Virtual machine structure and benefits

Structure

A virtual machine (VM) system (IBM VM/370) separates multiprogramming from the "extended machine" function. A bare-metal program called the virtual machine monitor (hypervisor) runs directly on the hardware and creates several virtual machines, each an exact copy of the real hardware (including kernel/user mode, I/O and interrupts). Each virtual machine runs its own operating system, which may be different on each VM.

 VM1         VM2         VM3
+-------+   +-------+   +-------+
| App   |   | App   |   | App   |
| Guest |   | Guest |   | Guest |
| OS 1  |   | OS 2  |   | OS 3  |
+-------+   +-------+   +-------+
+---------------------------------+
| Virtual machine monitor         |
+---------------------------------+
| Real hardware (CPU, RAM, disk)  |
+---------------------------------+

When an application makes a system call, it traps to the guest OS in its own VM. When that guest OS then executes a privileged instruction (for example, an I/O instruction), it traps to the monitor, which carries it out (emulates it) for that VM.

Benefits over other OS structures

• Complete protection and isolation: VMs are separate, so a crash or virus in one VM does not affect the others or the host.
• Different OSs on one machine: Linux, Windows and others run side by side, which helps portability testing.
• Safe system development: OS development and testing can be done on a VM without halting the production machine.
• Better hardware utilisation: many lightly loaded servers can be consolidated onto one physical machine.
• Easy backup and migration: a VM image can be saved, cloned or moved to another host.

The drawbacks are slower I/O and CPU performance because of the extra layer, and difficulty in sharing resources between VMs.