Virtual Machine

Published on January 2017 | Categories: Documents | Downloads: 40 | Comments: 0 | Views: 452
of 4
Download PDF   Embed   Report

Comments

Content

VIRTUALIZATION Virtualization has become an important tool in computer system design, and virtual machines are used in a number of sub disciplines ranging from operating systems to programming languages to processor architectures. By freeing developers and users from traditional interface and resource constraints, VMs enhance software interoperability, system impregnability, and platform versatility. Because VMs are the product of diverse groups with different goals, however, there has been relatively little unification of VM concepts. Consequently, it is useful to take a step back, consider the variety of VM architectures, and describe them in a unified way, putting both the notion of virtualization and the types of VMs in perspective. ABSTRACTION AND VIRTUALIZATION Despite their incredible complexity, computer systems exist and continue to evolve because they are designed as hierarchies with well defined interfaces that separate levels of abstraction. Using well defined interfaces facilitates independent subsystem development by both hardware and software design teams. The simplifying abstractions hide lower-level implementation details, thereby reducing the complexity of the design process. Figure 1a shows an example of abstraction applied to disk storage. The operating system abstracts hard-disk addressing details³for example, that it is comprised of sectors and tracks³so that the disk appears to application software as a set of variable-sized

files. Application programmers can then create, write, and read files without knowing the hard disk·s construction and physical organization. A computer·s instruction set architecture (ISA) clearly exemplifies the advantages of well-defined interfaces. Well-defined interfaces permit development of interacting computer subsystems not only in different organizations but also at different times, sometimes years apart. For example, Intel and AMD designers develop microprocessors that implement the Intel IA-32 (x86) instruction set, while Microsoft developers write software that is compiled to the same instruction set. Because both groups satisfy the ISA specification, the software can be expected to execute correctly on any PC built with an IA-32 microprocessor. Unfortunately, well-defined interfaces also have their limitations. Subsystems and components designed to specifications for one interface will not work with those designed for another. For example, application programs, when distributed as compiled binaries, are tied to a specific ISA and depend on a specific operating system interface. This lack of interoperability can be confining, especially in a world of networked computers where it is advantageous to move software as freely as data. Virtualization provides a way of getting around such constraints. Virtualizing a system or component³ such as a processor, memory, or an I/O device³at a given abstraction level maps its interface and visible resources onto the interface and resources of an underlying, possibly different, real system. Consequently, the real system appears as a different virtual system or even as multiple virtual systems. Unlike abstraction, virtualization does not necessarily aim to simplify or hide details. For example, in Figure 1b, virtualization transforms a single large disk into two smaller virtual disks, each of which appears to have its own tracks and sectors. Virtualizing software uses the file abstraction as an intermediate step to provide a mapping between the virtual and real disks. A write to a virtual disk is converted to a file write (and therefore to a real disk write). Note that the level of detail provided at the virtual disk interface³the

sector/track addressing³ is no different from that for a real disk; no abstraction takes place.

VIRTUAL MACHINES The concept of virtualization can be applied not only to subsystems such as disks but to an entire machine. To implement a virtual machine, developers add a software layer to a real machine to support the desired architecture. By doing so, a VM can circumvent real machine compatibility and hardware resource constraints. VM Definitions A virtual machine (VM) is a software implementation of a machine (i.e. a computer) that executes programs like a physical machine. Virtual machines are separated into two major categories, based on their use and degree of correspondence to any real machine. A system virtual machine provides a complete system platform which supports the execution of a complete operating system (OS). In contrast, a process virtual machine is designed to run a single program, which means that it supports a single process. An essential characteristic of a virtual machine is that the software running inside is limited to the resources and abstractions provided by the virtual machine³it cannot break out of its virtual world. Architected interfaces A discussion of VMs is also a discussion about computer architecture in the pure sense of the term. Because VM implementations lie at architected interfaces, a major consideration in the construction of a VM is the fidelity with which it implements these interfaces. Architecture, as applied to computer systems, refers to a formal specification of an interface in the system, including the logical

behavior of resources managed via the interface. Implementation describes the actual embodiment of an architecture. Abstraction levels correspond to implementation layers, whether in hardware or software, each associated with its own interface or architecture. Figure 2 shows some important interfaces and implementation layers in a typical computer system. Three of these interfaces at or near the HW/SW boundary³the instruction set architecture, the application binary interface, and the application programming interface³are especially important for VM construction.

Instruction set architecture The ISA marks the division between hardware and software, and consists of interfaces 3 and 4 in Figure 2. Interface 4 represents the user ISA and includes those aspects visible to an application program. Interface 3, the system ISA, is a superset of the user ISA and includes those aspects visible only to operating system software responsible for managing hardware resources.

Sponsor Documents

Or use your account on DocShare.tips

Hide

Forgot your password?

Or register your new account on DocShare.tips

Hide

Lost your password? Please enter your email address. You will receive a link to create a new password.

Back to log-in

Close