Current and Future Microsoft Virtualization Architectures

The virtualization support planned for Longhorn Server is very different from what exists today. The left side of the drawing shows the key elements of a Virtual Server 2005 R2-based system. The host OS is a stock version of Windows Server 2003, which loads the standard Windows drivers needed to communicate with the server’s physical hardware. The host OS runs Virtual Server 2005 R2 as a background service, which creates the virtual devices and loads and manages virtual machines (VMs). The guest OS on each VM loads device drivers for the virtual devices presented to it by Virtual Server 2005.

The right side of the drawing shows the key elements of the future Windows virtualization architecture. The server hardware must be running CPUs with the upcoming x64 versions of the Intel VT or AMD Pacifica virtualization technologies. When the system starts, Windows first loads the Windows hypervisor, which has no device driver model of its own and is the only Windows element that communicates directly with the hardware. The parent partition loads next. It must run Windows Longhorn Server, which will typically be installed in a minimal virtual server role that contains only the OS elements needed to create the virtualization environment. The parent partition controls the creation of a layer called the VMBus for both itself and all child partitions. The VMBuses sit at the bottom of each partition and communicate with each other through the hypervisor. The VMBus hosts virtualization service providers (VSPs), which provide interfaces for whole classes of physical devices (e.g., networking, storage, etc.), and virtualization service clients (VSCs), which expose virtualized device interfaces to the OSs of other partitions. Each VSP is controlled by the partition that “owns” the corresponding physical device, but the VMBus is configured to cross-connect each VSP to corresponding VSCs on other partitions.