Windows VM Tips

For using a windows VM via libvirt/KVM/qemu/virt-manager

See also this guru3d thread for something to investigate about interrupts.

Here is a thread about excessive context switching. There is a solution here (supposedly). Additionally it is proposed in this thread to remove the USB tablet device to reduce context switching (so I did that).

Only packages really needed (Ubuntu Server 22.04) are libvirt, qemu-kvm, libvirt-daemon-system. Add users who will use the VM to libvirt group.

If libvirtd.service is running (it should be) then just connect and create using virt-manager (either locally or remotely via ssh, e.g. virt-manager -c qemu+ssh://[ip]/system)

I also needed the virtio Windows drivers to add as a disc when booting so that Windows could see the virtio hard drive. After install I ran the setup file inside which enabled some GPU stuff.

Install SPICE guest tools to allow console resizing on Window resizing, and clipboard integration.

I found that virtio video driver still has some bugs - SPICE will stop resizing after rebooting after installing spice guest tools and will never work again. QXL video works perfectly.

By default libvirt/virt-manager/something puts the disk .qcow2s in /var/lib/libvirt/images (root access required to read).

Windows is a nightmare and sucks up craploads of CPU all the time because of course it does.

Useful commands to debug some of this:

• # perf kvm –host top -p `pidof qemu-system-x86_64`
  • Shows how often qemu is executing various functions, [k] for kernel space and [.] for user space.
  • There is also one function used for making the switch to guest space and it accounts for all time spent there. On a 4.14 kernel with intel cpu that function is vmx_vcpu_run but it might differ.
• # perf stat -e 'kvm:*' -a – sleep 1
  • This should show the reason a VM is doing VM_EXIT. Values should be roughly “less than 1000”.
• # perf kvm –host stat live
  • This one should show what the VM is doing? Apparently most time % should be HLT otherwise it's not idle.

It may also be a good idea to use powercfg to find out why the OS is waking up lots, if it is.

These tell Windows it's running in a VM.

In the XML for the machine, change the <features> and <clock> sections to the following:

Features:

<features>
  <acpi/>
  <apic/>
  <pae/>
  <hyperv>
    <relaxed state='on'/>
    <vapic state='on'/>
    <spinlocks state='on' retries='8191'/>
    <vpindex state='on'/>
    <synic state='on'/>
    <stimer state='on'/>
    <reset state='on'/>
    <vendor_id state="on" value="KVM Hv"/>
    <frequencies state="on"/>
  </hyperv>
</features>

Clock:

<clock offset="utc">
  <timer name="hpet" present="no"/>
  <timer name="hypervclock" present="yes"/>
</clock>

<timer name='rtc' tickpolicy='catchup'/>
<timer name='pit' tickpolicy='delay'/>

For me these enlightenments reduced idle CPU by 4-5x (from 45%+ to ~10-12% while running Milestone XProtect in the background, with no cams set up).