A guest operating system that executes within a virtual machine expects a zero-based physical address space, as provided by real hardware. ESX Server gives each VM this illusion, virtualizing physical memory by adding an extra level of address translation. Borrowing terminology from Disco [3], a machine address refers to actual hardware memory, while a physical address is a software abstraction used to provide the illusion of hardware memory to a virtual machine. We will often use ``physical'' in quotes to highlight this deviation from its usual meaning.
ESX Server maintains a pmap data structure for each VM to translate ``physical'' page numbers (PPNs) to machine page numbers (MPNs). VM instructions that manipulate guest OS page tables or TLB contents are intercepted, preventing updates to actual MMU state. Separate shadow page tables, which contain virtual-to-machine page mappings, are maintained for use by the processor and are kept consistent with the physical-to-machine mappings in the pmap.1This approach permits ordinary memory references to execute without additional overhead, since the hardware TLB will cache direct virtual-to-machine address translations read from the shadow page table.
The extra level of indirection in the memory system is extremely powerful. The server can remap a ``physical'' page by changing its PPN-to-MPN mapping, in a manner that is completely transparent to the VM. The server may also monitor or interpose on guest memory accesses.