Chapter 2 Server Virtualization
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Monolithic Hypervisor Monolithic hypervisor design involves using hypervisor-aware device drivers that are hosted within and managed by the hypervisor as shown in Figure 2-3. Monolithic VM1 Admin
VM2
VM3
Hypervisor Drivers Hardware FIGURE 2-3 Monolithic hypervisor platforms require hypervisor-aware device drivers.
The monolithic design choice results in some benefits but also some drawbacks. For example, monolithic hypervisors do not need a controlling, or parent, operating system because all guest operating systems interact directly with the underlying physical hardware of the host computer by using hypervisor-aware device drivers. This is an example of the benefit of the monolithic design. On the other hand, the fact that device drivers must be specifically developed for the hypervisor creates significant challenges because there are so many different types of motherboards, storage controllers, network adapters, and other types of hardware devices on the market. The result is that vendors of monolithic hypervisor platforms have to work closely with manufacturers of hardware devices to ensure these manufacturers develop hypervisor-aware versions of device drivers for their hardware. It also means that vendors of monolithic hypervisor platforms are dependent on manufacturers of hardware devices to supply such drivers for their products. The result is that the number of devices that can be used in virtualized operating system environments running on monolithic hypervisor platforms can be more limited than when those same operating system environments are run directly on physical computers. One important point is that in this design you’re ignoring one of the most important security tenets: defense in depth. With defense in depth, you provide multiple layers of defense to prevent against attacks. In this model, there is no defense in depth because everything is running in the most privileged part of the system. An example of a server virtualization product that uses a monolithic hypervisor design is VMware ESX Server.