1. In a computer system having virtualization software that supports execution of guest virtual machines, a method of supporting first and second read operations performed on a portion of guest memory, comprising: upon detecting a fault caused by the first read operation while the virtualization software and a guest driver are in a first mode, the first read operation being carried out by the guest driver accessing said portion of guest memory, invoking a first fault handler to process the fault in the virtualization software, and terminating the first read operation; and upon detecting a fault caused by the second read operation while the virtualization software and the guest driver are in a second mode, the second mode being established as a result of the guest driver calling into the virtualization software and the second read operation being carried out by the guest driver accessing said portion of guest memory, processing the fault by the guest driver in the guest software without invoking the first fault handler, and terminating the second read operation.