How to get CompareExchange128 support in VirtualBox

• CentOS Host
• Windows guest

The goal was to upgrade from Windows 7 to Windows 8 Pro then Windows 8.1.  This overview in gizmag is good - the link to get the upgrade assistant still works - but - if you are using VirtualBox with a Windows guest that has been in use for a long time, there is a good change that you will get an error because you do not have the system requirements.  In particular:

"You can't install Windows 8.1 because your processor doesn't support CompareExchange128."

There are instructions around for handling this from the command line but if "vboxmanage list vms" does not list ANY vms yet you have the VirtualBox manager GUI handy, this is your solution - very simple:

Go into Settings for the virtual machine (while it is powered off).  On the General tab, Basic, Version: select Windows 8.1 (not Windows 7, not Windows 8).  You have to use 8.1 and then the CompareExchange128 feature works automatically.

Enjoy.