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In
computing Computing is any goal-oriented activity requiring, benefiting from, or creating computing machinery. It includes the study and experimentation of algorithmic processes, and development of both hardware and software. Computing has scientific, ...
, hardware-assisted virtualization is a
platform virtualization Hardware virtualization is the virtualization of computers as complete hardware platforms, certain logical abstractions of their componentry, or only the functionality required to run various operating systems. Virtualization hides the physic ...
approach that enables efficient
full virtualization In computer science, virtualization is a modern technique developed in late 1990s and is different from simulation and emulation. Virtualization employs techniques used to create instances of an environment, as opposed to simulation, which mode ...
using help from hardware capabilities, primarily from the host processors. A full virtualization is used to emulate a complete hardware environment, or
virtual machine In computing, a virtual machine (VM) is the virtualization/ emulation of a computer system. Virtual machines are based on computer architectures and provide functionality of a physical computer. Their implementations may involve specialized har ...
, in which an unmodified guest
operating system An operating system (OS) is system software that manages computer hardware, software resources, and provides common daemon (computing), services for computer programs. Time-sharing operating systems scheduler (computing), schedule tasks for ef ...
(using the same
instruction set In computer science, an instruction set architecture (ISA), also called computer architecture, is an abstract model of a computer. A device that executes instructions described by that ISA, such as a central processing unit (CPU), is called a ...
as the host machine) effectively executes in complete isolation. Hardware-assisted virtualization was added to x86 processors (
Intel VT-x x86 virtualization is the use of hardware-assisted virtualization capabilities on an x86/x86-64 CPU. In the late 1990s x86 virtualization was achieved by complex software techniques, necessary to compensate for the processor's lack of hardware-a ...
,
AMD-V x86 virtualization is the use of hardware-assisted virtualization capabilities on an x86/x86-64 CPU. In the late 1990s x86 virtualization was achieved by complex software techniques, necessary to compensate for the processor's lack of hardware-a ...
or VIA VT) in 2005, 2006 and 2010 (respectively). Hardware-assisted virtualization is also known as accelerated virtualization; Xen calls it hardware virtual machine (HVM), and Virtual Iron calls it native virtualization.


History

Hardware-assisted virtualization first appeared on the IBM System/370 in 1972, for use with VM/370, the first virtual machine operating system. With the increasing demand for high-definition computer graphics (e.g. CAD), virtualization of mainframes lost some attention in the late 1970s, when the upcoming
minicomputer A minicomputer, or colloquially mini, is a class of smaller general purpose computers that developed in the mid-1960s and sold at a much lower price than mainframe and mid-size computers from IBM and its direct competitors. In a 1970 survey, ...
s fostered resource allocation through
distributed computing A distributed system is a system whose components are located on different networked computers, which communicate and coordinate their actions by passing messages to one another from any system. Distributed computing is a field of computer sci ...
, encompassing the commoditization of
microcomputer A microcomputer is a small, relatively inexpensive computer having a central processing unit (CPU) made out of a microprocessor. The computer also includes memory and input/output (I/O) circuitry together mounted on a printed circuit board (P ...
s. IBM offers hardware virtualization for its IBM Power Systems hardware for AIX,
Linux Linux ( or ) is a family of open-source Unix-like operating systems based on the Linux kernel, an operating system kernel first released on September 17, 1991, by Linus Torvalds. Linux is typically packaged as a Linux distribution, which i ...
and IBM i, and for its IBM- Mainframes System z. IBM refers to its specific form of hardware virtualization as "logical partition", or more commonly as LPAR. The increase in compute capacity per x86 server (and in particular the substantial increase in modern networks' bandwidths) rekindled interest in data-center based computing which is based on virtualization techniques. The primary driver was the potential for server consolidation: virtualization allowed a single server to cost-efficiently consolidate compute power on multiple underutilized dedicated servers. The most visible hallmark of a return to the roots of computing is
cloud computing Cloud computing is the on-demand availability of computer system resources, especially data storage ( cloud storage) and computing power, without direct active management by the user. Large clouds often have functions distributed over m ...
, which is a synonym for data center based computing (or mainframe-like computing) through high bandwidth networks. It is closely connected to virtualization. The initial implementation x86 architecture did not meet the Popek and Goldberg virtualization requirements to achieve "classical virtualization": * equivalence: a program running under the virtual machine monitor (VMM) should exhibit a behavior essentially identical to that demonstrated when running on an equivalent machine directly * resource control (also called safety): the VMM must be in complete control of the virtualized resources * efficiency: a statistically dominant fraction of machine instructions must be executed without VMM intervention This made it difficult to implement a virtual machine monitor for this type of processor. Specific limitations included the inability to
trap A trap is a mechanical device used to capture or restrain an animal for purposes such as hunting, pest control, or ecological research. Trap or TRAP may also refer to: Art and entertainment Films and television * ''Trap'' (2015 film), Fil ...
on some privileged instructions. To compensate for these architectural limitations, designers have accomplished virtualization of the x86 architecture through two methods: full virtualization or paravirtualization. Both create the illusion of physical hardware to achieve the goal of operating system independence from the hardware but present some trade-offs in performance and complexity. # Full virtualization was implemented in first-generation x86 VMMs. It relies on binary translation to trap and virtualize the execution of certain sensitive, non-virtualizable instructions. With this approach, critical instructions are discovered (statically or dynamically at run-time) and replaced with traps into the VMM to be emulated in software. Binary translation can incur a large performance overhead in comparison to a virtual machine running on natively virtualized architectures such as the IBM System/370. VirtualBox, VMware Workstation (for 32-bit guests only), and Microsoft Virtual PC, are well-known commercial implementations of full virtualization. # Paravirtualization is a technique in which the hypervisor provides an API and the OS of the guest virtual machine calls that API, requiring OS modifications. In 2005 and 2006,
Intel Intel Corporation is an American multinational corporation and technology company headquartered in Santa Clara, California, Santa Clara, California. It is the world's largest semiconductor chip manufacturer by revenue, and is one of the devel ...
and AMD (working independently) created new processor extensions to the
x86 architecture x86 (also known as 80x86 or the 8086 family) is a family of complex instruction set computer (CISC) instruction set architectures initially developed by Intel based on the Intel 8086 microprocessor and its 8088 variant. The 8086 was ...
called Intel VT-x and AMD-V, respectively. On the
Itanium Itanium ( ) is a discontinued family of 64-bit Intel microprocessors that implement the Intel Itanium architecture (formerly called IA-64). Launched in June 2001, Intel marketed the processors for enterprise servers and high-performance comp ...
architecture, hardware-assisted virtualization is known as VT-i. The first generation of x86 processors to support these extensions were released in late 2005 early 2006: *On November 13, 2005, Intel released two models of Pentium 4 (Model 662 and 672) as the first Intel processors to support VT-x. *On May 23, 2006, AMD released the Athlon 64 ("Orleans"), the Athlon 64 X2 ("Windsor") and the Athlon 64 FX ("Windsor") as the first AMD processors to support this technology. Well-known implementations of hardware-assisted x86 virtualization include VMware Workstation (for 64-bit guests only), XenCenter, Xen 3.x (including derivatives like Virtual Iron),
Linux KVM Kernel-based Virtual Machine (KVM) is a virtualization module in the Linux kernel that allows the Kernel (operating system), kernel to function as a hypervisor. It was merged into the Mainline Linux, mainline Linux kernel in version 2.6.20, whi ...
and
Microsoft Microsoft Corporation is an American multinational corporation, multinational technology company, technology corporation producing Software, computer software, consumer electronics, personal computers, and related services headquartered at th ...
Hyper-V.


Advantages

Hardware-assisted virtualization reduces the maintenance overhead of paravirtualization as it reduces (ideally, eliminates) the changes needed in the guest operating system. It is also considerably easier to obtain better performance. A practical benefit of hardware-assisted virtualization has been cited by VMware engineers and Virtual Iron.


Disadvantages

Hardware-assisted virtualization requires explicit support in the host CPU, which is available on not all x86/x86_64 processors. A "pure" hardware-assisted virtualization approach, using entirely unmodified guest operating systems, involves many VM traps, and thus high CPU overheads, limiting scalability and the efficiency of server consolidation. This performance hit can be mitigated by the use of paravirtualized drivers; the combination has been called "hybrid virtualization". In 2006 first-generation 32- and 64-bit x86 hardware support was found rarely to offer performance advantages over software virtualization.A Comparison of Software and Hardware Techniques for x86 Virtualization, Keith Adams and Ole Agesen, VMware, ASPLOS’06 October 21–25, 2006, San Jose, California, USA
Surprisingly, we find that the first-generation hardware support rarely offers performance advantages over existing software techniques. We ascribe this situation to high VMM/guest transition costs and a rigid programming model that leaves little room for software flexibility in managing either the frequency or cost of these transitions.


See also

* Intel VT-d * Further refinements of hardware-assisted virtualization are possible using an IOMMU; this allows native-speed access to dedicated hardware from a guest operating system, including
DMA DMA may refer to: Arts * ''DMA'' (magazine), a defunct dance music magazine * Dallas Museum of Art, an art museum in Texas, US * Danish Music Awards, an award show held in Denmark * BT Digital Music Awards, an annual event in the UK * Doctor of M ...
-capable hardware * Second Level Address Translation (SLAT), implementations including Rapid Virtualization Indexing or Extended Page Tables * Other virtualization techniques include operating system-level virtualization, as practiced by Parallels Virtuozzo Containers, and application virtualization. * Nanokernel * Hardware emulation * Emulator *
JTAG JTAG (named after the Joint Test Action Group which codified it) is an industry standard for verifying designs and testing printed circuit boards after manufacture. JTAG implements standards for on-chip instrumentation in electronic design autom ...
* Background Debug Mode interface * In-circuit emulator


References


Further reading

* * * * * * {{DEFAULTSORT:Hardware-Assisted Virtualization Hardware virtualization