When importing Windows VMs into XenServer, for example by using the Conversion Manager virtual appliance or the OVF import functionality in XenCenter, in some circumstances these VMs may not have been optimized for peak performance, as they may be missing certain Viridian optimization flags.

If you see a difference in performance in your Windows VMs after importing them into XenServer, you might benefit from the steps in this article. Performance impacts are most pronounced in imported Windows VMs running on very busy hosts. 

If you believe you may be affected by this, you can complete the following steps to ensure that the performance settings are optimized:

1. List all Windows VMs: xe vm-list os-version:distro=windows --minimal
   1. Note: This command will only list VMs which have previously been started and that have the XenServer VM Tools installed
2. List all VMs that have the expected Viridian optimization flags: xe vm-list platform:viridian=true platform:viridian_reference_tsc=true platform:viridian_stimer=true --minimal
3. If there are any VMs UUIDs listed in step 1 that do not appear in the list in step 2, these are VMs that are missing flags and can be optimized as follows:
   
   1. Update the viridian flags in the platform field. For example: 
      xe vm-param-set uuid=<vm-uuid> platform:viridian=true platform:viridian_time_ref_count=true platform:viridian_reference_tsc=true platform:viridian_apic_assist=true platform:viridian_crash_ctl=true platform:viridian_stimer=true
      Note: Only the viridian , viridian_reference_tsc and viridian_stimer flags are required to resolve the primary performance issue, but we advise that you set all available viridian flags.
   2. Restart the VM.

Note: You can ensure the flags are correct for all Windows VMs with a single command:
xe vm-list os-version:distro=windows --minimal | tr , '\n' | xargs -n 1 -I {} xe vm-param-set uuid={} platform:viridian=true platform:viridian_time_ref_count=true platform:viridian_reference_tsc=true platform:viridian_apic_assist=true platform:viridian_crash_ctl=true platform:viridian_stimer=true

Only use this command if you want to update all of your VMs. You must restart the VMs for the updates to take effect. This process does not update the snapshots associated with these VMs. Do not attempt to update existing snapshots. Instead create a new snapshot from your updated VM.

If you use your VMs with Citrix Virtual Apps and Desktops Machine Creation Services or Citrix Provisioning, ensure that you update your golden images.

Problem Cause

The imported Windows VMs may be missing certain Viridian optimization flags.