I recently came across a rather interesting issue that seems to be relatively unrecognised – since 18xx updates, the idling Windows guest VMs seem to be consuming about 30% of CPU on the Linux KVM host. This took me a little while to get to the bottom off, and after excluding the possibility of it being caused by any active processes from inside the VM, I eventually pinned it down to the way system timers are used.

Diagnosis

What seems to be happening is that the Windows kernel keeps polling the CPU timer all the time at a rather aggressive rate, which manifests as rather high CPU usage on the host even though the guest is not doing any productive work. On large virtualization server, this is obviously going to pointlessly burn through a huge amount of CPU for no benefit.

Solution

The solution is to expose an emulated Hyper-V clock. For all other clocks, the kernel seems to incessantly poll the timers, but for Hyper-V it recognises that this is a bad idea in a virtual machine, and starts to behave in a more sensible way.

To achieve this, add this to your libvirt XML guest definition:

<features>
    <hyperv>
      <relaxed state='on'/>
      <vapic state='on'/>
      <spinlocks state='on' retries='8191'/>
      <synic state='on'/>
      <stimer state='on'/>
    </hyperv>
</features>

This gets the VM’s idle CPU usage from 30%+ down to a much more reasonable 1%.