Intel VT

In computing, x86 virtualization refers to hardware virtualization for the x86 architecture. It allows multiple operating systems to simultaneously share x86 processor resources in a safe and efficient manner.

In the late 1990s x86 virtualization was achieved by complex software techniques, necessary to compensate for the processor's lack of virtualization support while attaining reasonable performance. In 2006, both Intel (VT-x) and AMD (AMD-V) introduced limited hardware virtualization support that allowed simpler virtualization software but offered very little speed benefits. Greater hardware support, which allowed substantial speed improvements, came with later processor models.

The following discussion focuses only on virtualization of the x86 architecture protected mode.

In protected mode the operating system kernel runs at a higher privilege such as ring 0, and applications at a lower privilege such as ring 3. In software-based virtualization, a host OS has direct access to hardware while the guest OSs have limited access to hardware, just like any other application of the host OS. One approach used in x86 software-based virtualization to overcome this limitation is called ring deprivileging, which involves running the guest OS at a ring higher (lesser privileged) than 0.

Three techniques made virtualization of protected mode possible:

These techniques incur some performance overhead due to lack of MMU virtualization support, as compared to a VM running on a natively virtualizable architecture such as the IBM System/370.

On traditional mainframes, the classic type 1 hypervisor was self-standing and did not depend on any operating system or run any user applications itself. In contrast, the first x86 virtualization products were aimed at workstation computers, and ran a guest OS inside a host OS by embedding the hypervisor in a kernel module that ran under the host OS (type 2 hypervisor).

There has been some controversy whether the x86 architecture with no hardware assistance is virtualizable as described by Popek and Goldberg. VMware researchers pointed out in a 2006 ASPLOS paper that the above techniques made the x86 platform virtualizable in the sense of meeting the three criteria of Popek and Goldberg, albeit not by the classic trap-and-emulate technique.

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