There is provided a data processing apparatus that includes processing circuitry for executing instructions relating to an active virtual processor in a plurality of virtual processors. Exception control circuitry receives an external exception associated with a target virtual processor in the plurality of virtual processors and when the target virtual processor is other than the active virtual processor, it issues a doorbell exception to cause a scheduling operation to schedule the target virtual processor to be the active virtual processor. Storage circuitry stores an indication of a set of masked virtual processors and the scheduling operation is adapted to disregard doorbell exceptions in respect of the set of masked virtual processors.