Memory management

A method of operation of a host data processing system which provides a virtual operating environment for one or more guest data processing systems comprises: initiating a transaction for translation of a guest system memory address to a host system physical address in response to a transaction request from a device overseen by a guest system for access to system memory at that guest system memory address; storing identification information relating to that transaction including at least data identifying device which requested the transaction; detecting a translation error condition in respect of that transaction; and handling a detected error condition by: (i) providing information indicative of the translation error condition to the guest system overseeing the device which requested the transaction; (ii) receiving a command from the guest system in respect of that transaction, the command from the guest system comprising information identifying the device which requested the transaction; and (iii) validating the received command for execution, by comparing the stored identification information for that transaction with at least the identity of the device identified by the command.