1. A system comprising: a physical machine comprising a hardware platform to host a plurality of virtual machines, the hardware platform comprising a memory, a processor and a memory controller, and the memory controller comprising a register; wherein: the processor is adapted to instruct a virtual machine monitor (VMM) to: execute instructions associated with the plurality of virtual machines associated with a plurality of contexts; in response to the processor changing from executing instructions associated with a first virtual machine of the plurality of virtual machines to executing instructions associated with a second virtual machine of the plurality of virtual machines, store a key associated with the second virtual machine in the register, the second virtual machine and the key being associated with at least one region of the memory less than the entire memory; and in response to termination of the second virtual machine, remove the key from the register; the memory controller is adapted to: use the key to encrypt content associated with the at least one region of the memory associated with the second virtual machine; and in response to a request to access the memory while the key is stored in the register, perform a cryptographic function on data associated with the request based on the key regardless of whether the request targets the at least one region of the memory associated with the second virtual machine or not; and decrypted content associated with a region of the memory not associated with the key is inaccessible to the second virtual machine.