VM/370 security and performance

[jbn at wdl1.UUCP]

     No, that's the beauty of VM; VM WILL RUN UNDER VM!  It is actually
possible, strange though it may sound to the UNIX community, to run a
full-fledged operating system, interrupt handlers, device drivers,
page table updating and all, UNDER THE CONTROL OF THE OPERATING SYSTEM.
There's a substantial performance penalty, but it's on the order of 30%,
not 300%.  So operating system development takes place along with other work.
     This technology was first developed for the IBM 360/67; a certain
amount of special hardware support is needed to make it work, but less than
one might think.  The key idea is that it must be possible to simulate
correctly all privileged operations.  Incidentally, operating systems of this 
type are called ``hypervisors''.  Few kinds of CPUs are capable of supporting a
hypervisor, but all 370s, 43xxs, and 303xs can do so.
     And yes, you can run VM under VM under VM ..., losing 30% performance
with each layer.