Introduction to Virtual Machines

Introduction to Virtual Machines

• Virtual Machines are an important technology that allow one physical computer to run multiple operating systems at the same time. Although the idea of virtual machines started in the 1960s, they have become extremely popular in recent years due to the need for better isolation, security and resource sharing in modern computing systems.

• A Virtual Machine (VM) is a software‒based environment that behaves like a complete computer system. It includes its own virtual CPU, memory, storage and operating system.

• A single physical computer (called the host) can run multiple VMs at the same time. Each VM runs its own operating system and applications as if it were a separate machine.

Examples of system‒level VMs :

■ IBM VM/370

■ VMware ESX Server

■ Xen

■ VirtualBox

1. Why Virtual Machines Became Popular?

• Virtual machines gained importance due to several reasons:

1. Improved isolation and security: Each VM is separated from others, so one VM cannot easily affect another.

2. Problems with traditional operating systems: Modern OSes sometimes have security and reliability issues; VMs provide an extra protection layer.

3. Resource Sharing (Cloud computing) : In environments like Amazon Web Services (AWS) EC2, VMs enable the efficient sharing of a single server among many customers, maximizing hardware utilization.

4. Faster processors : Modern processors are very powerful, so the additional overhead of running VMs is acceptable.

2. Components of Virtualization

• Virtual machine technology relies on specific software and hardware interactions :

1. Host and Guest

■ Host: Physical hardware platform (the real computer).

■ Guest: The Virtual Machine, including its operating system and applications, running on the host hardware.

2. Virtual Machine Monitor (VMM) or Hypervisor

■ The VMM (or Hypervisor) is the core piece of software that manages the VMs. It is much smaller than a traditional OS (sometimes only around 10,000 lines of code) and has three primary responsibilities:

♦ Interface presentation : Presents the illusion of a complete system to each guest OS.

♦ Resource mapping : Determines how virtual resources (like virtual CPU time, virtual disk space, virtual network) are mapped to the physical resources of the host. Resources can be time‒shared, partitioned or emulated in software.

♦ Isolation and protection : It must isolate the state of each guest from all others and protect itself from any malicious or faulty guest software.

3. Benefits of Virtual Machines

1. Software Management

■ VMs allow running old operating systems (e.g., DOS) alongside new ones.

■ Developers use VMs to test new software without affecting their main system.

2. Hardware Management

■ VMs allow multiple applications and OS versions to run on the same machine.

■ Some hypervisors can migrate a running VM to another physical computer without stopping it useful for load balancing and maintenance.

Example: Virtual Machines in Cloud Computing (AWS EC2)

• Amazon Web Services (AWS) uses VMs heavily in its EC2 cloud service. VMs help AWS to:

1. Isolate users from each other

2. Easily distribute software images

3. Stop or "kill" VMs to control resource usage

4. Hide the underlying hardware so old and new servers can coexist.

5. Offer different price categories of machines on the same hardware.

4. Virtualization Overhead

• The performance cost of virtualization depends on the type of program:

User Level CPU‒bound programs

■ Use mostly the processor and make few system calls.

■ The VMM rarely needs to intervene.

■ Almost zero overhead → runs at near native speed.

I/O‒intensive programs

■ Perform many system calls (disk, network, devices).

■ The VMM must check or emulate many privileged instructions.

■ High overhead.

I/O‒bound workloads

■ Spend most time waiting for slow I/O devices.

■ Processor is often idle, so virtualization cost is hidden.

■ The virtualization overhead depends on :

1. How many instructions the VMM must emulate.

2. How long each emulation takes.

• To reduce this overhead, if the virtual machine uses the same instruction set as the real hardware, the VMM tries to let most instructions run directly on the actual processor instead of emulating them. This makes the VM much faster.

5. Requirements of a Virtual Machine Monitor

• A Virtual Machine Monitor (VMM) has the responsibility of creating and managing virtual machines. It must:

1. Provide a software interface that looks like real hardware to the guest operating systems and applications.

2. Keep each guest VM isolated, so that the actions of one VM do not affect another.

3. Protect itself from any incorrect or harmful behavior of the guest software.

• The qualitative requirements are:

■ The guest software should behave as if it is running on real hardware, except for minor differences due to performance or sharing of limited resources.

■ The guest OS must not be able to directly change the actual hardware resources of the system.

• To fully virtualize the processor, the VMM must control almost all important activities, such as:

■ Access to privileged processor state

■ Input/Output (I/O) operations

■ Exceptions and interrupts

Even though the guest OS is running, the VMM remains in control.

Example : When a timer interrupt occurs :

1. The VMM stops the currently running VM.

2. It saves the VM's state.

3. It handles the interrupt.

4. It selects which VM should run next.

5. It restores that VM's state.

• Each guest VM is given a virtual timer and an emulated timer interrupt, provided by the VMM.

• The VMM must always be at a higher privilege level than the guest OS. The guest OS generally runs in user mode, ensuring that any attempt to run privileged instructions will cause a trap to the VMM.

Hardware Requirements for Virtualization

• For effective virtualization, the hardware must provide :

1. At least two modes of operation

■ System (kernel) mode

■ User mode

2. A set of privileged instructions that can be executed only in system mode. If these instructions are executed in user mode, they must generate a trap, allowing the VMM to take control.

6. ISA Support for Virtual Machines

• Virtual machines work best when the Instruction Set Architecture (ISA) is designed with virtualization in mind. If the ISA supports virtualization, fewer instructions need to be emulated by the Virtual Machine Monitor (VMM) and the virtual machine can run almost directly on the hardware. Such an ISA is called virtualizable. Examples include IBM 370 and RISC‒V.

• Many popular ISAs such as x86, ARMv7 and MIPS were originally designed without considering virtualization. These architectures contain some instructions that :

■ access sensitive hardware state

■ behave differently depending on the privilege level

■ do not trap when executed in user mode

• This creates problems for virtualization.

Handling Guest Operating Systems

• A guest operating system must run in user mode so that it cannot directly access real hardware. When the guest OS executes a privileged instruction (like enabling or disabling interrupts), it should generate a trap, allowing the VMM to:

1. intercept the instruction

2. update or simulate the virtual hardware state

3. return control to the guest OS

• This ensures that the guest OS interacts only with virtual resources, not real ones.

• If the ISA does not automatically trap on sensitive instructions executed in user mode :

■ The VMM must find and handle all such instructions manually.

■ It must rewrite, emulate, or modify them before execution.

■ This makes the VMM more complex, and

■ It reduces VM performance, because more instructions need software emulation.

7. Protection and Instruction Set Architecture

• Protection in a computer system is handled by both the hardware and the operating system. When virtual memory became common, some old instruction sets needed changes because certain instructions did not work correctly in virtual machines.

• Example : In x86, the instruction POPF updates the processor flags. In user mode, it Mupdates all flags except the Interrupt Enable (IE) flag, and it does not cause a trap. But a guest OS running inside a VM expects IE to change, so this causes incorrect behavior.

• To make virtual machines efficient, IBM mainframes historically used three techniques:

1. Lower the cost of processor virtualization.

2. Reduce interrupt overhead.

3. Deliver interrupts directly to the correct VM without involving the VMM.

Review Questions

1. What is a Virtual Machine? Why VMs became popular ?

2. Explain the components of virtualization.

3. Explain how the VMM provides isolation between multiple virtual machines.

4. What is a Hypervisor or VMM ?

5. State any two advantages of virtual machines.

6. What is virtualization overhead? Explain how CPU‒bound, I/O‒bound and I/O‒intensive programs behave under virtualization.

7. Discuss the requirements of a Virtual Machine Monitor (VMM).

8. Describe the hardware and ISA requirements necessary to support efficient virtualization.

9. Explain the protection issues related to instruction set architectures in virtual machines