In attached processor systeas, all real I/O operations and associated
interrupts are handled by the aain processor. Virtual I/O initiated on
the attached processor that is aapped to real devices must transfer
control to the main processor for real I/O execution. Therefore,
benefits may be realized in a virtual aachine "mix" by relegating those
virtual machines that have a high I/O-to-compute ratio to the main processor, and those virtual machines that have a high compute-to-I/O ratio to the attached processor. Such decisions should be carefully
weiahed as everv virtual machine is in contention with othp.r virtnal machines-for the system.--- ----- -------
A more important use of the affinity setting would be in applications
where there are virtual machine program requirements for special
hardware features that are available on one processor and not the other. Such features could be a performance enhancement such as virtual machine
assist (described later in the text) or a special RPQ that is a
requirement for a particular program's execution. VIRTUAL MACHINE ASSIST FEATURE The virtual machine assist feature is a processor hardware feature that
improves the performance of VM/370. Virtual storage operating systems. which run in problem state under the control of VM/310, use many privileged instructions and SVCs that cause interrupts that V!/310 aust handle. When the virtual machine assist feature is used. many of these
interrupts are intercepted and handled by the processor. Consequently. VM/310 performance is improved.
The Virtual Machine Assist Feature intercepts and handles
interruptions caused by SVCs (other than SVC 16), invalid page
conditions, and several privileged instructions. An SVC 16 is never
handled by the assist feature; it is always handled by CP. The
processing of the following privileged instructions is handled by this
feature:
LRA STCTL RRB ISK SSK IPK STNSM STOSM SSM LPSW SPKA (load real address)
(store control)
(reset reference bit)
(insert storage key)
(set storage key)
(insert PSW key)
(store then AND system mask)
(store then OR system mask) (set systea mask) (load PSW) (set PSW key froa address)
Although the assist feature was designed to improve the performance
of VM/370, virtual machines may see a performance improvement because aore resources are available for virtual machine users. For a list of
processors on which the Virtual Machine Assist Feature is available, see
the 98 IBM 1M/370 System Programmer:s Guide

Page of GC20-1807-7 As Updated April 1, 1981 by TNL GN25-0829 USING THE VIRTUAL MACHINE ASSIST FEATURE Whenever you IPL VM/370 on a processor with the virtual machine assist
feature, the feature is available for all VM/370 virtual machines.
However, the system operator's SET command can make the feature
unavailable to VM/370 and, subsequently, available again for all users.
If you do not know whether or not the virtual machine assist feature is
available to VM/370, use the class A and E QUERY command. For a
complete description of the Class A and E QUERY and SET commands, see
the !lXLJ1Q If the virtual machine assist feature is available to VM/370 when you
log on your virtual machine, it is also supported for your virtual
machine unless you are running a second-level VM/370 system in your
virtual If your VH/370 directory entry has the SVCOFF the SVC handling portion of the assist feature is not available when you
log on. The class G SET command can disable the assist feature (or only
disable SVC handling). It can also enable the assist feature, or if the
assist feature is available, enable the SVC handling. You can use the
class G QUERY SET command line to find whether you have full, partial,
or none of the assist feature available. For a complete description of
the Class G QUERY and SET commands, see the fR £2£ User.§. RESTRICTED USE OF THE VIRTUAL MACHINE ASSIST FEATURE Certain interrupts must be handled by VM/370. Consequently, the assist
feature is not available under certain circumstances. VM/370 automatically turns off the assist feature in a virtual machine that: • Has set an instruction address stop • Is tracing SVC and program interrupts Since an address stop is recognized by an SVC interrupt, VM/370 must
handle SVC interrupts while address stops are set. Whenever you issue
the ADSTOP command, VM/370 automatically turns off the SVC handling
portion of the assist feature for your virtual machine. The assist
feature is turned on again after the instruction is encountered and the
address stop removed. If you issue the QUERY SET command line while an
address stop is in effect, the response will indicate that the SVC handling portion of the assist feature is off. Whenever a virtual machine issues a TRACE command with the SVC, PRIV, BRANCH, INSTRUCT, or ALL operands, the virtual assist feature is
automatically turned off for that virtual machine. The assist feature
is turned on again when the tracing is completed. If the QUERY SET command line is issued while SVCs or program interrupts are being
traced, the response will indicate the assist feature is off.
The virtual machine assist feature is not available to a second-level
virtual machine, that is, a virtual machine that is running in a virtual
machine.
VM/370Extended Co-ntro-I .. P-rog-ram Support (ECPS) VM/370 Extended Control-Program Support (ECPS) extends, for specific
privileged instructions, the hardware assistance that the virtual
machine assist feature provides. ECPS also provides hardware assistance
Part 2. Control Program (CP) 99