Configuring a libvirt Windows VM

Jul 16, 2018

This was a massive pain, so I’ve written up this post for my reference. I went through lots of twists an turns to get to this point so it’s probably incomplete. You’ll be required to solve some problems on your own (the horror!), but it includes lots of links to documentation and should at least be a starting point. My goals were:


If you have suggestions or this is totally broken now, please let me know!

  1. Open up virt-manager, and use the default VM configuration to install Windows 10. Get to the point where the VM logs in to the default user on startup.
  2. Add a new SCSI hard drive to the VM. This is just to convince Windows it actually needs to use the new drivers, so a blank 100MiB QCOW2 image works great for this.
  3. Remove the Windows ISO from the CD drive, and replace it with virtio-win.iso. The documentation for this image can be found here.
  4. Install the following drivers from that disk. This is done by going to <drivername>/w10/amd64/<drivername>.inf, right-clicking, and selecting “Install”. Since the documentation for this stuff is mostly non-existant and scattered all over the place, I will provide explainations below:
    • vioscsi: SCSI driver, will be used for disks. Must be installed for thin-provisioned disks.
    • NetKVM: Network driver. Documentation is light on the details of why I should care, but it’s probably faster or something so might as well install it.
    • viorng: Prevents the OS from running out of random numbers. Doesn’t seem to do anything, but you might as well install it anyway.
    • vioser: Some kind of serial dirver? Apparently this is used for copy-paste, although copy-paste seems to work just fine without it; might as well install it.
    • Balloon: Allows memory to be allocated to other VMs when the host is running out of memory due to over-committing. Seems useful, might as well install it.
    • qxldod: Better graphics performance than VGA, according to the docs. Might as well install it.
  5. Install guest-agent (more docs) from the disk. This provides the host an interface to run commands and read files on the guest. We’re not going to need it, but extra features, right?
  6. Download and install the SPICE Windows guest tools. This will provide you with screen auto-resizing, a shared clipboard. According to the docs this also includes graphics drivers, although we’ve already installed qxldod above.
  7. Shutdown your system.
  8. Open the “Details” tab in virt-manager, and configure the following items:
    • CDROM 1: Advanced -> Disk Bus = SCSI
    • DISK 1: Advanced -> Disk Bus = SCSI
    • NIC: Device Model = virtio
    • Display: Listen Type = None (running over IP seems kinda experimental)
    • Video: Model = QXL
    • Controller SCSI 0: Model = VirtIO SCSI
  9. Open up the XML description of the VM for editing. No clue how to find out where it is, but mine was /etc/libvirt/qemu/win10.xml. Look for a line like <driver name='qemu' type='qcow2'/>. Add discard='unmap' so that it looks like <driver name='qemu' type='qcow2' discard='unmap'/>.
  10. Start up the VM again and hope that it works.
  11. Open up the defragmenter in Windows and confirm that the words “Thin Volume” show up somewhere.
  12. Click “Optimize” to run TRIM and sparsify the disk image. You can also shave off a few more bytes by running virt-sparsify afterwards.

Since you do not want to deal with all this BS again, make a copy of your disk and of your VM’s XML description. It should be pretty small (for Windows at least, <10GiB), and compresses very well. I did a little more to get rid of useless crap, and my image compresses (with gzip -9) from 6.4GiB to 4.7GiB.

For reference, here is the <devices> section of my win10.xml:

<disk type='file' device='cdrom'>
  <driver name='qemu' type='raw'/>
  <source file='/usr/share/virtio/virtio-win.iso'/>
  <target dev='sda' bus='scsi'/>
  <address type='drive' controller='0' bus='0' target='0' unit='0'/>
<disk type='file' device='disk'>
  <driver name='qemu' type='qcow2' cache='none'/>
  <source file='/var/lib/libvirt/images/win10.qcow2'/>
  <target dev='sdb' bus='scsi'/>
  <address type='drive' controller='0' bus='0' target='0' unit='1'/>
<controller type='usb' index='0' model='ich9-ehci1'>
  <address type='pci' domain='0x0000' bus='0x00' slot='0x05' function='0x7'/>
<controller type='usb' index='0' model='ich9-uhci1'>
  <master startport='0'/>
  <address type='pci' domain='0x0000' bus='0x00' slot='0x05' function='0x0' multifunction='on'/>
<controller type='usb' index='0' model='ich9-uhci2'>
  <master startport='2'/>
  <address type='pci' domain='0x0000' bus='0x00' slot='0x05' function='0x1'/>
<controller type='usb' index='0' model='ich9-uhci3'>
  <master startport='4'/>
  <address type='pci' domain='0x0000' bus='0x00' slot='0x05' function='0x2'/>
<controller type='pci' index='0' model='pci-root'/>
<controller type='scsi' index='0' model='virtio-scsi'>
  <address type='pci' domain='0x0000' bus='0x00' slot='0x06' function='0x0'/>
<controller type='virtio-serial' index='0'>
  <address type='pci' domain='0x0000' bus='0x00' slot='0x07' function='0x0'/>
<controller type='ide' index='0'>
  <address type='pci' domain='0x0000' bus='0x00' slot='0x01' function='0x1'/>
<interface type='network'>
  <mac address='52:54:00:6d:a3:d6'/>
  <source network='default'/>
  <model type='virtio'/>
  <address type='pci' domain='0x0000' bus='0x00' slot='0x03' function='0x0'/>
<serial type='pty'>
  <target type='isa-serial' port='0'>
    <model name='isa-serial'/>
<console type='pty'>
  <target type='serial' port='0'/>
<channel type='spicevmc'>
  <target type='virtio' name='com.redhat.spice.0'/>
  <address type='virtio-serial' controller='0' bus='0' port='1'/>
<input type='tablet' bus='usb'>
  <address type='usb' bus='0' port='1'/>
<input type='mouse' bus='ps2'/>
<input type='keyboard' bus='ps2'/>
<graphics type='spice' keymap='en-us'>
  <listen type='none'/>
  <image compression='off'/>
  <gl enable='no'/>
<sound model='ich6'>
  <address type='pci' domain='0x0000' bus='0x00' slot='0x04' function='0x0'/>
  <model type='qxl' ram='65536' vram='65536' vgamem='16384' heads='1' primary='yes'/>
  <address type='pci' domain='0x0000' bus='0x00' slot='0x02' function='0x0'/>
<redirdev bus='usb' type='spicevmc'>
  <address type='usb' bus='0' port='2'/>
<redirdev bus='usb' type='spicevmc'>
  <address type='usb' bus='0' port='3'/>
<memballoon model='virtio'>
  <address type='pci' domain='0x0000' bus='0x00' slot='0x08' function='0x0'/>