Linux as a Virtualization Guest VIrtual Machines

In this guide, you’ll learn how to manage virtual machines (VMs) on Linux using QEMU-KVM and Libvirt’s virsh CLI. By the end, you will be able to install dependencies, define VM domains via XML, control VM lifecycle, adjust resources, and clean up definitions—all from the command line. This workflow is fundamental for self-hosted virtualization and supports major cloud platforms like AWS, DigitalOcean, and Google Cloud.

Example: A single Linux server with 64 CPU cores and 1 TiB RAM can host dozens of isolated VMs, each with its own dedicated vCPU and memory allocation.

1. Installing Dependencies

Install QEMU, KVM, and Libvirt tools:

sudo dnf install -y qemu-kvm libvirt

• qemu-kvm: Hardware-accelerated virtualization
• libvirt: API and utilities for managing VMs

sudo systemctl enable --now libvirtd

2. Defining a Virtual Machine Domain

Libvirt uses XML to describe VM configurations (called domains). Create a file named testmachine.xml:

vim testmachine.xml

Paste this minimal configuration:

<domain type="kvm">
  <name>TestMachine</name>
  <memory unit="GiB">1</memory>
  <vcpu>1</vcpu>
  <os>
    <type arch="x86_64">hvm</type>
  </os>
</domain>

This sets:

• Name: TestMachine
• Memory: 1 GiB
• vCPUs: 1
• OS: Hardware VM on x86_64

Save and exit.

3. Defining and Listing Domains

Register the domain with Libvirt:

virsh define testmachine.xml

Expected output:

Domain 'TestMachine' defined from testmachine.xml

List all domains, including inactive:

virsh list --all

Sample:

4. VM Lifecycle Management

Use the following commands to control the VM. See the quick reference table below.

# Example: Start the VM
virsh start TestMachine

# Verify running VMs
virsh list

reset simulates pressing the reset button.
destroy cuts power immediately—like unplugging the VM.

5. Deleting a Domain

Remove the domain definition only:

virsh undefine TestMachine

virsh undefine TestMachine --remove-all-storage

6. Autostart Configuration

Enable the VM to start on host boot:

virsh autostart TestMachine

Disable autostart:

virsh autostart --disable TestMachine

7. Inspecting VM Details

Retrieve detailed VM information:

virsh dominfo TestMachine

Sample output:

Id:             -
Name:           TestMachine
UUID:           01a57937-3fbb-4191-9cec-a73383456fa8
OS Type:        hvm
State:          shut off
CPU(s):         1
Max memory:     1048576 KiB
Used memory:    1048576 KiB
Persistent:     yes
Autostart:      disable

8. Modifying VM Resources

Adjusting vCPUs

Increase maximum vCPUs:

virsh setvcpus TestMachine 2 --config --maximum

Set vCPUs for next boot:

virsh setvcpus TestMachine 2 --config

Adjusting Memory

Set maximum memory:

virsh setmaxmem TestMachine 2048M --config

Verify changes:

virsh dominfo TestMachine

9. Starting and Verifying Changes

Start the VM with updated resources:

virsh start TestMachine

Confirm the running state:

virsh dominfo TestMachine

Expected fields:

State:        running
CPU(s):       2
Max memory:   2097152 KiB
Used memory:  2097152 KiB

Quick Reference: Common virsh Commands

References

• Libvirt Documentation
• QEMU Official Website
• KVM on Linux
• Cloud Providers Comparison