Jeff Kell writes:
> Even IBM's
> VM/ESA which does emulate multiple virtual physical machines (albeit all
> IBM platforms) only allows them to co-exist, not co-operate.  They made a
> really big deal not that long ago about "shared code segments" that multiple
> users could share, and elaborated on converting your code to localize all of
> your data references to your assigned "data area" (non-trivial).  This
> seemingly "ground-breaking" breakthrough was years after MPE and it's
> explicit code/data sharing/separation.
 
VM is in a quite different class from MPE's code and data management. In MPE,
the code and data protection can be overridden by privileged programs. You
can't run an OS as a process under MPE without compromising system integrity.
The point of VM is that there must be no way that a program can tell that it's
being run in a virtual machine, even if the "program" is a privileged OS. So
sharing protected code is a lot harder in VM: if a privileged program running
in a virtual machine changes the code in a shared but protected page, the OS
must make a  private copy of that page for the virtual machine that changed
it. (One privileged program that's not only allowed but expected to do this is
a debugger.) All privileged operations have to be emulated, and the emulation
has to be invisible to other privileged code.
 
IBM did a very good job: you can even run VM in a VM vm. Sharing code within a
vm is easy, but sharing code across vm's requires takes a lot of careful
recordkeeping. In fact, nearly all known VM penetrations take advantage of
timing inconsistencies, the only way that a program can tell it's not really
privileged.
 
-- Bruce
 
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