1. A data processing apparatus comprising: processing circuitry configured to execute program instructions at a plurality of privilege levels; and a plurality of performance monitoring circuits, wherein the processing circuitry is configured to set, in response to at least one program instruction executed by the processing circuitry at a first privilege level, first configuration data for controlling performance monitoring by a first subset of performance monitoring circuits; the processing circuitry is configured to set a disable control flag in response to at least one program instruction executed by the processing circuitry at a second privilege level higher than the first privilege level; and the apparatus comprises performance monitoring control circuitry configured to disable performance monitoring by said first subset of performance monitoring circuits while the processing circuitry is executing instructions at the second privilege level if said disable control flag has a predetermined value, wherein the performance monitoring control circuitry is configured to enable performance monitoring by the first subset of performance monitoring circuits while the processing circuitry is executing program instructions at the first privilege level even if the disable control flag has the predetermined value.