Method and apparatus for emulating multiple virtual timers in a virtual computer system when the virtual timers fall behind the real time of a physical computer system

Abstract

A virtual computer system includes multiple timer emulators for emulating multiple virtual timers in a virtual machine (VM). A time coordinator keeps track of an apparent time that is provided to the multiple timer emulators for presentation to the VM through the virtual timers. In particular, the time coordinator ensures that timer events generated by the multiple timer emulators are presented to the VM in an appropriate sequence and with substantially appropriate relative apparent times. Also, when guest software reads a count from a virtual timer, the time coordinator ensures that the apparent time presented to the guest software is substantially consistent with the apparent times represented by preceding and succeeding timer events. When the apparent time falls behind the real time of the physical computer system, the time coordinator speeds up the apparent time until it catches up to the real time.