QEMU+KVM notes


  • We’re running in a debian system.
  • We’re running in an AMD processor (not much to change if using Intel).
  • We’re using the default directories for VMs (/usr/lib/libvirt/<IMAGE>/files...)

Debian pre-requisites

Make sure the CPU has virtualization capabilities:

grep -c '\(vmx\|svm\)' /proc/cpuinfo

Output should be > 0.

Install the required qemu and libvirt packages:

sudo apt-get install qemu-kvm qemu-system qemu-utils \
    libvirt-clients libvirt-daemon-system virtinst

Make sure libvirtd is running:

sudo systemctl status libvirtd.service

Make sure the kvm and kvm_intel or kvm_amd modules are loaded.

sudo virsh net-list --all
 Name      State      Autostart   Persistent
  default   inactive   no          yes

Start the network and configure it to start on boot:

sudo virsh net-start default
sudo virsh net-autostart default

VM Creation example (debian bookworm)

The example below may get outdated rapidly, as OS versions change all the time. Please adjust the download version and image, as well as the --os-variant and --os-type to match.


# Create directories for disk image and ISO
mkdir -p "${LIBVIRT_DIR?}"/{images,iso}/debian

# Fetch ISO file from debian
curl -L \
  https://cdimage.debian.org/debian-cd/current/amd64/iso-cd/debian-12.6.0-amd64-netinst.iso \

# Create a VM called "debian" and open the console.
sudo virt-install \
  --name debian \
  --os-variant debianbookworm \
  --ram 2048 \
  --cpu host \
  --disk "${LIBVIRT_DIR?}"/images/debian/debian.qcow2,device=disk,bus=virtio,size=10,format=qcow2 \
  --graphics spice \
  --hvm \
  --cdrom "${LIBVIRT_DIR?}"/iso/debian/debian.iso \
  --boot cdrom,hd

Note: If you get an apparmor error, this could be explained by libvirt bug #665531. To work around this bug, edit /etc/libvirt/qemu.conf and add security_driver=none to the file.

This will open a graphical console to your newly created VM so you can finish the OS installation. If the machine in question does not have graphical capabilities, add the --noautoconsole command-line option and open virt-viewer from a remote machine.

Basic VM commands

  • virsh list --all - Show all VMs (running and stopped).
  • virsh start vm - Start the VM.
  • virsh shutdown vm - Shutdown the VM.
  • virsh destroy vm - Forceful shutdown.
  • virsh undefine vm [--nvram] [--remove-all-storage] - Undefine (deletes) the VM.
  • virsh console vm - Serial console into the VM.
  • virsh autostart --domain domain - Enable autostart for the domain (VM).
  • virsh domiflist domain - List the network interfaces for the domain.

Pool management

  • virsh pool-list - List pools (a pool is a directory, mountpoint, device, etc and contains volumes).
  • virsh pool-destroy - Destroy (stop) the pool.
  • virsh pool-delete - Delete the ALL pool data (definition remains available).
  • virsh pool-undefine - Delete the pool from the list of pools (definition).

Volume management

  • virsh vol-list --pool pool_name - List volumes.
  • virsh vol-delete --pool pool_name - Delete a volume.

Check the VM manager using VNC or virt-viewer

sudo virsh vncdisplay vmname
vncviewer localhost:0


virt-viewer vmname

It’s also possible to use virt-manager and open a graphical console directly.

CPU notes

By default, qemu will use the qemu64/qemu32 virtualized CPUs which perform poorly and should be avoided. On the command-line, use --cpu host to pass use cpu-passthrough. This should improve guest performance but will make migrations harder. It’s also possible to use a named cpu model (E.g. --cpu Opteron_G5 for AMD hosts and --cpu Skylake-Server for intel.

For virsh (virsh edit domain), it’s possible to use the following snippet to enable cpu-passthrough:

<cpu mode='host-passthrough' check='partial'/>

Don’t forget to limit the number of CPUs in the guest with:

<vcpu placement='static'>2</vcpu>

It’s also possible to edit these directly in virt-manager, but I didn’t find a way to enable host-passthrough, only host-model, which is safer for migrations but slightly less performant. When specifying the CPU mode, pay attention to the check='partial' flag. Without it, virt-manager could set it to full which will cause failures as the guest does not have certain features of the host enabled (like CPU virtualization).

More info: https://www.berrange.com/posts/2018/06/29/cpu-model-configuration-for-qemu-kvm-on-x86-hosts/

Pool management

A pool is a quantity of storage put aside for “volumes”, which are then used by VMs. Pools can be Network mountpoints, directories, filesystems, etc.

Viewing pools:

virsh list-pools --all

To completely delete a pool:

virsh pool-destroy poolname
virsh pool-delete poolname
virsh pool-undefine poolname
  • destroy removes the pool from libvirt’s control, but does not remove the data. The data can be later recovered with pool-create.

  • delete deletes the resources (volumes) used by the pool. This operation is non-recoverable. The pool object will still exist after this command, ready for the creation of new volumes.

  • undefine completely removes the configuration for an inactive pool.

Allowing regular user access

If you prefer regular users to manage the VMs, add them to the libvirt and libvirt-qemu unix groups:

sudo adduser paganini libvirt
sudo adduser paganini libvirt-qemu

Also set the LIBVIRT_DEFAULT_URI environment variable to point to the global scope:

export LIBVIRT_DEFAULT_URI="qemu:///system"

or use --connect qemu:///system on the virsh command-line.

Missing 2560x1440 on Linux

For some reason the native mode of 2560x1440 did not show on xrandr using the QXL driver. The solution was to add it manually.

First, calculate the VESA modeline:

$ cvt 2560 1440
Modeline "2560x1440_60.00"  312.25  2560 2752 3024 3488  1440 1443 1448 1493 -hsync +vsync

Then create a xorg.conf snippet in /usr/share/x11/xorg.conf/10-monitor.conf:

section "Monitor"
    Identifier "Virtual-0 "
    Modeline "2560x1440_60.00"  312.25  2560 2752 3024 3488  1440 1443 1448 1493 -hsync +vsync
    Modeline "3840x2160_60.00"  712.75  3840 4160 4576 5312  2160 2163 2168 2237 -hsync +vsync
    Option "PreferredMode" "2560x1440_60.00"

Note: The snippet above worked until an upgrade. It appears that newer versions of X use “Virtual-1” instead of “Virtual-0” (there’s also the issue of the space). Another possibility (untested) is to force the name of the monitor on the default screen:

Section "Screen"
    Identifier "Default screen"
    Monitor "default-monitor"

section "Monitor"
    Identifier "default-monitor"
    Modeline "2560x1440_60.00"  312.25  2560 2752 3024 3488  1440 1443 1448 1493 -hsync +vsync
    Modeline "3840x2160_60.00"  712.75  3840 4160 4576 5312  2160 2163 2168 2237 -hsync +vsync
    Option "PreferredMode" "2560x1440_60.00"

Further reading: https://stafwag.github.io/blog/blog/2018/04/22/high-screen-resolution-on-a-kvm-virtual-machine-with-qxl/

Converting windows devices to VirtIO

The default SATA devices work well, but (supposedly) have worse performance than the virtio devices. Those devices can be installed at boot time (by passing the device path to Windows), or after installation with the steps below:

Source: https://superuser.com/questions/1057959/windows-10-in-kvm-change-boot-disk-to-virtio/1095112

  • Add both windows 10 DVD/CD ISO and virtio driver ISO to VM.
  • The latest driver ISO can be pulled out of the RPMs found at https://fedorapeople.org/groups/virt/virtio-win/repo/latest/
  • Boot off windows 10 DVD/CD and get into a command prompt from repair mode option. Load the driver via the CLI: drvload d:\viostor\w10\amd64\viostor.inf
    • In my case d: was where the virtio install ISO got assigned.
    • After loading the driver e: was where the windows install became mounted.
  • Use the DISM command to inject the storage controller driver
    • E.g. dism /image:f:\ /add-driver /driver:e:\viostor\w10\amd64\viostor.inf

As above, change drive letter assignments according to your own environment. Avoids needing to fiddle with making special windows boot CDs/Images and ‘patches’ the actual windows install image on the fly.

Clipboard sharing

Note that display type must be set to Spice.

Folder sharing

  • Linux guests can use NFS and a shared filesystem (but this does not work for Windows guests).
  • For Windows, that does not work and the alternative is to use SPICE:
    • In virt-manager in the details view click “Add Hardware” and select a “Channel” device.
    • Set the new device name to org.spice-spave.webdav.0, leave other fields as they are.
    • Start the guest machine and install spice-webdav on the guest. After installation make sure the “Spice webdav proxy” service is running (via services.msc).
    • Run C:\Program File\SPICE webdavd\map-drive.bat to map the shared folder, which by default is ~/.Public.
    • Errors like “System error 67 has occurred, the network name cannot be found” mean the webdav proxy is not running.
    • To change the shared folder, use virt-viewer instead of virt-manager and configure it under File->Preferences.

Source: https://www.guyrutenberg.com/2018/10/25/sharing-a-folder-a-windows-guest-under-virt-manager/

Another option is to mount a NFS share with Windows: https://graspingtech.com/mount-nfs-share-windows-10/

Bridged networking

Converting VDI images to qcow2

You can convert your virtualbox images to qcow2 and run them under KVM:

qemu-img convert -f vdi -O qcow2 [VBOX-IMAGE.vdi] [KVM-IMAGE.qcow2]

Shrinking qcow2 images

  • Run fstrim -av on the guest.
  • Shutdown the VM.
  • Copy the qcow2 file to a backup (ex: foo-backup.qcow2)
  • Convert the backup on top of the original:
qemu-img convert -o cluster_size=2M -O qcow2 foo-backup.qcow2 foo.qcow2

To use compression, add -c to the qemu-img command-line above. Note that compression does not write new data compressed(?) according to http://archive.vn/6sF4b

Windows 10 optimizations

Source: https://scribe.rip/@leduccc/improving-the-performance-of-a-windows-10-guest-on-qemu-a5b3f54d9cf5

Run virsh editxml and change/add the following items:

  • Change the disk driver to be virtio.
  • Turn off write caching.
  • Set features.
  • Pin the CPUs.
<domain type='kvm' ...>
      <vcpupin vcpu='0' cpuset='1'/>
      <vcpupin vcpu='1' cpuset='5'/>
      <vcpupin vcpu='2' cpuset='2'/>
      <vcpupin vcpu='3' cpuset='6'/>
      <vcpupin vcpu='4' cpuset='3'/>
      <vcpupin vcpu='5' cpuset='7'/>
      <emulatorpin cpuset='0,4'/>
      <iothreadpin iothread='1' cpuset='0,4'/>

    <hyperv mode='custom'>
      <relaxed state='on'/>
      <vapic state='on'/>
      <spinlocks state='on' retries='8191'/>
      <vpindex state='on'/>
      <synic state='on'/>
      <stimer state='on'>
        <direct state='on'/>
      <reset state='on'/>
      <frequencies state='on'/>
      <reenlightenment state='on'/>
      <tlbflush state='on'/>
      <ipi state='on'/>

  <cpu mode='host-passthrough' check='none' migratable='on'/>
  <clock offset='localtime'>
    <timer name='rtc' present='no' tickpolicy='catchup'/>
    <timer name='pit' present='no' tickpolicy='delay'/>
    <timer name='hpet' present='no'/>
    <timer name='kvmclock' present='no'/>
    <timer name='hypervclock' present='yes'/>

    <disk type='file' device='disk'>
      <driver name='qemu' type='qcow2' cache='none' io='threads' discard='unmap' iothread='1' queues='6'/>

Windows 10 only uses 2 CPUs.

From: http://www.openwebit.com/c/how-to-run-windows-vm-on-more-than-2-cores-under-kvm/

Windows guests do not recognize all of the available cores under KVM and will usually detect only 2 cores. This happens since KVM exposes the available virtual CPU cores as physical CPUs (sockets). So, if the physical host running KVM has 2 CPUs with 4 cores each (a total of 8 cores), and the guest is configured for 8 CPUs, Windows will see 8 physical CPUs and will run only on 2, due to the hard coded limits in some of the editions of Windows Server.

To make Windows use all available cores, we need to configure the guest to expose the CPUs as cores and not as physical CPUs (sockets):

In virt-manager:

Open the guest configuration screen:

  • Select Processor options tab
  • Expand the “Topology” setting
  • Set the sockets to 2
  • Set the cores to 4 (for the guest to have a total of 8 cores) or 3 (for the guest to have a total of 6 cores).
  • You can also expand the “Configration” settings and click on “copy host CPU configuration” to make the guest fully use all of the physical host’s CPU capabilities.

Note: This worked with 6 host CPU cores (2 sockets, 3 cores)

virt-manager fullscreen keyboard grabbing

  • Use Ctrl+Alt to release the keyboard on fullscreen mode.

Further reading and sources