Native virtualization
Encyclopedia
In computing
, hardware-assisted virtualization is a platform virtualization approach that enables efficient full virtualization
using help from hardware capabilities, primarily from the host processors. Full virtualization is used to simulate a complete hardware environment, or virtual machine
, in which an unmodified guest operating system
(using the same instruction set
as the host machine) executes in complete isolation. Hardware-assisted virtualization was added to x86 processors (Intel VT-x or AMD-V) in 2006.
Hardware-assisted virtualization is also known as accelerated virtualization; Xen
calls it hardware virtual machine (HVM), Virtual Iron
calls it native virtualization.
, the first virtual machine operating system. Virtualization was eclipsed in the late 1970s, with the advent of minicomputer
s that allowed for efficient timesharing, and later with the commoditization of microcomputer
s.
The proliferation of x86 servers rekindled interest in virtualization. The primary driver was the potential for server consolidation: virtualization allowed a single server to replace multiple underutilized dedicated servers.
However, the x86 architecture did not meet the Popek and Goldberg virtualization requirements
to achieve “classical virtualization″:
This made it difficult to implement a virtual machine monitor for this type of processor. Specific limitations included the inability to trap
on some privileged
instructions.
To compensate for these architectural limitations, virtualization of the x86 architecture has been accomplished through two methods: full virtualization or paravirtualization. Both create the illusion of physical hardware to achieve the goal of operating system independence from the hardware but present some trade-offs in performance and complexity.
Paravirtualization
is a technique in which the hypervisor provides an API and the OS of the guest virtual machine calls that API, requiring OS modifications. The best known commercial implementations of paravirtualization are modified Linux kernels from XenSource and GNU/Linux distributors.
Full virtualization
was implemented in first-generation x86 VMMs. It relies on binary translation
to trap and virtualize the execution of certain sensitive, non-virtualizable instructions. With this approach, critical instructions are discovered (statically or dynamically at run-time) and replaced with traps into the VMM to be emulated in software. Binary translation can incur a large performance overhead in comparison to a virtual machine running on natively virtualized architectures such as the IBM System/370. VirtualBox
, VMware Workstation
(for 32-bit guests only), and Microsoft Virtual PC
, are well-known commercial implementations of full virtualization.
With hardware-assisted virtualization, the VMM can efficiently virtualize the entire x86 instruction set by handling these sensitive instructions using a classic trap-and-emulate model in hardware, as opposed to software.
Intel and AMD came with distinct implementations of hardware-assisted x86 virtualization, Intel VT-x and AMD-V, respectively. On the Itanium
architecture, hardware-assisted virtualization is known as VT-i.
Well-known implementations of hardware-assisted x86 virtualization include VMware Workstation
(for 64-bit guests only), Xen
3.x (including derivatives like Virtual Iron
), Linux KVM
and Microsoft
Hyper-V
.
as it reduces (ideally, eliminates) the changes needed in the guest operating system. It is also considerably easier to obtain better performance. A practical benefit of hardware-assisted virtualization has been cited by VMware engineers and Virtual Iron.
A "pure" hardware-assisted virtualization approach, using entirely unmodified guest operating systems, involves many VM traps, and thus high CPU overheads, limiting scalability and the efficiency of server consolidation. This performance hit can be mitigated by the use of paravirtualized drivers; the combination has been called "hybrid virtualization".
In 2006 first-generation 32- and 64-bit x86 hardware support was found rarely to offer performance advantages over software virtualization.
Computing
Computing is usually defined as the activity of using and improving computer hardware and software. It is the computer-specific part of information technology...
, hardware-assisted virtualization is a platform virtualization approach that enables efficient full virtualization
Full virtualization
In computer science, full virtualization is a virtualization technique used to provide a certain kind of virtual machine environment, namely, one that is a complete simulation of the underlying hardware...
using help from hardware capabilities, primarily from the host processors. Full virtualization is used to simulate a complete hardware environment, or virtual machine
Virtual machine
A virtual machine is a "completely isolated guest operating system installation within a normal host operating system". Modern virtual machines are implemented with either software emulation or hardware virtualization or both together.-VM Definitions:A virtual machine is a software...
, in which an unmodified guest operating system
Operating system
An operating system is a set of programs that manage computer hardware resources and provide common services for application software. The operating system is the most important type of system software in a computer system...
(using the same instruction set
Instruction set
An instruction set, or instruction set architecture , is the part of the computer architecture related to programming, including the native data types, instructions, registers, addressing modes, memory architecture, interrupt and exception handling, and external I/O...
as the host machine) executes in complete isolation. Hardware-assisted virtualization was added to x86 processors (Intel VT-x or AMD-V) in 2006.
Hardware-assisted virtualization is also known as accelerated virtualization; Xen
Xen
Xen is a virtual-machine monitor providing services that allow multiple computer operating systems to execute on the same computer hardware concurrently....
calls it hardware virtual machine (HVM), Virtual Iron
Virtual Iron
Virtual Iron Software, was located in Lowell, Massachusetts, sold proprietary software for virtualization and management of a virtual infrastructure...
calls it native virtualization.
History
Hardware-assisted virtualization was first introduced on the IBM System/370 in 1972, for use with VM/370VM (operating system)
VM refers to a family of IBM virtual machine operating systems used on IBM mainframes System/370, System/390, zSeries, System z and compatible systems, including the Hercules emulator for personal computers. The first version, released in 1972, was VM/370, or officially Virtual Machine Facility/370...
, the first virtual machine operating system. Virtualization was eclipsed in the late 1970s, with the advent of minicomputer
Minicomputer
A minicomputer is a class of multi-user computers that lies in the middle range of the computing spectrum, in between the largest multi-user systems and the smallest single-user systems...
s that allowed for efficient timesharing, and later with the commoditization of microcomputer
Microcomputer
A microcomputer is a computer with a microprocessor as its central processing unit. They are physically small compared to mainframe and minicomputers...
s.
The proliferation of x86 servers rekindled interest in virtualization. The primary driver was the potential for server consolidation: virtualization allowed a single server to replace multiple underutilized dedicated servers.
However, the x86 architecture did not meet the Popek and Goldberg virtualization requirements
Popek and Goldberg virtualization requirements
The Popek and Goldberg virtualization requirements are a set of conditions sufficient for a computer architecture to support system virtualization efficiently. They were introduced by Gerald J. Popek and Robert P. Goldberg in their 1974 article "Formal Requirements for Virtualizable Third...
to achieve “classical virtualization″:
- equivalence: a program running under the virtual machine monitor(VMM)HypervisorIn computing, a hypervisor, also called virtual machine manager , is one of many hardware virtualization techniques that allow multiple operating systems, termed guests, to run concurrently on a host computer. It is so named because it is conceptually one level higher than a supervisory program...
should exhibit a behavior essentially identical to that demonstrated when running on an equivalent machine directly; - resource control (also called safety): the VMM must be in complete control of the virtualized resources;
- efficiency: a statistically dominant fraction of machine instructions must be executed without VMM intervention.
This made it difficult to implement a virtual machine monitor for this type of processor. Specific limitations included the inability to trap
Trap (computing)
In computing and operating systems, a trap, also known as an exception or a fault, is typicallyThere is a wide variation in the nomenclature...
on some privileged
Privilege (Computing)
In computing, privilege is defined as the delegation of authority over a computer system. A privilege is a permission to perform an action. Examples of various privileges include the ability to create a file in a directory, or to read or delete a file, access a device, or have read or write...
instructions.
To compensate for these architectural limitations, virtualization of the x86 architecture has been accomplished through two methods: full virtualization or paravirtualization. Both create the illusion of physical hardware to achieve the goal of operating system independence from the hardware but present some trade-offs in performance and complexity.
Paravirtualization
Paravirtualization
In computing, paravirtualization is a virtualization technique that presents a software interface to virtual machines that is similar but not identical to that of the underlying hardware....
is a technique in which the hypervisor provides an API and the OS of the guest virtual machine calls that API, requiring OS modifications. The best known commercial implementations of paravirtualization are modified Linux kernels from XenSource and GNU/Linux distributors.
Full virtualization
Full virtualization
In computer science, full virtualization is a virtualization technique used to provide a certain kind of virtual machine environment, namely, one that is a complete simulation of the underlying hardware...
was implemented in first-generation x86 VMMs. It relies on binary translation
Binary translation
In computing, binary translation is the emulation of one instruction set by another through translation of code. Sequences of instructions are translated from the source to the target instruction set...
to trap and virtualize the execution of certain sensitive, non-virtualizable instructions. With this approach, critical instructions are discovered (statically or dynamically at run-time) and replaced with traps into the VMM to be emulated in software. Binary translation can incur a large performance overhead in comparison to a virtual machine running on natively virtualized architectures such as the IBM System/370. VirtualBox
VirtualBox
Oracle VM VirtualBox is an x86 virtualization software package, originally created by software company Innotek GmbH, purchased by Sun Microsystems, and now developed by Oracle Corporation as part of its family of virtualization products...
, VMware Workstation
VMware Workstation
VMware Workstation is a virtual machine software suite for x86 and x86-64 computers from VMware, a division of EMC Corporation, which allows users to set up multiple x86 and x86-64 virtual machines and use one or more of these virtual machines simultaneously with the hosting operating system...
(for 32-bit guests only), and Microsoft Virtual PC
Microsoft Virtual PC
Windows Virtual PC is a virtualization program for Microsoft Windows. In July 2006 Microsoft released the Windows-hosted version as a free product...
, are well-known commercial implementations of full virtualization.
With hardware-assisted virtualization, the VMM can efficiently virtualize the entire x86 instruction set by handling these sensitive instructions using a classic trap-and-emulate model in hardware, as opposed to software.
Intel and AMD came with distinct implementations of hardware-assisted x86 virtualization, Intel VT-x and AMD-V, respectively. On the Itanium
Itanium
Itanium is a family of 64-bit Intel microprocessors that implement the Intel Itanium architecture . Intel markets the processors for enterprise servers and high-performance computing systems...
architecture, hardware-assisted virtualization is known as VT-i.
Well-known implementations of hardware-assisted x86 virtualization include VMware Workstation
VMware Workstation
VMware Workstation is a virtual machine software suite for x86 and x86-64 computers from VMware, a division of EMC Corporation, which allows users to set up multiple x86 and x86-64 virtual machines and use one or more of these virtual machines simultaneously with the hosting operating system...
(for 64-bit guests only), Xen
Xen
Xen is a virtual-machine monitor providing services that allow multiple computer operating systems to execute on the same computer hardware concurrently....
3.x (including derivatives like Virtual Iron
Virtual Iron
Virtual Iron Software, was located in Lowell, Massachusetts, sold proprietary software for virtualization and management of a virtual infrastructure...
), Linux KVM
Kernel-based Virtual Machine
Kernel-based Virtual Machine is a virtualization infrastructure for the Linux kernel. KVM supports native virtualization on processors with hardware virtualization extensions....
and Microsoft
Microsoft
Microsoft Corporation is an American public multinational corporation headquartered in Redmond, Washington, USA that develops, manufactures, licenses, and supports a wide range of products and services predominantly related to computing through its various product divisions...
Hyper-V
Hyper-V
Microsoft Hyper-V, codenamed Viridian and formerly known as Windows Server Virtualization, is a hypervisor-based virtualization system for x86-64 systems. A beta version of Hyper-V was shipped with certain x86-64 editions of Windows Server 2008, and the finalized version was released on June 26,...
.
Pros
Hardware-assisted virtualization reduces the maintenance overhead of paravirtualizationParavirtualization
In computing, paravirtualization is a virtualization technique that presents a software interface to virtual machines that is similar but not identical to that of the underlying hardware....
as it reduces (ideally, eliminates) the changes needed in the guest operating system. It is also considerably easier to obtain better performance. A practical benefit of hardware-assisted virtualization has been cited by VMware engineers and Virtual Iron.
Cons
Hardware-assisted virtualization requires explicit support in the host CPU, which is not available on all x86/x86_64 processors.A "pure" hardware-assisted virtualization approach, using entirely unmodified guest operating systems, involves many VM traps, and thus high CPU overheads, limiting scalability and the efficiency of server consolidation. This performance hit can be mitigated by the use of paravirtualized drivers; the combination has been called "hybrid virtualization".
In 2006 first-generation 32- and 64-bit x86 hardware support was found rarely to offer performance advantages over software virtualization.
See also
- Further refinements of hardware-assisted virtualization are possible using an IOMMUIOMMUIn computing, an input/output memory management unit is a memory management unit that connects a DMA-capable I/O bus to the main memory...
; this allows native-speed access to dedicated hardware from a guest operating system, including DMADirect memory accessDirect memory access is a feature of modern computers that allows certain hardware subsystems within the computer to access system memory independently of the central processing unit ....
-capable hardware - Rapid Virtualization IndexingRapid Virtualization IndexingRapid Virtualization Indexing is an AMD second generation hardware-assisted virtualization technology for the processor memory management unit ....
- Extended Page TableExtended Page TableExtended Page Tables is an Intel second generation x86 virtualization technology for the memory management unit . When this feature is active, the ordinary IA-32 page tables translate from linear addresses to guest-physical addresses...
- Other virtualization techniques include operating system-level virtualizationOperating system-level virtualizationOperating system-level virtualization is a server virtualization method where the kernel of an operating system allows for multiple isolated user-space instances, instead of just one. Such instances may look and feel like a real server, from the point of view of its owner...
, as practiced by Parallels Virtuozzo Containers, and application virtualizationApplication VirtualizationApplication virtualization is an umbrella term that describes software technologies that improve portability, manageability and compatibility of applications by encapsulating them from the underlying operating system on which they are executed. A fully virtualized application is not installed in...
. - Nanokernel
- Hardware emulationHardware emulationIn integrated circuit design, hardware emulation is the process of imitating the behavior of one or more pieces of hardware with another piece of hardware, typically a special purpose emulation system. The emulation model is usually based on RTL source code, which is compiled into the format...
- EmulatorEmulatorIn computing, an emulator is hardware or software or both that duplicates the functions of a first computer system in a different second computer system, so that the behavior of the second system closely resembles the behavior of the first system...
- Joint Test Action Group
- Background Debug Mode interfaceBackground Debug Mode interfaceBackground Debug Mode interface is an electronic interface that allows debugging of embedded systems. Specifically, it provides in-circuit debugging functionality in microcontrollers...
- In-circuit emulatorIn-circuit emulatorAn in-circuit emulator is a hardware device used to debug the software of an embedded system. It was historically in the form of bond-out processor which has many internal signals brought out for the purpose of debugging...