text: Binary translation is used to rewrite certain ring 0 instructions in terms of ring 3 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.