I/O Devices and Interrupt Model

Denali emulates a switched Ethernet LAN connecting all VMs. Each VM is assigned a virtual Ethernet NIC; from the perspective of external physical hosts, it appears as though each VM has its own physical Ethernet card. VMs interact with the virtual NIC using standard programmed I/O instructions, although the interface to the virtual NIC is drastically simpler than physical NICs, consisting only of a packet send and a packet receive operation. On the reception path, the isolation kernel emulates an Ethernet switch by demultiplexing incoming packets into a receive queue for the destination virtual machine. VMs can only process packets during their scheduler quantum, effectively implementing a form of lazy-receiver processing [11]. On the transmit path, the kernel maintains a per-machine queue of outbound packets which a VM can fill during its scheduler quantum. These transmit queues are drained according to a packet scheduler policy; Denali currently processes packets in round-robin order from the set of actively sending VMs.

Denali provides virtual disk support to VMs. The isolation kernel contains a simple file system in which it manages persistent, fixed-sized virtual disks. When a VM is instantiated, the isolation kernel exposes a set of virtual disks to it; the VM can initiate asynchronous reads and writes of 4KB blocks from the disks to which it has been given access. Because virtual disks exist independently of virtual machines, Denali trivially supports optimizations such as the read-only sharing of virtual disks across VMs. For example, if multiple VMs all use the same kernel boot image, that boot image can be stored on a single read-only virtual disk and shared by the VMs.

Denali also emulates a keyboard, console, and timer devices. These virtual devices do not differ significantly from physical hardware devices, and we do not describe them further.

Denali's batched interrupt model is implemented by maintaining a bitmask of pending interrupts for each VM. When a virtual interrupt arrives, the kernel posts the interrupt to the bitmask, activates the VM if it is idle, and clears any pending timeouts. When a VM begins its next quantum, the kernel uploads the bitmask to a virtual register, and transfers control to an interrupt handler. A VM can disable virtual interrupts by setting a virtual register value; VMs can never directly disable physical interrupts.