The term hypervisor apparently originated in IBM's CP-370 reimplementation of CP-67 for the System/370, released in 1972 as VM/370. The term hypervisor call, or hypercall, referred to the paravirtualization interface, by which a "guest" operating system could access services directly from the (higher-level) control program – analogous to making a "supervisor call" to the (same level) operating system. The term "supervisor" refers to the operating system kernel, which on IBM mainframes runs in supervisor state.
CP/CMS was part of IBM's attempt to build robust time-sharing systems for its mainframe computers. By running multiple operating systems simultaneously, the hypervisor increased system robustness and stability: Even if one operating system crashed, the others would continue working without interruption. Indeed, this even allowed beta or experimental versions of operating systems – or even of new hardware – to be deployed and debugged, without jeopardizing the stable main production system, and without requiring costly additional development systems.
IBM's System/370 series was announced in 1970 without any virtualization features, but these were added to the series in 1972, and have appeared in all successor systems. (All modern-day IBM mainframes, such as the zSeries line, continue to be backwards-compatible with the 1960s-era IBM S/360 line.) The 1972 announcement also included VM/370, a reimplementation of CP/CMS for the S/370. Unlike CP/CMS, IBM provided support for this version (though it was still distributed in source code form for several releases). VM stands for Virtual Machine, emphasizing that all, and not just some, of the hardware interfaces are virtualized. Both VM and CP/CMS enjoyed early acceptance and rapid development by universities, corporate users, and time-sharing vendors, as well as within IBM. Users played an active role in ongoing development, anticipating trends seen in modern open source projects. However, in a series of disputed and bitter battles, time-sharing lost out to batch processing through IBM political infighting, and VM remained IBM's "other" mainframe operating system for decades, losing over MVS. It has enjoyed a resurgence of popularity and support in recent years as the current z/VM product, e.g. as the platform for Linux for zSeries.
As mentioned above, the VM control program includes a hypervisor call handler which intercepts DIAG ("Diagnose") 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 in CP/CMS release 3.1, this use of DIAG provided an operating system interface that was analogous to the System/360 SVC ("supervisor call") instruction, but that did not require altering or extending the system's virtualization of SVC.
The major UNIX vendors, including Sun Microsystems, HP, IBM, and SGI, have been selling virtualized hardware since before 2000. These have generally been large systems with hefty, server-class price tags (in the multi-million dollar range at the high end), although virtualization is also available on some mid-range systems, such as IBM's System-P servers, Sun's CoolThreads T1000, T2000 and T5x00 servers and HP 9000 Superdome series.
Multiple host operating systems have been modified to run as guest OSes on Sun's Logical Domains Hypervisor. As of late 2006, Solaris, Linux (Ubuntu and Gentoo), and FreeBSD have been ported to run on top of Hypervisor (and can all run simultaneously on the same processor, as fully-virtualized independent guest OSes). Wind River "Carrier Grade Linux" also plans to run on Sun's Hypervisor. Full virtualization on SPARC processors was not difficult because the SPARC architecture, since its inception in the mid-1980s, was deliberately kept clean of artifacts that would have impeded virtualization. (Compare with virtualization on x86 processors below)
HP's technology to host multiple OS technology on its Itanium powered systems (Integrity) is called Integrity Virtual Machines (Integrity VM). Since Itanium is capable of running HP-UX, Linux, and Windows - 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 which allows for many important 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 dyanmic kernel updates without system reboot. HP also provides more rigid partitioning of their Integrity and HP9000 systems by way of VPAR and NPAR technology, the former offering shared resource partitioning and the later offering complete I/O and processing isolation. The flexibility of VSE has given way to its use more frequently in newer deployments.
Similar trends have been seen with x86/x64 server platforms, where virtualization efforts have been led by open source projects such as Xen. 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.
The x86 architecture used in most PC systems is particularly difficult to virtualize. Full virtualization (presenting the illusion of a complete set of standard hardware) on x86 has significant costs in hypervisor complexity and runtime performance.
An alternative approach requires that the guest operating system be modified to make system calls to the hypervisor, rather than executing machine I/O instructions which are then simulated by the hypervisor. This is called paravirtualization in Xen, a "hypercall" in Parallels Workstation, and a "DIAGNOSE code" in IBM's VM. VMware supplements the slowest rough corners of virtualization with device drivers for the guest. All are really the same thing, a system call to the hypervisor below. Some microkernels such as Mach and L4 are flexible enough such that "paravirtualization" of guest operating systems is possible.
CPU vendors have added hardware virtualization assistance to their products. Intel's is called VT (codenamed Vanderpool), AMD's is referred to as AMD Virtualization or AMD-V (codename: Pacifica). These extensions address the parts of x86 that are difficult or inefficient to virtualize, providing additional support to the hypervisor. This enables simpler virtualization code and a higher performance for full virtualization.
Others, like Xen, are implemented as software-only virtual machines. Xen runs on a normal host operating system such as Linux, and is able to run both paravirtualized and fully virtualized (i.e. unmodified) operating systems with the help of the hardware virtualization extensions Intel VTx. In fact, Xen has successfully demonstrated Windows XP running unmodified. The Xen distribution already contains versions of FreeBSD, Linux, NetBSD, and Plan 9 from Bell Labs that have been so modified. User programs will continue to work on Xen without change. Also, Xen has been re-implemented on the OpenSolaris operating system as of build 75 — the result is called Sun xVM Server.
In June 2008, Microsoft delivered a new Type 1 hypervisor called Hyper-V (codenamed "Viridian" and previously referred to as Windows Server virtualization); the design features OS integration at the lowest level. New versions of the Windows operating system beginning with Windows Vista include extensions to boost performance when running on top of the Viridian hypervisor.
Virtual machines have recently appeared in embedded systems, such as mobile phones. This is driven by the desire to provide a high-level operating-system interface for application programming, such as Linux or Microsoft Windows, while at the same time maintaining traditional real-time operating system (RTOS) APIs. The low-level RTOS environments need to be retained for legacy support, and because the real-time capabilities of high-level OSes are insufficient for many embedded applications.
Hypervisors for embedded use must therefore be real-time capable, a design criterion not present for hypervisors used in other domains. The resource-constrained nature of many 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. Support for virtualization requires memory protection (in the form of a memory management unit or at least a memory protection unit) and a distinction between user mode and privileged mode, which rules out most microcontrollers. This still leaves x86, MIPS, ARM and PowerPC as widely-deployed architectures on medium- to high-end embedded systems.
As embedded-system manufacturers usually have source code to their operating systems, there is less need for full virtualization in this space. Instead, the performance advantages of paravirtualization make this usually the virtualization technology of choice. Nevertheless, ARM has recently added a limited form of support for full virtualization (single guest only) with their TrustZone technology.
Other differences between virtualization in server/desktop and embedded environments are 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 information-flow control.
The first (and so far only) hypervisor deployed in a commercially-sold mobile embedded system (a Toshiba mobile phone) is OKL4, a commercial member of the L4 microkernel family. It supports x86, ARM and MIPS processors.
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