> -----Original Message----- > From: IBM Mainframe Assembler List [mailto:ASSEMBLER- > l...@listserv.uga.edu] On Behalf Of glen herrmannsfeldt > Sent: 27 October 2015 05:44 > To: ASSEMBLER-LIST@LISTSERV.UGA.EDU > Subject: Re: Question of curiosity: Why are IVSK and TPROT instrictions > privileged? > > As mentioned previously, the ability to virtualize depends on instructions > that can tell problem from supervisor state need to trap in problem state. > > The instruction I remember wondering most about when I was first learning > S/370 assembler is STIDP. But that was before I knew about virtualization. I > suppose virtual STIDP is useful. > > On the other hand, STCK not being privileged made Y2K testing harder, as > one couldn't virtualize the clock. > > Also, and back to the 360/67, why is LRA privileged? Again, I presume > virtualization, but I don't see any security problems with LRA, but on the > other hand, there isn't any real use for it. >
It is needed to allow VM's to have their own virtual storage. So the storage a VM sees as real is actual level-1 virtual storage. When a VM creates its own Virtual store that's Level-2 virtual. VM needs to be able to intercept LRA in order to pass the calling program the "fake" level-1 virtual address, not the underlying real address. There was a lot of smoke and mirrors in VM/370 and running virtual machines that used their own virtual store was very slow because of the effort needed to construct and manage fake page tables. > -- glen Dave
