Hypervisor Software. Virtualization is a technique where an abstraction of the physical hardware is created in order to run applications and operating systems while hiding the details of the hardware used. The software that manages this abstraction is often called a Hypervisor or a Virtual Machine Monitor (VMM), and the abstractions created are called Virtual Machines (VM). Using a Hypervisor, one can run multiple operating systems on the same machine, at the same time. Each operating system is run under its own Virtual Machine and accesses physical hardware which is abstracted with the help of the Hypervisor. It is common to classify virtual machines as Native Virtual Machines (Type I) and Hosted Virtual Machines (Type II). In the former case the Hypervisor is run directly on the hardware and can load different virtual machines side by side. In the latter case however, the Hypervisor is run as an application under an existing operating system, which is called Host. Virtual machines are run alongside the regular processes of the host and are called guests. Not all Hypervisors are alike, since there is more than one way to do virtualization. For example a Virtual Machine may be designed to run software not intended for the hardware architecture used, as is the case with various emulators, or with software such as the Java Virtual Machine. However, we are mostly interested in Virtual Machines that can run the same software as the hardware architecture, unchanged or with minimal changes. Hypervisors that can run complete operating systems intended for the underlying hardware, with no changes to the code, are said to implement Full Virtualization; the software running under the VM is under the illusion it runs on real hardware. At the same time, there are Hypervisors that require the cooperation of the Operating System running under the VM; in this case the operating system needs to be patched to run under a virtual architecture slightly different than the real hardware. This kind of virtualization is called paravirtualization. One of the most significant barriers to efficiently virtualize a given architecture is the presence of instructions that are sensitive to the current mode of operation of the processor. Typically an operating system running under a VM executes in a lower privilege mode than what it was designed for, and attempts to use instructions that control the state of the hardware. If these do not cause a trap to the Hypervisor and fail silently, or just behave differ...
Hypervisor Software. It is common to classify virtual machines as Native Virtual Machines (Type I) and Hosted Virtual Machines (Type II). In the former case the Hypervisor is run directly on the hardware and can load different virtual machines side to side. In the latter case however, the Hypervisor is run as an application under an existing operating system, which is called Host. Virtual machines are run alongside the regular processes of the host and are called guests. Not all Hypervisors are alike, since there is more than one way to do virtualization. For example a Virtual Machine may be designed to run software not intended for the hardware architecture used, as is the case with various emulators or with the Java Virtual Machine. However we are mostly interested in Virtual Machines that can run the same software as the hardware architecture, unchanged or with minimal changes. Hypervisors that can run complete operating systems intended for the underlying hardware, with no changes to the code, are said to implement Full Virtualization; the software running under the VM is under the illusion it runs under real hardware. At the same time, there are Hypervisors that require the cooperation of the Operating System running under the VM; in this case the operating system needs to be patched to run under a virtual architecture slightly different than the real hardware. This kind of virtualization is called Paravirtualization. One of the most significant barriers to efficiently virtualizing an architecture is the presence of instructions that are sensitive to the current mode of operation of the processor. Typically an operating system running under a VM executes in a lower privilege mode than what it is designed for, and attempts to use instructions that control the state of the hardware. If these do not cause a trap to the Hypervisor, but instead fail silently, or just behave differently in the lower privilege mode, then the Hypervisor would have to implement complicated binary patching techniques to intercept those instructions. Other challenges to efficiently virtualizing an architecture include the way memory management and virtual memory is implemented, which mean that often Hypervisors have to maintain Shadow Page Tables incurring additional overheads. In order to overcome these performance overheads, one solution is to implement paravirtualization instead of full virtualization. However running unmodified guest operating systems is desirable, so hardware vendors have start...
