Start Interpretive Execution (SIE) is very much still at the core of
z/VM virtualisation (as it is at the core of LPAR itself - every
partition is dispatched on the real hardware using SIE). Although
entering and exiting SIE is expensive, once dispatched a z/VM virtual
machine runs at native speed. How a guest runs, the facilities available
to it, and what gets intercepted, are all controlled by the information
contained in and around the SIE Sate Description. This includes
architecture level and the varying responses to STFLE according to the
relocation zone. As I recall (I've been retired a couple of years),
STFLE causes a SIE interception, and z/VM then stores the appropriate
response in guest memory to reflect the facilities available to it.
Although this is a high overhead, it is assumed the STFLE itself will
rarely be executed (ideally just once during system initialisation), so
the overhead is inconsequential.
There is lots more to the story, but much of what happens under the LGR
covers is proprietary information.
Ray
On 3/28/2019 16:54, Gary Weinhold wrote:
This is probably part of Start Interpretive Execution (assuming
that's still used). VM used to be based on running users in problem
state so every privop is intercepted , but SIE allows more selective
trapping of which ops are intercepted especially for operating systems
like z/OS. I think VM was trapping STCK during the Y2K testing era as
an example of non-privileged ops.
On 2019-03-28 4:42 p.m., Charles Mills wrote:
Is this one of those situations that is controlled by a control
register bit? VM does not interpret every instruction like Hercules.
It must be getting an interrupt on STFLE or whatever.
Charles
Gary Weinhold Senior Application Architect
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-----Original Message-----
From: IBM Mainframe Assembler List
[mailto:[email protected]] On Behalf Of Tom Marchant
Sent: Thursday, March 28, 2019 11:48 AM
To: [email protected]
Subject: Re: Determing the Presence of an Instruction
On Thu, 28 Mar 2019 09:56:23 -0700, Charles Mills wrote:
VM does a fair amount of "interception" and instruction simulation
in any event.
For privileged instructions, yes. In this case, something has to
happen for non-privileged instructions too.
