1. A method of probing first and second domains of a computer system having virtual machines running therein, comprising: instrumenting the first domain with a first probe and the second domain with a second probe; executing the first probe in the first domain, wherein a value of a variable that is shared by the first and second probes is updated as a result of execution of the first probe; and executing the second probe in the second domain, wherein the updated value of the shared variable is read during execution of the second probe and used to direct the execution of the second probe.