Process VM state mapping

It refers to the mapping of register and memory of guest process to the host address space. In the host addres space, guest registers could be mapped to the host regisers or in register conext block in memory (runtime data) of the host address space. The guest code and data will be map into memory, together with the emulatior (runtime code).

Memory mapping from guest to host address space conceptually uses a mapping table. The mechanism is similar tothe virtual-to-real address translation (base address translation then forming the address by adding offset). This emulation using software has high overhead. This approach is most flexible as consecutive memory blocks in guest can be dispersed in non-consecutive blocks in guest address space. To simplify the translation, we can cosnider address space mapping methods that rely more on the inderlying hardware than VM. Both cases assuming the host address space is larget than the guest:

(1) the guest address space is map continuously in the host address space above the runtime code. In this case, the host address = guest address + (length of runtime)
(2) the guest address space is map continuously in the host address space starting at the same offset. in this case, host address = guest address. Runtime is relocated to a location above the guest address space.

It is apparent that the relative size of guest and host address space has significant implicaiton to the choice of mapping method. Whether the run time can be placed in arbitrary area outside the confine of guest address apce is also another improtant factor.

Emulator needs to deal with memory model as it mimic the OS the guest process thought run in. For example, guest process may allocate a memory block with some protection setting and emulator needs to mimic the memory model to be compatible. In general, user application sees 3 main features:
(1) overall structure of the address space e.g. segment or flat
(2) access privilege (R, W, E)
(3) protection and allocation granularity - smallest unit that the OS can allocate and protect for the application.

The complexity of mapping of a page in guest to host depends on the relative page size and protection types available in both platform. If the host page size is larger and protection types is more comprehensive than the guest, it is possible tomap the guest page to host page directly thus letting the underlying hardware to enforce the allocation and protection. Otherwise, some software mapping and interfence from EM is required which is more complex and slow.