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Virtual machine

virt-manager running the Haiku operating system in a window In computing , a virtual machine ( VM ) is the virtualization or emulation of a computer system . Virtual machines ar...

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virt-manager running the Haiku operating system in a window

In computing, a virtual machine (VM) is the virtualization or emulation of a computer system. Virtual machines are based on computer architectures and provide the functionality of a physical computer. Their implementations may involve specialized hardware, software, or a combination of the two. Virtual machines differ and are organized by their function, shown here:

Some virtual machine emulators, such as QEMU and video game console emulators, are designed to also emulate (or "virtually imitate") different system architectures, thus allowing execution of software applications and operating systems written for another CPU or architecture. OS-level virtualization allows the resources of a computer to be partitioned via the kernel. The terms are not universally interchangeable.

Definitions

System virtual machines

A 'virtual machine' was originally defined by Popek and Goldberg as "an efficient, isolated duplicate of a real computer machine."[2] Current use includes virtual machines that have no direct correspondence to any real hardware.[3] The physical, "real-world" hardware running the VM is generally referred to as the 'host', and the virtual machine emulated on that machine is generally referred to as the 'guest'. A host can emulate several guests, each of which can emulate different operating systems and hardware platforms.

The desire to run multiple operating systems was the initial motive for virtual machines, so as to allow time-sharing among several single-tasking operating systems. In some respects, a system virtual machine can be considered a generalization of the concept of virtual memory that historically preceded it. IBM's CP/CMS, the first systems to allow full virtualization, implemented time sharing by providing each user with a single-user operating system, the Conversational Monitor System (CMS). Unlike virtual memory, a system virtual machine entitled the user to write privileged instructions in their code. This approach had certain advantages, such as adding input/output devices not allowed by the standard system.[3]

As technology evolves virtual memory for purposes of virtualization, new systems of memory overcommitment may be applied to manage memory sharing among multiple virtual machines on one computer operating system. It may be possible to share memory pages that have identical contents among multiple virtual machines that run on the same physical machine, what may result in mapping them to the same physical page by a technique termed kernel same-page merging (KSM). This is especially useful for read-only pages, such as those holding code segments, which is the case for multiple virtual machines running the same or similar software, software libraries, web servers, middleware components, etc. The guest operating systems do not need to be compliant with the host hardware, thus making it possible to run different operating systems on the same computer (e.g., Windows, Linux, or prior versions of an operating system) to support future software.[4]

The use of virtual machines to support separate guest operating systems is popular in regard to embedded systems. A typical use would be to run a real-time operating system simultaneously with a preferred complex operating system, such as Linux or Windows. Another use would be for novel and unproven software still in the developmental stage, so it runs inside a sandbox. Virtual machines have other advantages for operating system development and may include improved debugging access and faster reboots.[5]

Multiple VMs running their own guest operating system are frequently engaged for server consolidation.[6]

Process virtual machines

A process virtual machine, sometimes called an application virtual machine, or Managed Runtime Environment (MRE), runs as a normal application inside a host OS and supports a single process. It is created when that process is started and deleted when it is closed. Its purpose is to provide a platform-independent programming environment that abstracts away details of the underlying hardware or operating system and allows a program to execute in the same way on any platform.[7]

A process VM provides a high-level abstraction – that of a high-level programming language (compared to the low-level ISA abstraction of the system VM). Process VMs are implemented using an interpreter; performance comparable to compiled programming languages can be achieved by the use of just-in-time compilation.

This type of VM has become popular with the Java programming language, which is implemented using the Java virtual machine. Other examples include the Parrot virtual machine and the .NET Framework, which runs on a VM called the Common Language Runtime. All of them can serve as an abstraction layer for any computer language.

A special case of process VMs are systems that abstract over the communication mechanisms of a (potentially heterogeneous) computer cluster. Such a VM does not consist of a single process, but one process per physical machine in the cluster. They are designed to ease the task of programming concurrent applications by letting the programmer focus on algorithms rather than the communication mechanisms provided by the interconnect and the OS. They do not hide the fact that communication takes place, and as such do not attempt to present the cluster as a single machine.

Unlike other process VMs, these systems do not provide a specific programming language, but are embedded in an existing language; typically such a system provides bindings for several languages (e.g., C and Fortran). Examples are Parallel Virtual Machine (PVM) and Message Passing Interface (MPI).

History

Both system virtual machines and process virtual machines date to the 1960s and remain areas of active development.

Las máquinas virtuales de sistema surgieron del tiempo compartido , implementado notablemente en el Sistema de Tiempo Compartido Compatible (CTSS). El tiempo compartido permitía que varios usuarios utilizaran un ordenador simultáneamente : cada programa parecía tener acceso completo a la máquina, pero solo se ejecutaba uno a la vez, y el sistema alternaba entre programas en intervalos de tiempo, guardando y restaurando el estado en cada ocasión. Esto evolucionó hacia las máquinas virtuales, especialmente a través de los sistemas de investigación de IBM: el M44/44X , que utilizaba virtualización parcial , y el CP-40 y el SIMMON , que utilizaban virtualización completa y fueron ejemplos tempranos de hipervisores . La primera arquitectura de máquina virtual ampliamente disponible fue el CP-67 /CMS (véase la Historia del CP/CMS para más detalles). Una distinción importante radicaba en el uso de varias máquinas virtuales en un mismo sistema anfitrión para el tiempo compartido, como en el M44/44X y el CP-40, y el uso de una sola máquina virtual en un sistema anfitrión para la creación de prototipos, como en el SIMMON. Los emuladores , con emulación de hardware de sistemas anteriores para compatibilidad, se remontan al IBM System/360 en 1963, [ 8 ] [ 9 ] mientras que la emulación de software (entonces llamada "simulación") es anterior a él.

Las máquinas virtuales de proceso surgieron originalmente como plataformas abstractas para un lenguaje intermedio utilizado como representación intermedia de un programa por un compilador ; los primeros ejemplos datan de alrededor de 1964 con el sistema de escritura de compiladores META II, que lo utilizaba tanto para la descripción de la sintaxis como para la generación de código de destino. Un ejemplo notable de 1966 fue la máquina de código O , una máquina virtual que ejecuta código O (código objeto) emitido por el front-end del compilador BCPL . Esta abstracción permitió que el compilador se portara fácilmente a una nueva arquitectura mediante la implementación de un nuevo back-end que tomaba el código O existente y lo compilaba a código máquina para la máquina física subyacente. El lenguaje Euler utilizó un diseño similar, con el lenguaje intermedio llamado P (portable). [ 10 ] Esto se popularizó alrededor de 1970 con Pascal , notablemente en el sistema Pascal-P (1973) y el compilador Pascal-S (1975), en el que se denominó código p y la máquina resultante como máquina de código p . Esto ha sido influyente, y las máquinas virtuales en este sentido se han denominado a menudo máquinas de código P. Además de ser un lenguaje intermedio, el código P de Pascal también se ejecutaba directamente mediante un intérprete que implementaba la máquina virtual, especialmente en UCSD Pascal (1978); esto influyó en intérpretes posteriores, en particular en la máquina virtual Java (JVM). Otro ejemplo temprano fue SNOBOL4 (1967), escrito en el lenguaje de implementación SNOBOL (SIL), un lenguaje ensamblador para una máquina virtual, que luego se adaptaba a máquinas físicas mediante la transpilación a su ensamblador nativo a través de un ensamblador de macros . [ 11 ] Sin embargo, las macros han caído en desuso, por lo que este enfoque ha sido menos influyente. Las máquinas virtuales de procesos fueron un enfoque popular para implementar software para microcomputadoras primitivas, incluyendo Tiny BASIC y juegos de aventuras, desde implementaciones únicas como Pyramid 2000 hasta un motor de propósito general como la z-machine de Infocom , que Graham Nelson argumenta que es "posiblemente la máquina virtual más portátil jamás creada". [ 12 ]

Significant advances occurred in the implementation of Smalltalk-80,[13] particularly the Deutsch/Schiffmann implementation[14] which pushed just-in-time (JIT) compilation forward as an implementation approach that uses process virtual machine.[15] Later notable Smalltalk VMs were VisualWorks, the Squeak Virtual Machine,[16] and Strongtalk.[17] A related language that produced a lot of virtual machine innovation was the Self programming language,[18] which pioneered adaptive optimization[19] and generational garbage collection. These techniques proved commercially successful in 1999 in the HotSpot Java virtual machine.[20] Other innovations include a register-based virtual machine, to better match the underlying hardware, rather than a stack-based virtual machine, which is a closer match for the programming language; in 1995, this was pioneered by the Dis virtual machine for the Limbo language.

Virtualization techniques

Logical diagram of full virtualization

Full virtualization

In full virtualization, the virtual machine simulates enough hardware to allow an unmodified "guest" OS (one designed for the same instruction set) to be run in isolation. This approach was pioneered in 1966 with the IBM CP-40 and CP-67, predecessors of the VM family.

Examples outside the mainframe field include Parallels Workstation, Parallels Desktop for Mac, VirtualBox, Virtual Iron, Oracle VM, Virtual PC, Virtual Server, Hyper-V, VMware Fusion, VMware Workstation, VMware Server (discontinued, formerly called GSX Server), VMware ESXi, QEMU, Adeos, Mac-on-Linux, Win4BSD, Win4Lin Pro, and Egenera vBlade technology.

Hardware-assisted virtualization

En la virtualización asistida por hardware, el hardware proporciona soporte arquitectónico que facilita la creación de un monitor de máquina virtual y permite que los sistemas operativos invitados se ejecuten de forma aislada. [ 21 ] La virtualización asistida por hardware se introdujo por primera vez en el IBM System/370 en 1972, para su uso con VM/370 , el primer sistema operativo de máquina virtual ofrecido por IBM como producto oficial. [ 22 ]

En 2005 y 2006, Intel y AMD proporcionaron hardware adicional para dar soporte a la virtualización. Sun Microsystems (adquirida por Oracle Corporation ) añadió características similares a sus procesadores UltraSPARC Serie T en 2005. Algunos ejemplos de plataformas de virtualización adaptadas a este hardware son KVM , VMware Workstation , VMware Fusion , Hyper-V , Windows Virtual PC , Xen , Parallels Desktop para Mac , Oracle VM Server para SPARC , VirtualBox y Parallels Workstation .

En 2006, se descubrió que la compatibilidad con hardware x86 de 32 y 64 bits de primera generación rara vez ofrecía ventajas de rendimiento sobre la virtualización por software. [ 23 ]

Virtualización a nivel de sistema operativo

En la virtualización a nivel de sistema operativo, un servidor físico se virtualiza a nivel de sistema operativo, lo que permite que varios servidores virtualizados, aislados y seguros, se ejecuten en un único servidor físico. Los entornos de sistema operativo "invitados" comparten la misma instancia del sistema operativo que el sistema anfitrión. Por lo tanto, el mismo núcleo del sistema operativo se utiliza para implementar los entornos "invitados", y las aplicaciones que se ejecutan en un entorno "invitado" determinado lo consideran un sistema independiente. La implementación pionera fue FreeBSD jails ; otros ejemplos incluyen Docker , Solaris Containers , OpenVZ , Linux-VServer , LXC , AIX Workload Partitions , Parallels Virtuozzo Containers e iCore Virtual Accounts.

Instantáneas

Una instantánea es un estado de una máquina virtual, y generalmente de sus dispositivos de almacenamiento, en un momento exacto. Una instantánea permite restaurar posteriormente el estado de la máquina virtual en el momento de la instantánea, deshaciendo así cualquier cambio posterior. Esta capacidad resulta útil como técnica de copia de seguridad , por ejemplo, antes de realizar una operación de riesgo. [ 24 ]

Virtual machines frequently use virtual disks for their storage; in a very simple example, a 10-gigabytehard disk drive is simulated with a 10-gigabyte flat file. Any requests by the VM for a location on its physical disk are transparently translated into an operation on the corresponding file. Once such a translation layer is present, however, it is possible to intercept the operations and send them to different files, depending on various criteria. Every time a snapshot is taken, a new file is created, and used as an overlay for its predecessors. New data is written to the topmost overlay; reading existing data, however, needs the overlay hierarchy to be scanned, resulting in accessing the most recent version. Thus, the entire stack of snapshots is virtually a single coherent disk; in that sense, creating snapshots works similarly to the incremental backup technique.

Other components of a virtual machine can also be included in a snapshot, such as the contents of its random-access memory (RAM), BIOS settings, or its configuration settings. "Save state" feature in video game console emulators is an example of such snapshots.

Restoring a snapshot consists of discarding or disregarding all overlay layers that are added after that snapshot, and directing all new changes to a new overlay.

Migration

The snapshots described above can be moved to another host machine with its own hypervisor; when the VM is temporarily stopped, snapshotted, moved, and then resumed on the new host, this is known as migration. If the older snapshots are kept in sync regularly, this operation can be quite fast, and allow the VM to provide uninterrupted service while its prior physical host is, for example, taken down for physical maintenance.

Failover

Similar to the migration mechanism described above, failover allows the VM to continue operations if the host fails. Generally it occurs if the migration has stopped working. However, in this case, the VM continues operation from the last-known coherent state, rather than the current state, based on whatever materials the backup server was last provided with.

Nested virtualization

Nested virtualization refers to the ability of running a virtual machine within another, having this general concept extendable to an arbitrary depth. In other words, nested virtualization refers to running one or more hypervisors inside another hypervisor. The nature of a nested guest virtual machine does not need to be homogeneous with its host virtual machine; for example, application virtualization can be deployed within a virtual machine created by using hardware virtualization.[25]

Nested virtualization becomes more necessary as widespread operating systems gain built-in hypervisor functionality, which in a virtualized environment can be used only if the surrounding hypervisor supports nested virtualization; for example, Windows 7 is capable of running Windows XP applications inside a built-in virtual machine. Furthermore, moving already existing virtualized environments into a cloud, following the Infrastructure as a Service (IaaS) approach, is much more complicated if the destination IaaS platform does not support nested virtualization.[26][27]

The way nested virtualization can be implemented on a particular computer architecture depends on supported hardware-assisted virtualization capabilities. If a particular architecture does not provide hardware support required for nested virtualization, various software techniques are employed to enable it.[26] Over time, more architectures gain required hardware support; for example, since the Haswell microarchitecture (announced in 2013), Intel started to include VMCS shadowing as a technology that accelerates nested virtualization.[28]

Security

Qubes OS Architecture Design Diagram shows how compromise of Firefox or Thunderbird (in AppVM 1) could not lead to compromise of the user's KeePass password manager (in AppVM 2), due to design using distinct VMs to obtain a compartmentalization architecture.

Virtual machines are frequently used to isolate applications from one another for security.

This architectural design (used by Qubes OS, Whonix, KickSecure, and Dangerzone) can prevent the spread of malware from one compromised system (e.g. an email client opening an infected document) to another system (e.g. a password manager).[29][30]

See also

References

  1. ↑Dittamo, Cristian (2010). On Expressing Different Concurrency Paradigms on Virtual Execution Systems (Ph.D. thesis). University of Pisa. Retrieved 2025-05-12.
  2. ↑Popek, Gerald J.; Goldberg, Robert P. (1974). "Formal requirements for virtualizable third generation architectures"(PDF). Communications of the ACM. 17 (7): 412–421. doi:10.1145/361011.361073. S2CID 12680060.
  3. 12Smith, James E.; Nair, Ravi (2005). "The Architecture of Virtual Machines". Computer. 38 (5): 32–38, 395–396. Bibcode:2005Compr..38e..32S. doi:10.1109/MC.2005.173. S2CID 6578280.
  4. ↑Oliphant, Patrick. "Virtual Machines". VirtualComputing. Archived from the original on 2016-07-29. Retrieved 2015-09-23. Some people use that capability to set up a separate virtual machine running Windows on a Mac, giving them access to the full range of applications available for both platforms.
  5. ↑"Super Fast Server Reboots – Another reason Virtualization rocks". vmwarez.com. 2006-05-09. Archived from the original on 2006-06-14. Retrieved 2013-06-14.
  6. ↑"Server Consolidation and Containment With Virtual Infrastructure"(PDF). VMware. 2007. Archived(PDF) from the original on 2013-12-28. Retrieved 2015-09-29.
  7. ↑"About the Java Technology". Oracle. Retrieved 2026-05-22.
  8. ↑Pugh, Emerson W. (1995). Building IBM: Shaping an Industry and Its Technology. MIT. p. 274. ISBN 978-0-262-16147-3.
  9. ↑Pugh, Emerson W.; et al. (1991). IBM's 360 and Early 370 Systems. MIT. pp. 160–161. ISBN 978-0-262-16123-7.
  10. ↑Wirth, Niklaus Emil; Weber, Helmut (1966). EULER: a generalization of ALGOL, and its formal definition: Part II, Communications of the Association for Computing Machinery. Vol. 9. New York: ACM. pp. 89–99.
  11. ↑Griswold, Ralph E.The Macro Implementation of SNOBOL4. San Francisco, CA: W. H. Freeman and Company, 1972 (ISBN 0-7167-0447-1), Chapter 1.
  12. ↑Nelson, Graham A."About Interpreters". Inform website. Archived from the original on 2009-12-03. Retrieved 2009-11-07.
  13. ↑Goldberg, Adele; Robson, David (1983). Smalltalk-80: The Language and its Implementation. Addison-Wesley Series in Computer Science. Addison-Wesley. ISBN 978-0-201-11371-6.
  14. ↑Deutsch, L. Peter; Schiffman, Allan M. (1984). "Efficient implementation of the Smalltalk-80 system". POPL. Salt Lake City, Utah: ACM. doi:10.1145/800017.800542. ISBN 0-89791-125-3.
  15. ↑Aycock, John (2003). "A brief history of just-in-time". ACM Comput. Surv.35 (2): 97–113. doi:10.1145/857076.857077. S2CID 15345671.
  16. ↑Ingalls Jr., Daniel "Dan" Henry Holmes; Kaehler, Ted; Maloney, John; Wallace, Scott; Kay, Alan Curtis (1997). "Back to the future: the story of Squeak, a practical Smalltalk written in itself". OOPSLA '97: Proceedings of the 12th ACM SIGPLAN conference on Object-oriented programming, systems, languages, and applications. New York, NY, US: ACM Press. pp. 318–326. doi:10.1145/263698.263754. ISBN 0-89791-908-4.
  17. ↑Bracha, Gilad; Griswold, David (1993). "Strongtalk: Typechecking Smalltalk in a Production Environment". Proceedings of the Eighth Annual Conference on Object-oriented Programming Systems, Languages, and Applications. OOPSLA '93. New York, NY, US: ACM. pp. 215–230. doi:10.1145/165854.165893. ISBN 978-0-89791-587-8.
  18. ↑ Ungar, David Michael ; Smith, Randall B. (diciembre de 1987). "El yo: el poder de la simplicidad". ACM SIGPLAN Notices . 22 (12): 227– 242. doi : 10.1145/38807.38828 . ISSN 0362-1340 . 
  19. ↑ Hölzle, Urs ; Ungar, David Michael (1994). "Optimizing dynamically-dispatched calls with run-time type feedback" . PLDI . Orlando, Florida, Estados Unidos: ACM. pp. 326–336 . doi : 10.1145/178243.178478 . ISBN  0-89791-662-X.
  20. ↑ Paleczny, Michael; Vick, Christopher; Click, Cliff (2001). "El compilador del servidor Java HotSpot" (PDF) . Actas del Simposio de Investigación y Tecnología de la Máquina Virtual Java . Vol. 1. Monterey, California: USENIX Association. 
  21. ↑ Uhlig, Rich; Neiger, Gil; Rodgers, Dion; Santoni, Amy L.; Martins, Fernando CM; Anderson, Andrew V.; Bennett, Steven M. ; Kägi, Alain; Leung, Felix H.; Smith, Larry (mayo de 2005). "Tecnología de virtualización de Intel". Computer . 38 (5): 48– 56. Bibcode : 2005Compr..38e..48U . doi : 10.1109/MC.2005.163 . S2CID 18514555 . 
  22. ↑ Randal, A. (2019). Lo ideal frente a lo real: una revisión de la historia de las máquinas virtuales y los contenedores.
  23. ↑ Adams, Keith; Agesen, Ole (21 de octubre de 2006). Una comparación de técnicas de software y hardware para la virtualización x86 (PDF) . ASPLOS'06, 21-25 de octubre de 2006. San José, California, EE. UU. Archivado (PDF) del original el 20 de agosto de 2010. Sorprendentemente, encontramos que el soporte de hardware de primera generación rara vez ofrece ventajas de rendimiento sobre las técnicas de software existentes. Atribuimos esta situación a los altos costos de transición VMM/invitado y a un modelo de programación rígido que deja poco margen para la flexibilidad del software en la gestión de la frecuencia o el costo de estas transiciones.
  24. ↑ Ackerman, Pascal (07/10/2021). Ciberseguridad industrial: Supervise eficazmente la postura de ciberseguridad de su entorno ICS . Packt Publishing Ltd. pág. 57. ISBN  978-1-80020-582-6. Consultado el 17 de noviembre de 2025 .
  25. ↑Orit Wasserman, Red Hat (2013). "Nested virtualization: Shadow turtles"(PDF). KVM forum. Retrieved 2021-05-07.
  26. 12Muli Ben-Yehuda; Michael D. Day; Zvi Dubitzky; Michael Factor; Nadav Har’El; Abel Gordon; Anthony Liguori; Orit Wasserman; Ben-Ami Yassour (2010-09-23). "The Turtles Project: Design and Implementation of Nested Virtualization"(PDF). usenix.org. Retrieved 2014-12-16.
  27. ↑Alex Fishman; Mike Rapoport; Evgeny Budilovsky; Izik Eidus (2013-06-25). "HVX: Virtualizing the Cloud"(PDF). rackcdn.com. Retrieved 2014-12-16.
  28. ↑"4th-Gen Intel Core vPro Processors with Intel VMCS Shadowing"(PDF). Intel. 2013. Retrieved 2014-12-16.
  29. ↑"Introduction". Why Qubes OS?.
  30. ↑"When security matters: working with Qubes OS at the Guardian". The Guardian Engineering Blog.

Further reading

  • James E. Smith, Ravi Nair, Virtual Machines: Versatile Platforms For Systems And Processes, Morgan Kaufmann, May 2005, ISBN 1-55860-910-5, 656 pages (covers both process and system virtual machines)
  • Craig, Iain D. Virtual Machines. Springer, 2006, ISBN 1-85233-969-1, 269 pages (covers only process virtual machines)
  • Mendel Rosenblum (2004-08-31). "The Reincarnation of Virtual Machines". ACM Queue. Vol. 2, no. 5.
  • Sandia National Laboratories Runs 1 Million Linux Kernels as Virtual Machines
  • The design of the Inferno virtual machine by Phil Winterbottom and Rob Pike