The principle of virtualization technology

Virtual machine is the abstraction and simulation of real computing environment. VMM needs to allocate a set of data structures for each virtual machine to manage their state, including the full set of registers of the virtual processor, the use of physical memory, the state of virtual devices and so on. When VMM schedules a virtual machine, it will restore part of its state to the host system. Not all states need to be restored. For example, the physical address of the page table set by VMM is stored in the host CR3 register instead of the value set by the guest operating system. The host processor directly executes the machine instructions of the guest operating system. Because the Guest OS runs at a low privilege level, when accessing the privileged state of the host system (such as writing GDT registers), the host processor will be abnormal due to insufficient authority, and the running right will automatically return to VMM. In addition, the arrival of external interrupts will also lead to the operation of VMM.

VMM may need to write the current state of the virtual machine back to the state data structure, analyze the reason why the virtual machine hangs, and then perform the corresponding privileged operation on behalf of the guest operating system. In the simplest case, for example, when the guest operating system modifies the CR3 register, it only needs to update the state data structure of the virtual machine. Generally speaking, in most cases, VMM needs to go through a complicated process to complete the original simple operation. Finally, the VMM returns the running right to the guest operating system, and the guest operating system resumes execution from the last interruption point, or handles virtual interrupts and exceptions that VMM "inserts". This classic virtual machine operation mode is called "capture and simulate". The virtual machine is completely transparent to the guest operating system, and the guest operating system does not need any modification, but the design of VMM will be more complicated and the overall performance of the system will be obviously damaged.