1. A computer system comprising: a memory-manage subsystem configured to maintain and use a temporary-read-only bit in tables used to convert pseudo-physical addresses of a hypervisor's guest operating system (OS) to physical addresses, and to use the temporary read-only bit in conjunction with another read-only bit to trigger a trap by which normal read-only page violations and violations for checkpointing are distinguishable by the hypervisor; and a rollback and recovery subsystem configured to periodically checkpoint a state of one of more guest OS's of the hypervisor.