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6 / 23Design Aspectes◮ Guest OSes have to be modified to run at a lowerpriv. level, i.e. ring 1 instead of 0 on x86.◮ Privileged instructions are paravirtualized by requiringthem to be validated within Xen.◮ Exception handlers generally stay the same, exceptfor the Page Fault handler⇒ CR2 not accessible so we write to the stack frame◮ Only page faults and syscalls occur often enough toaffect performance⇒ a ‘fast’ exception handler is made available⇒ only pagefaults must be delivered via Xen◮ I/O via a clean set of device abstractions