KVM, QEMU, and libvirt

This post is taking a look at another suite of software to create and manage Virtual Machines. For a background on computer virtualization, check out a previous post.

Linux distributions support virtualization through the Linux kernel which is why the software is called KVM. This stands for “Kernel-based virtual machine”. It consists of a loadable kernel module, kvm.ko, that provides the core virtualization infrastructure and a processor specific module, kvm-intel.ko or kvm-amd.ko.

In order to leverage the kernel module, we will use another software called QEMU which is an open source machine emulator. QEMU emulates “hardware” such as a processor, memory, network interface, and storage. While qemu has a command line interface, it is mostly used by engineers who are developing the platform. Libvirt provides an abstraction from specific versions and hypervisors and encapsulates some workarounds and best practices.

These three softwares come together to form a relatively user-friendly way to create and manage virtual machines on Linux systems.

Installation

These softwares are packaged in most Linux distributions for an easy Installation. Below is a simple ansible playbook to install the software packages on Ubuntu and Red Hat distributions. If you are not familiar with ansible, check out a previous post to get started.

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---
hosts: ubuntu
become: yes
vars:
  ubuntu_pkgs:
    - "qemu-kvm"
    - "libvirt-daemon-system"
    - "libvirt-clients"
    - "bridge-utils"
  redhat_pkgs:
    - "qemu-kvm"
    - "libvirt"
    - "virt-install"
    - "bridge-utils"
  # make sure to specify a user below
  libvirt_user: ""
  libvirt_group: "libvirt"
tasks:
  - name: install kvm on ubuntu
    apt:
      name: "{{ ubuntu_pkgs }}"
      state: present
  - name: install kvm on red hat
    dnf:
      name: "redhat_pkgs"
      state: present
  - name: add user to libvirt group
    user:
      name: "{{ libvirt_user }}"
      groups: "{{ libvirt_group }}"
      append: yes

Playbook execution

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ansible-playbook playbook.yml -i your_inventory.yml --extra-vars "libvirt_user=YOUR_USER" -K

Enter the user account you want to use to create virtual machines as an extra variable and adding the -K will ask for a sudo password as this playbook installs new packages and requires superuser access.

Install with package manager

An alternative way to install qemu/kvm is to use a distribution package manager. For example on Ubuntu, install the packages and add your user to the libvirt group.

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sudo apt update
sudo apt install qemu-kvm libvirt-daemon-system libvirt-clients bridge-utils

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sudo usermod -aG libvirt $USER

libvirt command line usage

Ensure that the libvirt service is running if you encounter any error messages:

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sudo systemctl status libvirtd

CLI create vm

Use virt-install to create a vm from the shell:

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virt-install \
# name your vm
--name=YOUR_VM \
# you can add a string comment describing the vm
--description='YOUR DESCRIPTION' \
# specify the memory in mb
--ram=2048 \
# you can alter the number of cpu threads given to the vm
--vcpus=1 \
# specify a path where the vm storage resides and size in gb
--disk path =/var/lib/libvirt/images/YOUR_VM/YOUR_VM.qcow2,size=32 \
# with this option you can add an .iso image file mounted inside the vm
--cdrom /var/lib/libvirt/images/YOUR_VM/YOUR.iso \
# create a virtual graphics adapter to connect to the vm with vnc
--graphics vnc

virsh commands

There are several utilities available to manage virtual machines and libvirt. The virsh utility can be used from the command line:

To list running virtual machines:

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virsh list --all

To start a virtual machine:

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virsh start <vm_name>

To restart a virtual machine:

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virsh reboot <vm_name>

GUI create vm

If you have a desktop environment installed, the virt-manager utility provides a gui to create, manage, and connect to virtual machines.

Ubuntu virt-manager

If it was not installed with other libvirt tools, install:

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sudo apt update
sudo apt install virt-manager

Ensure your user is in the libvirt group or run virt-manager as sudo.

For more details and how to use this tool on other distributions, check out the official site.