I don't see how this explanation can be correct given the fact that the code using host_vmxon / host_vmxoff in the revision you point to is guarded by #if VBOX_WITH_HOST_VMX, which the changeset also shows is false. It wasn't turned on until the following revision: <https://www.virtualbox.org/changeset/43380/vbox>. I can't see how any change in behavior of host_vmxon / host_vmxoff in 10.8.2 could be the cause if they weren't even being called prior to this.