David Kreuter wrote:
Due to double SIE emulation do not run 3rd level guest in production; i.e. do not use a 2nd level vm system as
a home for production linux servers of any sort.
linux production servers belong under vm in an lpar
production = any services wanted by anyone but you
Double SIE emulation ?
AFAIK, the penalty for running a 3rd level VM is minimal.
z/VM doesn't *emulate* SIE.. It just alters the SIEBK and redrives the
SIE to the underlying level... You could be running 15 levels - and
you're SIE will eventually be executed by the lowest level.. no
emulation there.. no 'software' SIE !
It's even possible current implementation don't even intercept SIE
(there is nothing precluding SIE from executing SIE.. SIE was never
itself subject to a mandatory intercept).
That was especially true for preferred guests.. *was* because we're no
longer allowed preferred guests anyway.. but when they existed, even
some I/Os didn't get an intercept (assuming you had SIE I/O Assist, your
device was dedicated, etc, etc..).
For all I know, when running in an LPAR, you're already running under
SIE anyway (no doubt PR/SM uses SIE to run its LPARs !)
Now.. The problem with running n Levels is probably more with I/Os.. and
those privileged instructions that SIE can't handle..
--Ivan
