1. A data processing apparatus, comprising: processing circuitry for executing hypervisor software to support the execution of multiple virtual machines on the processing circuitry; a memory system for storing data for access by the processing circuitry, the memory system comprising secure memory for storing secure data and non-secure memory for storing non-secure data, the secure memory only being accessible via a secure access request; address translation circuitry, responsive to an access request issued by a current virtual machine executing on the processing circuitry and specifying a virtual address, to perform an address translation process to identify a physical address in said memory containing data seeking to be accessed by the current virtual machine, and to cause to be issued to the memory system a modified access request specifying the physical address; and a storage for storing a trusted virtual machine identifier managed by the hypervisor software, the hypervisor software being arranged to set the trusted virtual machine identifier if the current virtual machine is to be trusted to access the secure memory; in response to the access request issued by the current virtual machine, the address translation circuitry only being able to cause the modified access request to be issued as said secure access request specifying a physical address within the secure memory if the trusted virtual machine identifier is set.