Virtual machine monitors (VMM) emulate most guest access to interrupts and the advanced programmable interrupt controller (APIC) in a virtual environment. They also virtualize all guest interrupts. These activities require the exit and reentry of the virtual machines (VM), but they are time consuming and are a major source of overhead. in order to minimize that effect, the latest Intel(R) Xeon(R) processors, such as the Intel(R) Xeon(R) E5-2600 v2 Product Family, emulate those activities in the hardware.
Figure 1 shows that all virtualized activities relating interrupts and APIC to and from the guest OS have to go through VMM in systems without APICv; however, in systems with APICv, they are executed in the hardware, not in the VMM. This way all activities can stay inside the VM, thus eliminating the need to issue the “VM exit” command; this change results in reduced overhead and increased I/O throughput.