![]() ![]() 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. There has been some controversy whether the x86 architecture with no hardware assistance is virtualizable as described by Popek and Goldberg. 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). 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. 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. I/O device emulation: Unsupported devices on the guest OS must be emulated by a device emulator that runs in the host OS.Shadow descriptor tables must therefore be used to track changes made to the descriptor tables by the guest OS. 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. ![]() : 5 : 2 This involves denying the guest OS any access to the actual page table entries by trapping access attempts and emulating them instead in software. ![]() #Cpuinfo mac softwareBecause 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.
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