virtual machine (VM)
Virtual Machine
Virtualization:
The term virtualization has many meanings, and aspects of virtualization permeate all aspects of computing. Virtual machines are one instance of this trend. Generally, with a virtual machine, guest operating systems and applications run in an environment that appears to them to be native hardware and that behaves toward them as native hardware would but that also protects, manages, and limits them.
Overview:
The fundamental idea behind a virtual machine is to abstract the hardware of a single computer (the CPU, memory, disk drives, network interface cards, and so forth) into several different execution environments, thereby creating the illusion that each separate environment is running on its own private computer. In the case of virtualization, there is a layer that creates a virtual system on which operating systems or applications can run.
Virtual machine implementations involve several components. At the base is the host, the underlying hardware system that runs the virtual machines. The virtual machine manager(VMM) (also known as a hypervisor) creates and runs virtual machines by providing an interface that is identical to the host (except in the case of paravirtualization, discussed later). Each guest process is provided with a virtual copy of the host (Figure 16.1). Usually, the guest process is in fact an operating system. A single physical machine can thus run multiple operating systems concurrently, each in its own virtual machine.
The implementation of VMMs varies greatly. Options include the following:
1.Hardware-based solutions that provide support for virtual machine creation and management via firmware. These VMMs, which are commonly found in mainframe and large to midsized servers, are generally known as type 0 hypervisors. IBM LPARs and Oracle LDOMs are examples.
2.Operating-system-like software built to provide virtualization, including VMware ESX(mentioned above), Joyent SmartOS, and Citrix XenServer. These VMMs are known as type 1 hypervisors.
3.General-purpose operating systems that provide standard functions as well as VMM functions, including Microsoft Windows Server with HyperV and RedHat Linux with the KVM feature. Because such systems have a feature set similar to type 1 hypervisors, they are also known as type 1.
4.Applications that run on standard operating systems but provide VMM features to guest operating systems. These applications, which include VMware Workstation and Fusion, Parallels Desktop, and Oracle Virtual[1]Box, are type 2 hypervisors.
5.Paravirtualization, a technique in which the guest operating system is modified to work in cooperation with the VMM to optimize performance.
System models. (a) Nonvirtual machine. (b) Virtual machine.
6.Programming-environment virtualization, in which VMMs do not virtu[1]alize real hardware but instead create an optimized virtual system. This technique is used by Oracle Java and Microsoft.Net.
7.Emulators that allow applications written for one hardware environment to run on a very different hardware environment, such as a different type of CPU.
8.Application containment, which is not virtualization at all but rather provides virtualization-like features by segregating applications from the operating system. Oracle Solaris Zones, BSD Jails, and IBM AIX WPARs “contain” applications, making them more secure and manageable.
The variety of virtualization techniques in use today is a testament to the breadth, depth, and importance of virtualization in modern computing. Virtualization is invaluable for data-centre operations, efficient application development, and software testing, among many other uses.
The virtualization requirements stated that:
1. A VMM provides an environment for programs that is essentially identical to the original machine.
2. Programs running within that environment show only minor performance decreases.
3. The VMM is in complete control of system resources
Hardware Assistance :
Without some level of hardware support, virtualization would be impossible. The more hardware support available within a system, the more feature-rich and stable the virtual machines can be and the better they can perform. In the Intel x86 CPU family, Intel added new virtualization support in successive generations (the VT-x instructions) beginning in 2005. Now, binary translation is no longer needed. In fact, all major general-purpose CPUs are providing extended amounts of hardware support for virtualization.
For example, AMD virtualization technology (AMD-V) has appeared in several AMD processors starting in 2006. It defines two new modes of operation—host and guest—thus moving from a dual-mode to a multimode processor. The VMM can enable host mode, define the characteristics of each guest virtual machine, and then switch the system to guest mode, passing control of the system to a guest operating system that is running in the virtual machine. In guest mode, the virtualized operating system thinks it is running on native hardware and sees whatever devices are included in the host’s definition of the guest. If the guest tries to access a 16.5 Types of Virtual Machines and Their Implementations 721 virtualized resource, then control is passed to the VMM to manage that interaction.
Types of Virtual Machines and Their Implementations:
We’ve now looked at some of the techniques used to implement virtualization. Next, we consider the major types of virtual machines, their implementation, their functionality, and how they use the building blocks just described to create a virtual environment. Of course, the hardware on which the virtual machines are running can cause great variation in implementation methods. Here, we discuss the implementations in general, with the understanding that VMMs take advantage of hardware assistance where it is available.
Type 0 Hypervisor:
Type 0 hypervisors have existed for many years under many names, including “partitions” and “domains”. They are a hardware feature, and that brings its own positives and negatives. Operating systems need do nothing special to take advantage of their features. The VMM itself is encoded in the firmware and loaded at boot time. In turn, it loads the guest images to run in each partition. The feature set of a type 0 hypervisor tends to be smaller than those of the other types because it is implemented in hardware. For example, a system might be split into four virtual systems, each with dedicated CPUs, memory, and I/O devices. Each guest believes that it has dedicated hardware because it does, simplifying many implementation details.
Type 1 hypervisors are commonly found in company data centres and are in a sense becoming “the data-centre operating system.” They are special-purpose operating systems that run natively on the hardware, but rather than providing system calls and other interfaces for running programs, they create, run, and manage guest operating systems. In addition to running on standard hardware, they can run on type 0 hypervisors, but not on other type 1 hypervisors. Whatever the platform, guests generally do not know they are running on anything but the native hardware.
Type 2 Hypervisor:
Type 2 hypervisors are less interesting to us as operating-system explorers, because there is very little operating-system involvement in these application[1]level virtual machine managers. This type of VMM is simply another process run and managed by the host, and even the host does not know virtualization is happening within the VMM. Type 2 hypervisors have limits not associated with some of the other types. For example, a user needs administrative privileges to access many of the hardware assistance features of modern CPUs. If the VMM is being run by a standard user without additional privileges, the VMM cannot take advantage of these features. Due to this limitation, as well as the extra overhead of running a general-purpose operating system as well as guest operating systems, type 2 hypervisors tend to have poorer overall performance than type 0 or 1.
Paravirtualization:
As we’ve seen, paravirtualization takes a different tack than the other types of virtualization. Rather than try to trick a guest operating system into believing it has a system to itself, paravirtualization presents the guest with a system that is similar but not identical to the guest’s preferred system. The guest must be modified to run on the paravirtualized virtual hardware. The gain for this extra work is more efficient use of resources and a smaller virtualization layer. The Xen VMM, which is the leader in paravirtualization, has implemented several techniques to optimize the performance of guests as well as of the host system. For example, as we have seen, some VMMs present virtual devices to guests that appear to be real devices. Instead of taking that approach, the Xen VMM presents clean and simple device abstractions that allow efficient I/O, as well as good communication between the guest and the VMM about device I/O. For each device used by each guest, there is a circular buffer shared by the guest and the VMM via shared memory.
Virtualization and Protection Rings:
Examples:
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