There are 3 components:
(1) VMM-n (native) - This component runs in native mode. It is the component that intercepts traps or patch critical instruction in VM. this may provide drivers for performance purpose or those not available in host OS.
(2) VMM-u (user) - This component runs in user mode, similar to an applicaiton to the host OS. This component issues resource requests to the host OS, in particular memory and I/O request on behalf of VMM-n. VMM-u issue requests using the system library fucntion provided by the host OS.
(3) VMM-d (driver) - This component provides a means for communication between VMM-n and VMM-u. This makes VMM-n appears as a special device to VMM-u.
The advantage of a hosted VMM is easy installation and the fact that the actual device drivers do not have to be incorporated into VMM. The disadvanatages are
(1) VMM-n operates in privileged mode alongside with the host OS. There could be compatibilty problem which may corrupt each other's memory.
(2) The allocaiton of resources is completely under the control of host OS. The effects of allocation prolicies in the VMM are less predictable because the VMM has insifficient knowledge and control over the use of resources.
Performance disadvantage compared to a native VM because the need to go back and forth between VMM-n and host Os. The perofrmnace degratdation is more significant in an I/O intensive workload than in a user-mode CPU-intensive workload.
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