Editing Virtual machine (section)

== 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 [[Hypervisor|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]].<ref>{{Cite web |author=Orit Wasserman, [[Red Hat]] |year=2013 |title=Nested virtualization: Shadow turtles |url=https://www.linux-kvm.org/images/e/e9/Kvm-forum-2013-nested-virtualization-shadow-turtles.pdf |access-date=2021-05-07 |publisher=KVM forum}}</ref>

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.<ref name="usenix-turtles">{{Cite web |author1=Muli Ben-Yehuda |author2=Michael D. Day |author3=Zvi Dubitzky |author4=Michael Factor |author5=Nadav Har’El |author6=Abel Gordon |author7=Anthony Liguori |author8=Orit Wasserman |author9=Ben-Ami Yassour |date=2010-09-23 |title=The Turtles Project: Design and Implementation of Nested Virtualization |url=https://www.usenix.org/legacy/event/osdi10/tech/full_papers/Ben-Yehuda.pdf |access-date=2014-12-16 |website=usenix.org}}</ref><ref>{{Cite web |author1=Alex Fishman |author2=Mike Rapoport |author3=Evgeny Budilovsky |author4=Izik Eidus |date=2013-06-25 |title=HVX: Virtualizing the Cloud |url=http://0b4af6cdc2f0c5998459-c0245c5c937c5dedcca3f1764ecc9b2f.r43.cf2.rackcdn.com/11552-hotcloud13-fishman.pdf |access-date=2014-12-16 |website=rackcdn.com}}</ref>

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.<ref name="usenix-turtles" />  Over time, more architectures gain required hardware support; for example, since the [[Haswell (microarchitecture)|Haswell]] microarchitecture (announced in 2013), Intel started to include [[VMCS shadowing]] as a technology that accelerates nested virtualization.<ref>{{cite web |year=2013 |title=4th-Gen Intel Core vPro Processors with Intel VMCS Shadowing |url=https://www-ssl.intel.com/content/dam/www/public/us/en/documents/white-papers/intel-vmcs-shadowing-paper.pdf |access-date=2014-12-16 |publisher=[[Intel]]}}</ref>