Deferred shadowing of segment descriptors in a virtual machine monitor for a segmented computer architecture

Abstract

One or more virtual machines (VM's) run via a virtual machine monitor (VMM) on a hardware platform that has a segmented memory architecture. Each VM has at least one VM descriptor table that has, as entries, VM segment descriptors. At least one VMM descriptor table is established, including at least one shadow descriptor table that stores shadow descriptors for certain of the VM segment descriptors, which are then shadowed descriptors. The VMM compares the shadow descriptors with their respective corresponding shadowed VM descriptors, detects any lack of correspondence between the shadow descriptor table and the corresponding VM descriptor table, and updates and thereby synchronizes each shadow descriptor with its respective shadowed VM descriptor no later than, and preferably only upon the first use of, the respective descriptor by the VM. Whenever the VMM detects any attempt by the VM to load an usynchronized shadowed descriptor, the VMM verifies that the VM is allowed to load it, and then establishes synchronization for the pair of descriptors. One detection mechanism is the tracing of entire memory pages in which VM descriptors are stored; another involves sensing and setting the state of a segment present bit for individual descriptors. The invention improves virtualization performance by reducing the number of descriptors that need to be kept coherent. The VMM also has a flexible mechanism to invalidate descriptors, for example, when the VM unmaps or remaps the memory page it is located in, or when the VM sets a new segment descriptor table.