KVM/libvirt Virtualization

Check Hardware Virtualization Support

Before installing KVM, you must confirm that the CPU supports hardware virtualization (Intel VT-x or AMD-V).

# Check if the CPU supports virtualization extensions
grep -Ec '(vmx|svm)' /proc/cpuinfo

If the return value is greater than 0, the CPU supports virtualization.

# Check if KVM kernel modules are loaded
lsmod | grep kvm

You should see kvm and either kvm_intel (Intel CPU) or kvm_amd (AMD CPU).

Caution

If running inside a virtual machine (nested virtualization), the host must have nested virtualization enabled. On physical machines, ensure VT-x or AMD-V is enabled in the BIOS/UEFI.

Install KVM and Related Tools

EL 9 / EL 10 (RHEL / Alma / Rocky)

sudo dnf install -y qemu-kvm libvirt virt-install virt-viewer \
  libvirt-client
# bridge-utils (brctl) is deprecated since EL8; the nmcli steps below do not need it

EL 8 (RHEL 8; CentOS Stream 8 is EOL)

sudo dnf install -y @virt qemu-kvm libvirt virt-install \
  virt-viewer bridge-utils

CentOS 7

sudo yum install -y qemu-kvm libvirt virt-install virt-viewer \
  bridge-utils libvirt-client

If you need a graphical management tool:

sudo dnf install -y virt-manager

Start the libvirtd Service

sudo systemctl enable --now libvirtd

Verify:

sudo systemctl status libvirtd
virsh version

Add Your User to the libvirt Group

sudo usermod -aG libvirt $(whoami)

Log out and back in for the change to take effect. After that, you can use virsh as a regular user to connect to system-level VMs.

virsh Basic Commands

virsh is the command-line management tool for libvirt, used to manage the full lifecycle of virtual machines.

VM Lifecycle Management

# List running VMs
virsh list

# List all VMs (including powered-off ones)
virsh list --all

# Start a VM
virsh start myvm

# Graceful shutdown
virsh shutdown myvm

# Force power off (equivalent to pulling the power plug)
virsh destroy myvm

# Reboot a VM
virsh reboot myvm

# Suspend/resume a VM
virsh suspend myvm
virsh resume myvm

VM Information Queries

# View VM details
virsh dominfo myvm

# View VM XML configuration
virsh dumpxml myvm

# View VM VNC display address
virsh vncdisplay myvm

# View disks used by a VM
virsh domblklist myvm

# View VM network interfaces
virsh domiflist myvm

VM Configuration Modifications

# Edit VM XML configuration
virsh edit myvm

# Set VM to auto-start on boot
virsh autostart myvm

# Disable auto-start
virsh autostart --disable myvm

# Modify VM memory (requires powered-off state)
virsh setmaxmem myvm 4G --config
virsh setmem myvm 4G --config

# Modify vCPU count (requires powered-off state)
virsh setvcpus myvm 4 --config --maximum
virsh setvcpus myvm 4 --config

Create VMs with virt-install

Install from ISO

sudo virt-install \
  --name almalinux9 \
  --ram 2048 \
  --vcpus 2 \
  --disk path=/var/lib/libvirt/images/almalinux9.qcow2,size=20,format=qcow2 \
  --os-variant almalinux9 \
  --network network=default \
  --graphics vnc,listen=0.0.0.0 \
  --cdrom /data/iso/AlmaLinux-9-latest-x86_64-minimal.iso \
  --boot uefi

Tip

Use the osinfo-query os command to view all available --os-variant values. If the command is not available, install libosinfo:

sudo dnf install -y libosinfo

Automated Installation with Kickstart

sudo virt-install \
  --name almalinux9-auto \
  --ram 2048 \
  --vcpus 2 \
  --disk path=/var/lib/libvirt/images/alma9-auto.qcow2,size=20,format=qcow2 \
  --os-variant almalinux9 \
  --network network=default \
  --graphics none \
  --location /data/iso/AlmaLinux-9-latest-x86_64-minimal.iso \
  --initrd-inject /data/ks/alma9-ks.cfg \
  --extra-args "inst.ks=file:/alma9-ks.cfg console=ttyS0"

Quick Creation from Cloud Images

# Download the cloud image
wget https://repo.almalinux.org/almalinux/9/cloud/x86_64/images/AlmaLinux-9-GenericCloud-latest.x86_64.qcow2

# Copy and resize the disk
cp AlmaLinux-9-GenericCloud-latest.x86_64.qcow2 /var/lib/libvirt/images/myvm.qcow2
qemu-img resize /var/lib/libvirt/images/myvm.qcow2 20G

# Create a VM using cloud-init configuration
sudo virt-install \
  --name myvm \
  --ram 2048 \
  --vcpus 2 \
  --disk /var/lib/libvirt/images/myvm.qcow2 \
  --os-variant almalinux9 \
  --network network=default \
  --import \
  --cloud-init root-password-generate=on,disable=on \
  --graphics none

Managing VMs

Snapshot Management

# Create a snapshot
virsh snapshot-create-as myvm --name "before-update" --description "Before system update"

# List snapshots
virsh snapshot-list myvm

# Revert to a snapshot
virsh snapshot-revert myvm --snapshotname "before-update"

# Delete a snapshot
virsh snapshot-delete myvm --snapshotname "before-update"

Disk Management

# Create a new disk
qemu-img create -f qcow2 /var/lib/libvirt/images/data-disk.qcow2 50G

# Hot-add a disk to a VM
virsh attach-disk myvm /var/lib/libvirt/images/data-disk.qcow2 vdb \
  --driver qemu --subdriver qcow2 --persistent

# View disk information
qemu-img info /var/lib/libvirt/images/myvm.qcow2

# Detach a disk
virsh detach-disk myvm vdb --persistent

Clone a VM

# Power off the source VM first
virsh shutdown myvm

# Clone
virt-clone --original myvm --name myvm-clone \
  --file /var/lib/libvirt/images/myvm-clone.qcow2

Delete a VM

# Power off the VM
virsh destroy myvm

# Remove the VM definition
virsh undefine myvm

# Remove along with all storage
virsh undefine myvm --remove-all-storage

virt-manager Graphical Management

virt-manager provides a graphical interface for VM management.

Local Usage

Launch directly in a desktop environment:

virt-manager

Remote Management via SSH

Connect to a remote server’s libvirt from your local workstation:

virt-manager -c qemu+ssh://root@192.168.1.100/system

Or in the virt-manager interface, select “File > Add Connection”, choose QEMU/KVM, and check “Connect to remote host over SSH”.

Tip

If you don’t use a desktop environment, you can install virt-manager locally and manage VMs on a remote server via an SSH tunnel, without needing a graphical interface on the server.

Network Configuration

Default NAT Network

libvirt creates a default NAT network named default (typically 192.168.122.0/24).

# List networks
virsh net-list --all

# View network details
virsh net-info default

# Start the default network
virsh net-start default

# Set the default network to auto-start
virsh net-autostart default

Configure a Bridged Network

A bridged network allows VMs to connect directly to the physical network, obtaining IP addresses in the same subnet as the host.

1. Create a bridge using nmcli:

   # Create bridge br0
   sudo nmcli connection add type bridge con-name br0 ifname br0
   
   # Configure bridge IP (modify according to your network environment)
   sudo nmcli connection modify br0 \
     ipv4.addresses 192.168.1.100/24 \
     ipv4.gateway 192.168.1.1 \
     ipv4.dns "223.5.5.5,8.8.8.8" \
     ipv4.method manual
   
   # Add the physical NIC to the bridge
   sudo nmcli connection add type bridge-slave con-name br0-ens192 \
     ifname ens192 master br0
   
   # Activate the bridge (note: remote operation may disconnect the session)
   sudo nmcli connection up br0
   sudo nmcli connection down ens192

2. Create a libvirt bridge network definition file /tmp/bridge-net.xml:

   <network>
     <name>host-bridge</name>
     <forward mode="bridge"/>
     <bridge name="br0"/>
   </network>

3. Register the bridge network in libvirt:

   virsh net-define /tmp/bridge-net.xml
   virsh net-start host-bridge
   virsh net-autostart host-bridge

4. Use the bridged network when creating a VM:

   sudo virt-install \
     --name bridged-vm \
     --ram 2048 \
     --vcpus 2 \
     --disk path=/var/lib/libvirt/images/bridged-vm.qcow2,size=20 \
     --os-variant almalinux9 \
     --network network=host-bridge \
     --cdrom /data/iso/AlmaLinux-9-latest-x86_64-minimal.iso

Caution

When configuring bridged networking remotely via SSH, switching network settings may cause the connection to drop. It is recommended to perform this operation through an out-of-band management tool such as IPMI/iDRAC/iLO or from a local console.

Switch Networks for an Existing VM

# Power off the VM
virsh shutdown myvm

# Edit the VM configuration
virsh edit myvm

In the XML, find the <interface> section and change it to:

<interface type='network'>
  <source network='host-bridge'/>
  <model type='virtio'/>
</interface>

Start the VM:

virsh start myvm

Performance Optimization Tips

# Enable KSM (Kernel Same-page Merging) to save memory
sudo systemctl enable --now ksm
sudo systemctl enable --now ksmtuned

# Check KSM status
cat /sys/kernel/mm/ksm/pages_shared

VM configuration recommendations:

• Use virtio drivers for the best disk and network performance
• Prefer the qcow2 disk format (supports snapshots and thin provisioning)
• Consider using raw format disks for I/O-intensive workloads
• Allocate Hugepages for VMs to improve memory performance