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X86 virtualization
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.
The following discussion focuses only on virtualization of the x86 architecture protected mode.
In protected mode the operating system kernel runs in kernel space at the most privileged level (ring 0) that allows it to configure the MMU, manage physical memory, and directly control I/O peripherals, while applications run in user space at a lower privilege level (such as ring 3), where they are confined to their own virtual address spaces and must invoke system calls to request I/O operations or other privileged services from the kernel.
In software-based virtualization, a host OS has direct access to hardware while the guest operating systems, have limited access to hardware, similarly to other user space application of the host OS. One approach used in x86 software-based virtualization to implement this mechanism is called ring deprivileging, which involves running the guest OS at a ring higher (lesser privileged) than 0, so that attempts to execute privileged instructions can be intercepted and handled by the hypervisor.
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).
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X86 virtualization AI simulator
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X86 virtualization
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.
The following discussion focuses only on virtualization of the x86 architecture protected mode.
In protected mode the operating system kernel runs in kernel space at the most privileged level (ring 0) that allows it to configure the MMU, manage physical memory, and directly control I/O peripherals, while applications run in user space at a lower privilege level (such as ring 3), where they are confined to their own virtual address spaces and must invoke system calls to request I/O operations or other privileged services from the kernel.
In software-based virtualization, a host OS has direct access to hardware while the guest operating systems, have limited access to hardware, similarly to other user space application of the host OS. One approach used in x86 software-based virtualization to implement this mechanism is called ring deprivileging, which involves running the guest OS at a ring higher (lesser privileged) than 0, so that attempts to execute privileged instructions can be intercepted and handled by the hypervisor.
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).