The virtual-machine concept provides complete protection of system resources since each virtual machine is isolated from all other virtual machines. What might be bad about this?
This isolation, however, permits no direct sharing of resources.
A virtual-machine system is a perfect vehicle for operating-systems research and development. System development is done on the virtual machine, instead of on a physical machine and so does not disrupt normal system operation.
The virtual machine concept is difficult to implement due to the effort required to provide an exact duplicate to the underlying machine.
Explicit calls into the hypervisor by the guest OS. Used by guest OS for things like updating hardware page tables.
Time and timers
Guest OSs made aware of real time and virtual time.
Virtual time advances only when the guest OS is executing. Used for scheduling by the guest OS.
Xen-provided timers are used by guest OS.
Solves one efficiency problem with VMware Workstation.
Guest XP causes host to perform poorly, because must constantly deliver timer interrupts to XP to do things like smooth transition animations (like minimizing a window, etc.). Forcing the guest to use XP provided timer would eliminate the need to virtualize these timer interrupts.
Reserved for each guest OS instance at time of creation.
Provides strong isolation.
Isolation is tricky, but batching of requests provides some degree of it.