Windows has support for several paravirt features that it will use when running on Hyper-V, Microsoft's hypervisor. These features are called enlightenments. Many of the features are similar to paravirt functionality that exists with Linux on KVM (virtio, kvmclock, PV EOI, etc.)

Nowadays QEMU/KVM can also enable support for several Hyper-V enlightenments. When enabled, Windows VMs running on KVM will use many of the same paravirt optimizations they would use when running on Hyper-V. For detailed info, see Vadim's presentation from KVM Forum 2012.

From the QEMU/KVM developers, the recommended configuration is:

-cpu ...,hv_relaxed,hv_spinlocks=0x1fff,hv_vapic,hv_time

Which maps to the libvirt XML:

   <relaxed state='on'/>
   <vapic state='on'/>
   <spinlocks state='on' retries='8191'/>

 <clock ...>
  <timer name='hypervclock' present='yes'/>

Some details about the individual features:

  • relaxed/hv_relaxed. Available in libvirt 1.0.0+ (commit) and qemu 1.1+ (commit). This bit disables a Windows sanity check that commonly results in a BSOD when the VM is running on a heavily loaded host (example bugs here, here, and here). Sounds similar to the Linux kernel option no_timer_check, which is automatically enabled when Linux is running on KVM.
  • vapic/hv_vapic. Available in libvirt 1.1.0+ (commit) and qemu 1.1+ (commit).
  • spinlocks/hv_spinlocks. Available in libvirt 1.1.0+ (commit) and qemu 1.1+ (commit)
  • hypervclock/hv_time. Available in libvirt 1.2.2+ (commit) and qemu 2.0+ (commit). Sounds similar to kvmclock, a paravirt time source which is used when Linux is running on KVM.

It should be safe to enable these bits for all Windows VM, though only Vista/Server 2008 and later will actually make use of the features.

(In fact, Linux also has support for using these Hyper-V features, like the paravirt device drivers and hyperv_clocksource. Though these are really only for running Linux on top of Hyper-V. With Linux on KVM, the natively developed paravirt extensions are understandably preferred).

The next version of virt-manager will enable Hyper-V enlightenments when creating a Windows VM (git commit). virt-xml can also be used to enable these bits easily from the command line for an existing VM:

$ sudo virt-xml $VMNAME --edit --features hyperv_relaxed=on,hyperv_vapic=on,hyperv_spinlocks=on,hyperv_spinlocks_retries=8191
$ sudo virt-xml $VMNAME --edit --clock hypervclock_present=yes

The first invocation will work with virt-manager 1.0.1, the second invocation requires virt-manager.git. In my testing this didn't upset my existing Windows VMs and they worked fine after a reboot.

Other tools aren't enabling these features yet, though there are bugs tracking this for the big ones:

(edit 2014-09-08: This change was released in virt-manager-1.1.0)