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Denali's virtual Ethernet has been streamlined for simplicity and performance. Real hardware network adapters often require multiple programmed I/O instructions to transmit or receive a single packet. For example, the Linux pcnet32 driver used by VMWare workstation [31] issues 10 PIOs to receive a packet and 12 PIOs to transmit a packet. VMMs which support unmodified legacy OSs must trap and emulate these PIOs, resulting in additional overhead. By contrast, Denali's virtual Ethernet requires only a single PIO to send or receive a packet.
To estimate the benefit of Denali's simple virtual Ethernet, we modified the guest OS device driver to perform as many PIOs as the pcnet32 driver. Doing so increased the packet reception cost by 18,381 cycles (10.9 ms) and the packet transmission cost by 22,955 cycles (13.7 ms). This increases the overhead of receiving a single 100-byte UDP packet by 42%.