virtual address translation for those I/O operations meeting first criteria and to invoke the memory management unit to perform virtual address translation for those I/O operations which do not meet the first criteria.