Ahypervisor, also known as avirtual machine monitor (VMM) orvirtualizer, is a type of computersoftware,firmware orhardware that creates and runsvirtual machines. A computer on which a hypervisor runs one or more virtual machines is called ahost machine, and each virtual machine is called aguest machine. The hypervisor presents the guest operating systems with avirtual operating platform and manages the execution of the guest operating systems. Unlike anemulator, the guest executes most instructions on the native hardware.[1] Multiple instances of a variety of operating systems may share the virtualized hardware resources: for example,Linux,Windows, andmacOS instances can all run on a single physicalx86 machine. This contrasts withoperating-system–level virtualization, where all instances (usually calledcontainers) must share a single kernel, though the guest operating systems can differ inuser space, such as differentLinux distributions with the same kernel.
The termhypervisor is a variant ofsupervisor, a traditional term for thekernel of anoperating system: the hypervisor is the supervisor of the supervisors,[2] withhyper- used as a stronger variant ofsuper-.[a] The term dates to circa 1970;[3] IBM coined it for software that ranOS/360 and the 7090 emulator concurrently on the360/65[4] and later used it for the DIAG handler of CP-67. In the earlierCP/CMS (1967) system, the termControl Program was used instead.
Some literature, especially inmicrokernel contexts, makes a distinction betweenhypervisor andvirtual machine monitor (VMM). There, both components form the overallvirtualization stack of a certain system.Hypervisor refers tokernel-space functionality and VMM touser-space functionality. Specifically in these contexts, ahypervisor is a microkernel implementing virtualization infrastructure that must run in kernel-space for technical reasons, such asIntel VMX. Microkernels implementing virtualization mechanisms are also referred to asmicrohypervisor.[5][6] Applying this terminology toLinux,KVM is ahypervisor andQEMU orCloud Hypervisor are VMMs utilizing KVM as hypervisor.[7]
In his 1973 thesis, "Architectural Principles for Virtual Computer Systems,"Robert P. Goldberg classified two types of hypervisor:[1]
The distinction between these two types is not always clear. For instance,KVM andbhyve arekernel modules[9] that effectively convert the host operating system to a type-1 hypervisor.[10]
The first hypervisors providingfull virtualization were the test toolSIMMON and the one-offIBM CP-40 research system, which began production use in January 1967 and became the first version of the IBMCP/CMS operating system. CP-40 ran on aS/360-40 modified at theCambridge Scientific Center to supportdynamic address translation, a feature that enabled virtualization. Prior to this time, computer hardware had only been virtualized to the extent to allow multiple user applications to run concurrently, such as inCTSS andIBM M44/44X. With CP-40, the hardware'ssupervisor state was virtualized as well, allowing multiple operating systems to run concurrently in separatevirtual machine contexts.
Programmers soon implemented CP-40 (asCP-67) for theIBM System/360-67, the first production computer system capable of full virtualization. IBM shipped this machine in 1966; it includedpage-translation-table hardware for virtual memory and other techniques that allowed a full virtualization of all kernel tasks, including I/O and interrupt handling. (The "official" operating system, the ill-fatedTSS/360, did not employ full virtualization.) Both CP-40 and CP-67 began production use in 1967.CP/CMS was available to IBM customers from 1968 to early 1970s, in source code form without support.
CP/CMS formed part of IBM's attempt to build robusttime-sharing systems for itsmainframe computers. By running multiple operating systems concurrently, the hypervisor increased system robustness and stability: Even if one operating system crashed, the others would continue working without interruption. Indeed, this even allowedbeta or experimental versions of operating systems—or even of new hardware[11]—to be deployed and debugged, without jeopardizing the stable main production system, and without requiring costly additional development systems.
IBM announced itsSystem/370 series in 1970 without thevirtual memory feature needed for virtualization, but added it in the August 1972 Advanced Function announcement. Virtualization has been featured in all successor systems, such that all modern-day IBM mainframes, including thezSeries line, retain backward compatibility with the 1960s-era IBM S/360 line. The 1972 announcement also includedVM/370, a reimplementation ofCP/CMS for the S/370. UnlikeCP/CMS, IBM provided support for this version (though it was still distributed in source code form for several releases).VM stands forVirtual Machine, emphasizing that all, not just some, of the hardware interfaces are virtualized. Both VM and CP/CMS enjoyed early acceptance and rapid development by universities, corporate users, andtime-sharing vendors, as well as within IBM. Users played an active role in ongoing development, anticipating trends seen in modernopen source projects. However, in a series of disputed and bitter battles[citation needed], time-sharing lost out tobatch processing through IBM political infighting, and VM remained IBM's "other" mainframe operating system for decades, losing toMVS. It enjoyed a resurgence of popularity and support from 2000 as thez/VM product, for example as the platform forLinux on IBM Z.
As mentioned above, the VM control program includes ahypervisor-call handler that intercepts DIAG ("Diagnose", opcode x'83') instructions used within a virtual machine. This provides fast-path non-virtualized execution of file-system access and other operations (DIAG is a model-dependent privileged instruction, not used in normal programming, and thus is not virtualized. It is therefore available for use as a signal to the "host" operating system). When first implemented inCP/CMS release 3.1, this use of DIAG provided an operating system interface that was analogous to theSystem/360Supervisor Call instruction (SVC), but that did not require altering or extending the system's virtualization of SVC.
In 1985 IBM introduced thePR/SM hypervisor to managelogical partitions (LPAR).
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Several factors led to a resurgence around 2005 in the use ofvirtualization technology amongUnix,Linux, and otherUnix-like operating systems:[12]
Major Unix vendors, includingHP,IBM,SGI, andSun Microsystems, have been selling virtualized hardware since before 2000. These have generally been large, expensive systems (in the multimillion-dollar range at the high end), although virtualization has also been available on some low- and mid-range systems, such as IBMpSeries servers,HP Superdome series machines, andSun/Oracle T-series CoolThreads servers.
AlthoughSolaris has always been the only guest domain OS officially supported by Sun/Oracle on theirLogical Domains hypervisor, as of late 2006[update],Linux (Ubuntu and Gentoo), andFreeBSD have been ported to run on top of the hypervisor (and can all run simultaneously on the same processor, as fully virtualized independent guest OSes). Wind River "Carrier Grade Linux" also runs on Sun's Hypervisor.[13] Full virtualization onSPARC processors proved straightforward: since its inception in the mid-1980s Sun deliberately kept the SPARC architecture clean of artifacts that would have impeded virtualization. (Compare with virtualization on x86 processors below.)[14]
HPE providesHP Integrity Virtual Machines (Integrity VM) to host multiple operating systems on theirItanium powered Integrity systems. Itanium can runHP-UX, Linux, Windows andOpenVMS, and these environments are also supported as virtual servers on HP's Integrity VM platform. The HP-UX operating system hosts the Integrity VM hypervisor layer that allows for multiple features of HP-UX to be taken advantage of and provides major differentiation between this platform and other commodity platforms - such as processor hotswap, memory hotswap, and dynamic kernel updates without system reboot. While it heavily leverages HP-UX, the Integrity VM hypervisor is really a hybrid that runs on bare-metal while guests are executing. Running normal HP-UX applications on an Integrity VM host is heavily discouraged,[by whom?] because Integrity VM implements its own memory management, scheduling and I/O policies that are tuned for virtual machines and are not as effective for normal applications. HPE also provides more rigid partitioning of their Integrity and HP9000 systems by way of VPAR andnPar technology, the former offering shared resource partitioning and the latter offering complete I/O and processing isolation. The flexibility of virtual server environment (VSE) has given way to its use more frequently in newer deployments.[citation needed]
IBM provides virtualization partition technology known aslogical partitioning (LPAR) onSystem/390,zSeries,pSeries andIBM AS/400 systems. For IBM's Power Systems, the POWER Hypervisor (PHYP) is a native (bare-metal) hypervisor in firmware and provides isolation between LPARs. Processor capacity is provided to LPARs in either a dedicated fashion or on an entitlement basis where unused capacity is harvested and can be re-allocated to busy workloads. Groups of LPARs can have their processor capacity managed as if they were in a "pool" - IBM refers to this capability as Multiple Shared-Processor Pools (MSPPs) and implements it in servers with thePOWER6 processor. LPAR and MSPP capacity allocations can be dynamically changed. Memory is allocated to each LPAR (at LPAR initiation or dynamically) and is address-controlled by the POWER Hypervisor. For real-mode addressing by operating systems (AIX,Linux,IBM i), thePower processors (POWER4 onwards) have designed virtualization capabilities where a hardware address-offset is evaluated with the OS address-offset to arrive at the physical memory address. Input/Output (I/O) adapters can be exclusively "owned" by LPARs or shared by LPARs through an appliance partition known as the Virtual I/O Server (VIOS). The Power Hypervisor provides for high levels of reliability, availability and serviceability (RAS) by facilitating hot add/replace of multiple parts (model dependent: processors, memory, I/O adapters, blowers, power units, disks, system controllers, etc.)
Similar trends have occurred with x86/x86-64 server platforms, whereopen-source projects such asXen have led virtualization efforts. These include hypervisors built on Linux and Solaris kernels as well as custom kernels. Since these technologies span from large systems down to desktops, they are described in the next section.
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X86 virtualization was introduced in the 1990s, with its emulation being included inBochs.[15] Intel and AMD released their first x86 processors with hardware virtualisation in 2005 withIntel VT-x (code-named Vanderpool) andAMD-V (code-named Pacifica).
An alternative approach requires modifying the guest operating system to make asystem call to the underlying hypervisor, rather than executing machine I/O instructions that the hypervisor simulates. This is calledparavirtualization inXen, a "hypercall" inParallels Workstation, and a "DIAGNOSE code" in IBMVM. Some microkernels, such asMach andL4, are flexible enough to allow paravirtualization of guest operating systems.
Embedded hypervisors, targetingembedded systems and certainreal-time operating system (RTOS) environments, are designed with different requirements when compared to desktop and enterprise systems, including robustness, security andreal-time capabilities. The resource-constrained nature of multiple embedded systems, especially battery-powered mobile systems, imposes a further requirement for small memory-size and low overhead. Finally, in contrast to the ubiquity of the x86 architecture in the PC world, the embedded world uses a wider variety of architectures and less standardized environments. Support for virtualization requiresmemory protection (in the form of amemory management unit or at least a memory protection unit) and a distinction betweenuser mode andprivileged mode, which rules out mostmicrocontrollers. This still leavesx86,MIPS,ARM andPowerPC as widely deployed architectures on medium- to high-end embedded systems.[16]
As manufacturers of embedded systems usually have the source code to their operating systems, they have less need for full virtualization in this space. Instead, the performance advantages ofparavirtualization make this usually the virtualization technology of choice. Nevertheless, ARM and MIPS have recently added full virtualization support as an IP option and has included it in their latest high-end processors and architecture versions, such asARM Cortex-A15 MPCore and ARMv8 EL2.
Other differences between virtualization in server/desktop and embedded environments include requirements for efficient sharing of resources across virtual machines, high-bandwidth, low-latency inter-VM communication, a global view of scheduling and power management, and fine-grained control of information flows.[17]
The use of hypervisor technology bymalware androotkits installing themselves as a hypervisor below the operating system, known ashyperjacking, can make them more difficult to detect because the malware could intercept any operations of the operating system (such as someone entering a password) without the anti-malware software necessarily detecting it (since the malware runs below the entire operating system). Implementation of the concept has allegedly occurred in theSubVirt laboratory rootkit (developed jointly byMicrosoft andUniversity of Michigan researchers[18]) as well as in theBlue Pill malware package. However, such assertions have been disputed by others who claim that it would be possible to detect the presence of a hypervisor-based rootkit.[19]
In 2009, researchers from Microsoft andNorth Carolina State University demonstrated a hypervisor-layer anti-rootkit calledHooksafe that can provide generic protection against kernel-moderootkits.[20]
