
510 Large Scale and Big Data
enable ag, can be altered only in system mode. If POPF is executed in user mode
by a program that attempts to pop the interrupt enable ag, POPF will act as a no-op
(i.e., no operation) instruction. Therefore, the behavior of POPF depends on the mode
of operation, thus rendering behavior-sensitive. Finally, if the instruction is neither
control-sensitive nor behavior-sensitive, it is innocuous.
According to Popek and Goldberg [48], a hypervisor can be constructed if it satis-
es three properties, efciency, resource control, and equivalence. Efciency entails
executing all innocuous instructions directly on hardware ...