The Anatomy of a Virtual Machine
Consider the fundamental objectives of a virtualization environment. One objective is
to enable multiple operating system instances to execute simultaneously on a single
physical system, which enables the many benefits covered in Chapter 1. Another
objective is to divide up and share the resources available on the physical host to
many virtual machines, which act as their own, self-contained systems that are
completely isolated from the other virtualized systems running on the host. Each
virtual machine believes it is the sole user of the hardware on which it is running.
Within each virtual machine, an operating system is installed, and into that operating
system, applications are installed and configurations implemented to enable services
to the organization.
Operating systems are written to run on hardware and expect certain components to
be present that can be interacted with, such as the computer’s BIOS, storage
controller, input/output systems, and network device. Drivers are included in the
operating system to see certain types of devices, such as network and storage
controllers, to enable installation and startup of the operating system. It’s also
possible to add drivers for hardware that does not have drivers included as part of the
operating system. This fundamental presence of hardware components does not apply
to a virtual machine. The entire environment of a virtual machine is synthetic, with
abstracted resources allotted to the virtual machine and many resources utilizing the
VMBus, as previously explained. However, the key aspects of a computer must be
present for an operating system to install and function. Note that when using SR-IOV
networking configurations or Discrete Device Assignment (DDA) in Windows Server
2016, physical hardware is mapped directly to a VM. These are specialized
configurations, however, and the core resources of the VM, such as chipset, processor,
and memory, are still virtualized.
Generation 1 Virtual Machine
All of the synthetic resources and devices that are exposed by Hyper-V provide the
highest level of performance and functionality, but if an operating system cannot
natively use them, that operating system cannot be installed or started on that
synthetic hardware. Even today with Hyper-V, many Windows 2000, Windows 2003,
and Windows XP virtual machines are running virtualized, and these operating
systems are not virtualization aware. The use of the VMBus architecture within a
guest operating system requires deep integration with other operating system
components; it’s not as simple as installing an additional storage or network driver
during installation.
It is therefore often required to provide certain types of hardware as emulated, which
means Hyper-V components provide to virtual machines what appear to be standard
types of hardware, such as an Intel 82371AB/EB IDE controller, an Intel 21140