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4.5 Virtual Machine Monitor
During the whole VM lifetime, the VMM has access to VM (physical) memory, which
can be used for efficient shared memory communication between the VM and the VMM.
An alternative to allocating all memory at once is demand paging. Whenever the VM
accesses a page that has not been used before, the VMM receives a page fault and maps
a fresh page into the VM.
In this thesis I chose the former approach because it allows the mapped memory region
to be contiguous in physical memory. Whereas this allocation scheme is more wasteful
on system memory, it simplifies the address translation if the guest is allowed to directly
drive DMA enabled devices.
4.5.2 Platform Virtualization
As described in Chapter 2.2.4, the VMM has to implement platform devices such as an
interrupt controller (IC) and a timer device.
For the design of the IC, it is reasonable to align it with the high level abstractions of
Linux. If done so, the Linux operations correspond directly to IC ones. As for a virtual
device, interrupts are flagged by helper threads in a shared data structure, and delivered
to the VM by the VMM’s control loop.
The timer thread periodically blocks with a timeout, upon which it flags the timer
interrupt at the virtual IC.
4.5.3 Peripheral Devices
For a VM to be useful, it has to be provided with virtual devices. The L4 system
includes device managers, servers that multiplex access to hardware. For example Ankh
implements a virtual network switch, thereby multiplexing the physical network card.
Similarly, we have a secure console that securely multiplexes the system’s frame buffer
and maintains a secure input indicator, which cannot be forged by clients. These device
managers consist of a device driver and multiplexing logic. Client communication with
device managers is done with IPC and shared memory. These services are a natural
choice for the back ends of virtual devices.
The VMM makes use of to the device managers to present the VM with a custom
interface. The guest OS needs special drivers to communicate with the virtual devices.
Because these drivers implement an abstracted interface only, I will call them stub
drivers. The VMM’s task is limited to translating the stub driver’s commands into IPC
messages, which are forwarded to the device manager.
Asynchronous events, such as the availability of packets, are signaled using IPC or
asynchronously using User-Irqs. For each device manager, the VMM employs a blocking
helper thread to receive the notification and flag the event as interrupt at the virtual IC.
Virtio provides a para-virtualization interface, which is used by Lguest and by KVM
(optionally). It implements device drivers for a network card, a serial console and a
generic block driver [Rus08]. It is up to the VMM to implement back ends to the virtio
device interfaces. By using the virtio interface, I could benefit from existing drivers,
but would have to implement complex mappings to the L4 device manager’s interfaces,
which would significantly increase the complexity of the VMM.
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