Hardware virtualization techniques
Virtualization of the underlying raw hardware (native execution)
This approach is described as full virtualization of the hardware, and can be implemented using a type 1 or type 2 hypervisor: a type 1 hypervisor runs directly on the hardware, and a type 2 hypervisor runs on another operating system, such as Linux or Windows. Each virtual machine can run any operating system supported by the underlying hardware. Users can thus run two or more different "guest" operating systems simultaneously, in separate "private" virtual computers.The pioneer system using this concept was IBM's CP-40, the first (1967) version of IBM's CP/CMS (1967–1972) and the precursor to IBM's LPAR VM family (1972–present). With the VM architecture, most users run a relatively simple interactive computing single-user operating system, CMS, as a "guest" on top of the VM control program (VM-CP). This approach kept the CMS design simple, as if it were running alone; the control program quietly provides multitasking and resource management services "behind the scenes". In addition to CMS communication and other system tasks are performed by multitasking VMs (RSCS, GCS, TCP/IP, UNIX), and users can run any of the other IBM operating systems, such as MVS, even a new CP itself or now z/OS. Even the simple CMS could be run in a threaded environment (LISTSERV, TRICKLE). z/VM is the current version of VM, and is used to support hundreds or thousands of virtual machines on a given mainframe. Some installations use Linux on z Systems to run Web servers, where Linux runs as the operating system within many virtual machines.
Full virtualization is particularly helpful in operating system development, when experimental new code can be run at the same time as older, more stable, versions, each in a separate virtual machine. The process can even be recursive: IBM debugged new versions of its virtual machine operating system, VM, in a virtual machine running under an older version of VM, and even used this technique to simulate new hardware.[11]
The standard x86 processor architecture as used in the modern PCs does not actually meet the Popek and Goldberg virtualization requirements. Notably, there is no execution mode where all sensitive machine instructions always trap, which would allow per-instruction virtualization.
Despite these limitations, several software packages have managed to provide virtualization on the x86 architecture, even though dynamic recompilation of privileged code, as first implemented by VMware, incurs some performance overhead as compared to a VM running on a natively virtualizable architecture such as the IBM System/370 or Motorola MC68020. By now, several other software packages such as Virtual PC, VirtualBox, Parallels Workstation and Virtual Iron manage to implement virtualization on x86 hardware.
Intel and AMD have introduced features to their x86 processors to enable virtualization in hardware.
As well as virtualization of the resources of a single machine, multiple independent nodes in a cluster can be combined and accessed as a single virtual NUMA machine.[12]
Emulation of a non-native system
Virtual machines can also perform the role of an emulator, allowing software applications and operating systems written for another computer processor architecture to be run. Emulation may be entirely in software, or may also include a hardware component that may also involve use of microcode.
Operating-system-level virtualization
Operating-system-level virtualization is a server virtualization technology which virtualizes servers on an operating system (kernel) layer. It can be thought of as partitioning: a single physical server is sliced into multiple small partitions (otherwise called virtual environments (VE), virtual private servers (VPS), guests, zones, etc.); each such partition looks and feels like a real server, from the point of view of its users.
For example, Solaris Zones supports multiple guest operating systems running under the same operating system such as Solaris 10.[13] Guest operating systems can use the same kernel level with the same operating system version, or can be a separate copy of the operating system with a different kernel version using Solaris Kernel Zones.[14] Solaris native Zones also requires that the host operating system is a version of Solaris; other operating systems from other manufacturers are not supported.[citation needed] However, Solaris Branded Zones would need to used to have other operating systems as zones.[citation needed]
Another example is System Workload Partitions (WPARs), introduced in version 6.1 of the IBM AIX operating system. System WPARs are software partitions running under one instance of the global AIX OS environment.
The operating system level architecture has low overhead that helps to maximize efficient use of server resources. The virtualization introduces only a negligible overhead and allows running hundreds of virtual private servers on a single physical server. In contrast, approaches such as full virtualization (like VMware) and paravirtualization (like Xen or UML) cannot achieve such level of density, due to overhead of running multiple kernels. From the other side, operating system-level virtualization does not allow running different operating systems (i.e. different kernels), although different libraries, distributions, etc. are possible.
Abstract virtual machine techniques
Virtual machines that execute code for an abstract machine, which is given by a detailed specification rather than an existing physical device, are traditionally known as p-code machines.
These virtual machines can be used either as abstract platforms for an intermediate language used as the intermediate representation of a program by a compiler, or be executed directly by an interpreter implementing the virtual machine. The use of an intermediate language improves compiler portability, dividing it into a front end, which compiles to the intermediate language, and a back end, which compiles the intermediate language to machine code for the underlying physical machine. Interpreting the source code or bytecode provides the same portability benefit as only the virtual machine software itself must be written separately for each type of computer on which it runs, provides additional benefits such as avoiding needing to compile code before execution (ahead-of-time (AOT) compilation), and allows more complex behavior at runtime such as supporting many features of dynamic programming languages (reflections, for example) and runtime optimization (adaptive optimization by dynamic recompilation).
Additionally, a single intermediate language can be targeted by many high-level languages, such as Java, Scala and Clojure all translating to the JVM bytecode, and many languages (such as C#, F#, Visual Basic.NET, etc.) translating to the .NET Framework bytecode.
Notable examples include the following:
One of the first was the p-code machine specification, which allowed programmers to write Pascal programs that would run on any computer running virtual machine software that correctly implemented the specification.
The specification of the Java virtual machine
The Common Language Infrastructure virtual machine at the heart of the Microsoft's .NET initiative.
Open Firmware allows plug-in hardware to include boot-time diagnostics, configuration code, and device drivers that can run on any kind of a CPU.
Implementation techniques for virtual machines include just-in-time (JIT) compilation and dynamic recompilation of hot spots (trace-based JIT), notably in the HotSpot virtual machine.
Virtual machine design must trade off closeness to the source language (simplifying the front end) and closeness to the machine language (simplifying the back end), and its common relation to multiple source or target languages. An important distinction is between stack-based and register-based virtual machines: stack-based machines are closer to the source language since most programming languages use a recursion that maps naturally to a stack, while register-based virtual machines are closer to the target machine language since most processors use registers internally (register machines, not stack machines). Earlier virtual machines typically were stack-based, including Pascal p-code (1970) through the Java virtual machine (1995), but newer virtual machines are generally register-based, which simplifies memory-to-memory mapping of the interpreter, avoiding excessive copying of values and reducing the total number of instructions per function. The register-based design dates to the Dis virtual machine in 1995, and has since been widely used in the Lua virtual machine (since the Lua 5 in 2003), Perl 6's Parrot, and Android's Dalvik.
Virtualization-enabled hardware
Alcatel-Lucent 3B20D/3B21D emulated on commercial off-the-shelf computers with 3B2OE or 3B21E system
AMD-V (formerly code-named Pacifica)
ARM TrustZone
Boston Circuits gCore (grid-on-chip) with 16 ARC 750D cores and Time-machine hardware virtualization module.
Freescale PowerPC MPC8572 and MPC8641D
IBM System/370, System/390, and zSeries mainframes
IBM Power Systems
Intel VT-x (formerly code-named Vanderpool)
See also: x86 virtualization § Hardware support
HP vPAR and cell based nPAR
GE Project MAC then
Honeywell Multics systems
Honeywell 200/2000 systems Liberator replacing IBM 14xx systems, Level 62/64/66 GCOS
IBM System/360 Model 145 Hardware emulator for Honeywell 200/2000 systems
RCA Spectra/70 Series emulated IBM System/360
NAS CPUs emulated IBM and Amdahl machines
Honeywell Level 6 minicomputers emulated predecessor 316/516/716 minis
Oracle Corporation (previously Sun Microsystems) SPARC sun4v (SPARC M6, T5, T4, T3, UltraSPARC T1 and T2) – utilized by Oracle VM Server for SPARC, also known as "Logical Domains"
Xerox Sigma 6 CPUs were modified to emulate GE/Honeywell 600/6000 systems[citation needed]
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