Peter,
You define relocation domains that may contain all or just some of the
different processors available. The directory entry can state which
domain the guest is to use.
z/VM automatically determines the lowest common facilities within each
relocation domain.
When the guest signs on and IPL's, since z/VM already knows what the
lowest common facilities are available, it automatically tells the guest
such when the guest issues the STFLE instruction. z/VM also modifies the
results for other 'query' type functions, such as KM, to only return the
lowest common value.
So, yes, it is fairly automatic. The z/VM programmers did a really good
job on this.
Tony Thigpen
Peter Relson wrote on 3/28/19 9:43 AM:
I'll admit that I have no idea what I'm talking about w/r/t LGR, but it
seems to me that "dial down" is exactly what the function does.
Tony mentioned that it is "automatic". I don't think it can be fully
automatic; the minimum target level has to be defined to z/VM (and I had
wondered whether it could be defined to z/VM on a per-guest basis).
So imagine a z/VM definition while running on a z14: Guest G is to be able
to use LGR to a z13.
Now you IPL z/OS as guest G.
z/OS would see the facilities available as those for the z13. A program
would see the facilities available as those for the z13 (I assume).
Isn't that what you would think "dial down" should do?
Peter Relson
z/OS Core Technology Design
