On Thu, 28 Mar 2019 11:07:34 -0400, Tony Thigpen wrote: >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.
z/VM also has to tell the processor, every time a guest is dispatched on that processor, what facilities it is to process, or which it is to pretend do not exist. So that if the guest issues an instruction that was introduced on a z14 after zVM told it that it was to behave like a z12, a PIC 1 would occur. The processor would have to be given this information every time a guest that was running on a different emulated level of hardware was dispatched on that processor. And, of course, at any given moment, different processors might be running at different levels. If a spare CP was switched in by the hardware to handle an error, that information would have to be transferred as well. It seems like there would be a fair amount of overhead in this. Still, it is an intriguing capability. -- Tom Marchant
