Jeremy Fitzhardinge wrote:

It might be interesting if the two cases could be unified in some way, 
so that the VMMs could use a common usermode mechanism to achieve the 
same end, which is what Carsten was proposing.  But its not obvious to 
me how much common mechanism can be pulled out, since its a pretty 
deeply architecture-specific operation.

The big difference here is that LinuxOnLinux does represent guest 
virtual addressing in these mm structs where all other kernel based 
VMMs do represent guest physical in the user address space. That 
somewhat disqualifies LinuxOnLinux to share the commonality.
Whether or not proposed patch makes sense for shaddow page tables is 
unknown to me, since we have nested paging on s390.