The Kernel Virtual Machine is a hypervisor for Linux on hardware with virtualization extensions (Intel VT or AMD-V). It is deployed as a loadable kernel modules, kvm.ko, and either kvm-intel.ko or kvm-amd.ko.

The KVM Debian Wiki rocks, and provides details on the basics including a great performance tuning section.

Install⌗

Easy instructions to get QEMU/KVM and virt-manager up and running on Arch.

Check CPU supports virtualisation grep -E "(vmx|svm)" --color=always /proc/cpuinfo
Make sure VT CPU extension is enabled in BIOS.
User access to /dev/kvm is required, so add users into kvm(78) group with sudo gpasswd -a USER_NAME kvm
Loading kernel modules kvm_intel or kvm_amd depend on your CPU, Add module name in /etc/modules-load.d/kvm.conf either kvm_intel or kvm_amd
Install qemu, virt-manager, dnsmasq and iptables with sudo pacman -S qemu virt-manager dnsmasq iptables ebtables dnsmasq
Run and enable boot up start libvirtd daemon with systemctl start libvirtd and systemctl enable libvirtd
Use PolicyKit authorization create /etc/polkit-1/rules.d/50-libvirt.rules with the example policy below.
Create the libvirt group and add users with groupadd libvirt then sudo gpasswd -a USER_NAME libvirt
Check network interface status sudo virsh net-list --all. If it is inactive start it using sudo virsh net-start default
Now you can use virt-manager GUI to build some VMs.

PolicyKit that allows the kvm group to manage libvirt:

/* Allow users in kvm group to manage the libvirt
daemon without authentication */
polkit.addRule(function(action, subject) {
    if (action.id == "org.libvirt.unix.manage" &&
        subject.isInGroup("kvm")) {
            return polkit.Result.YES;
    }
});

Administration tasks⌗

User specific vs system wide VMs⌗

virsh when not run as root, will default the connection to libvirt using qemu://session, sandboxing the view of VMs per user.

To manage system (i.e. root) level VMs either run virsh as root, or run virsh with a custom connect string virsh --connect qemu:///system list --all.

To change the default bind string can set the LIBVIRT_DEFAULT_URI env var.

$ export LIBVIRT_DEFAULT_URI='qemu:///system'

List VMs⌗

List user specific VMs:

$ virsh list --all

Or system wide VMs:

$ virsh --connect qemu:///system list --all

Or:

# virsh list --all

Start VM⌗

# virsh start ARCHBOX

Shutdown VM⌗

# virsh shutdown ARCHBOX

Murder (hung) VM⌗

# virsh destroy UBUNTUBOX

Autostart default NATed bridged network⌗

sudo virsh net-autostart default

What if the `default` network interface is not listed⌗

If virsh net-list is not listing any network interface just reinitialize it with:

 sudo virsh net-define /usr/share/libvirt/networks/default.xml

How to extend / increase a partition⌗

Shutdown the VM virsh shutdown hostname
Increase the qcow2 image. Find the qcow2 file of the VM and take a backup (just in case). cp hostname.qcow2 hostname.qcow2.backup and qemu-img resize hostname.qcow2 +100GB
Start the VM virsh start hostname
Extend the partition in Window

Use network ISO source for new VMs⌗

Using the awesome --location option:

virt-install --virt-type kvm --name buster-amd64 \
--location http://deb.debian.org/debian/dists/buster/main/installer-amd64/ \
--os-variant debian10 \
--disk size=10 --memory 1000

Windows VM disk driver⌗

Check and install fast IO driver virtio-win on guest Windows VM.

Create new VM guest with below configuration: IDE storage for Windows OS container WINDOWS.qcow2, IDE CDROM attach Windows OS ISO.
Start VM guest and install the Windows OS as usual
Shutdown VM guest
Reconfigure VM guest: Add a dummy VirtIO / VirtIO SCSI storage with 100MB size, e.g. DUMMY.qcow2, then attach VirtIO driver CD ISO to the IDE CDROM.
Restart VM guest
Install the VirtIO driver from the IDE CDROM when Windows prompt for new hardware driver
For VM guest of Windows 10 and above, run “cmd” as Administrator and run: bcdedit /set {current} safeboot minimal
Shutdown VM guest
Reconfigure VM guest with below configuration: Remove IDE storage for Windows OS DONT delete WINDOWS.qcow2, remove VirtIO storage for dummy storage you can delete DUMMY.qcow2, remove IDE storage for CD ROM, then add a new VirtIO/VirtIO SCSI storage and attach WINDOWS.qcow2 to it.
Restart the VM guest
For VM guest of Windows 10 and above, run command: bcdedit /deletevalue {current} safeboot

KVM virtualisation