- Better protection in case of system compromise. If an attacker gains access to one OS, other OSes are not affected. Furthermore, for a hypervisor that supports snapshotting, if a system is compromised, its image can be restored to previously known good state.
- Consolidation of hardware and flexible control. An OS image can move from machine to machine. This can be used to keep more machines utilized; the other underutilized machines can powered off to save energy.
- Power management (ACPI)
- Disk controllers (Parallel ATA, AHCI)
- Video display (vendor specific, but most have VBE BIOS for basic non-accelerated 2D graphics)
- Networking (vendor specific)
- Sound card (also vendor specific), and
- USB or Firewire peripherals (OHCI and UHCI).
A hypervisor is sometimes designed to run on top of a host OS, and its anti-device drivers must translate raw I/O request back to the high-level I/O interface that a host OS provides. This translation is necessary so it can run unmodified OS that supplies a wealth of device drivers. But this translation is a waste of effort for operating system researchers who want to get an OS up and running quickly. They do not have resources to hire developers to write device drivers. Their own time can be better spent than reading device specifications.
When Sony introduced PlayStation 3, one of its more interesting capability is to be able to run Linux on it. Linux doesn't have direct access to most of the devices on PlayStation 3 (most notably the video card RSX hardware). However, hypervisor on PlayStation 3 provides a hypervisor call API that allows restricted frame buffer, hard drive, and USB access. Most of the Linux device drivers on PlayStation 3 are straightforward hypervisor calls. These hypervisor calls are undocumented, but their functionality can be deduced by how a call is used in the open source Linux kernel.
Back in the MS-DOS days, IBM ROM BIOS provides rudimentary drivers to interface with devices via BIOS call.
- A jiffy counter in the BIOS data area around 0x400, updated by IRQ1/Int 8h handler.
- A keyboard driver handles key-presses on IRQ1/Int 9h by putting keystrokes in a simple ring buffer, which is read using the keyboard service software interrupt Int 16h.
- Floppy and fixed disk (hard disk) controller handling IRQ5 and IRQ6. It also provides a synchronous, blocking disk I/O service via Int 13h.
MS-DOS provide basic file system, character I/O devices (serial port, printer, and console), memory and process management. Most applications still use BIOS calls if they want to. Computer viruses were particularly interested in intercepting BIOS calls to find opportunities to infect disks. However, games had to write their own video and sound drivers because these hardware types were not as standardized.
Sony PlayStation 3 hypervisor call looks very much like BIOS calls.
The PlayStation 3 hypervisor does not run multiple OSes at the same time, so it does not arbitrate resource allocation. However, its purpose is to restrict access to their hardware, so the hardware interface can remain proprietary.
A general purpose hypervisor, however, can also provide hypervisor calls. This allows better integration of guest OS with the host. For example, users in the host OS could resize the virtual machine window, and the guest OS is notified of resizing. The hypervisor could also provide OpenGL support for 3D acceleration. Many commercial virtual machine products accomplish this by supplying a proprietary guest-addition driver that bypasses hardware emulation and uses undocumented hypervisor API.
Now, it would be nice to have a standardized hypervisor API, so operating system researchers can put together a fully functional OS quickly to gain support, focusing only on the architecture and less on device drivers. Such experimental OS can also be run on hardware with a non-multitasking hypervisor (like Sony PlayStation 3), providing abstract hardware access. In essence, it would be extremely convenient for operating system research and development if we have something like ROM BIOS back in the days.