Intel VT

x86 virtualization is the use of hardware-assisted virtualization capabilities on an x86/x86-64 CPU.

In the late 1990s x86 virtualization was achieved by complex software techniques, necessary to compensate for the processor's lack of hardware-assisted virtualization capabilities while attaining reasonable performance. In 2005 and 2006, both Intel ( VT-x) and AMD (AMD-V) introduced limited hardware virtualization support that allowed simpler virtualization software but offered very few speed benefits. Greater hardware support, which allowed substantial speed improvements, came with later processor models.

Software-based virtualization

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:
* Binary translation is used to rewrite in terms of ring 3 instructions certain ring 0 instructions, such as POPF, that would otherwise fail silently or behave differently when executed above ring 0, making the classic trap-and-emulate virtualization impossible. To improve performance, the translated basic blocks need to be cached in a coherent way that detects code patching (used in VxDs for instance), the reuse of pages by the guest OS, or even self-modifying code.
* A number of key data structures used by a processor need to be shadowed. Because most operating systems use paged virtual memory, and granting the guest OS direct access to the MMU would mean loss of control by the virtualization manager, some of the work of the x86 MMU needs to be duplicated in software for the guest OS using a technique known as ''shadow page tables''. This involves denying the guest OS any access to the actual page table entries by trapping access attempts and emulating them instead in software. The x86 architecture uses hidden state to store segment descriptors in the processor, so once the segment descriptors have been loaded into the processor, the memory from which they have been loaded may be overwritten and there is no way to get the descriptors back from the processor. ''Shadow descriptor tables'' must therefore be used to track changes made to the descriptor tables by the guest OS.
* I/O device emulation: Unsupported devices on the guest OS must be emulated by a device emulator that runs in the host OS.

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.

A different route was taken by other systems like Denali, L4, and Xen, known as paravirtualization, which involves porting operating systems to run on the resulting virtual machine, which does not implement the parts of the actual x86 instruction set that are hard to virtualize. The paravirtualized I/O has significant performance benefits as demonstrated in the original SOSP'03 Xen paper.

The initial version of x86-64 (AMD64) did not allow for a software-only full virtualization due to the lack of segmentation support in long mode, which made the protection of the hypervisor's memory impossible, in particular, the protection of the trap handler that runs in the guest kernel address space. Revision D and later 64-bit AMD processors (as a rule of thumb, those manufactured in 90 nm or less) added basic support for segmentation in long mode, making it possible to run 64-bit guests in 64-bit hosts via binary translation. Intel did not add segmentation support to its x86-64 implementation (Intel 64), making 64-bit software-only virtualization impossible on Intel CPUs, but Intel VT-x support makes 64-bit hardware assisted virtualization possible on the Intel platform.

On some platforms, it is possible to run a 64-bit guest on a 32-bit host OS if the underlying processor is 64-bit and supports the necessary virtualization extensions.

Hardware-assisted virtualization

In 2005 and 2006, Intel and AMD (working independently) created new processor extensions to the x86 architecture. The first generation of x86 hardware virtualization addressed the issue of privileged instructions. The issue of low performance of virtualized system memory was addressed with MMU virtualization that was added to the chipset later.

Central processing unit

Virtual 8086 mode

Based on painful experiences with the 80286 protected mode, which by itself was not suitable enough to run concurrent DOS applications well, Intel introduced the virtual 8086 mode in their 80386

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