operation for a virtual machine means that the program of that virtual machine will be executed on the selected or named processor. It does
not imply that supervisory functions and the CP housekeeping functions
associated with that virtual machine will be handled by the same
processor.
In attached processor systems, all real I/O operations and associated
interrupts are handled by the main processor. Virtual I/O initiated on
the attached processor that is mapped to real devices must transfer
control to the main processor for real I/O execution. Therefore,
benefits may be realized in a virtual machine "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
weighed as every virtual machine is in contention with other virtual aachines for resources of 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. It
i.proves the performance of VM/370. Virtual storage operating systems,
which run in problem state under the control of V8/370, use aany privileged instructions and SVCs that cause interrupts that V8/370 must handlee ihen the virtual machine assist feature is used, many of these
interrupts are intercepted and handled by the processor; and,
consequently, V!/370 performance is improved. See for a list of the processors on which virtual machine assist is available. The virtual machine assist feature intercepts and handles
interruptions caused by SVCs (other than SVC 76), invalid page
conditions, and several privileged instructions. An SVC 76 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 STNS! STOS! SSK LPSi SPKA (load real address)
(store control)
(reset reference bit)
(insert storage key)
(set storage key)
(insert PSi key)
(store then AND system mask) (store then OR system mask)
(set system mask) (load PSi) (set PSi key from address)
Although the assist feature was designed to improve the performance of YK/370, virtual aachines may see a performance improvement because more resources are available for virtual.machine users. CP Introduction 1-35

Whenever you IPL '8/370 on a processor with the virtual machine assist
feature, the feature is available for all V8/370 virtual aachines.
However, the systea operator's SET coamand can make the feature
unavailable to '8/370 and, subsequently, available again for all users.
The foraat of the systea operator's SET command is: SET SASSIST {ON} [[PROC] xx] OFF If you do not know whether or not the virtual machine assist feature
is available to Y8/370, use the class A and E QUERY coamand. For a
coap1ete description of the Class A and E QUERY and SET commands, see
the V8LJ70 If the virtual machine assist feature is available to V8/370 when you log on !onr vir+uAl .Achinp; i+ for your virtual aachine. If your V8/370 directory entry has the SVCOFF option, the SVC handling portion of the assist feature is not available when you log on.
The class G SET co •• and 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. The format of the co.mand is: SET ASSIST {[ ON] [SVC ] [T8R ]} [NOSVC] [NOT8R] OFF 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 co.p1ete
description of the Class G QUERY and SET coamands, see the !8/37Q CP for 2!iU!era1 Certain interrupts aust be handled by V8/370. Consequently, the assist
feature is not available under certain circuastances. V8/370 autoaatica11y turns off the assist feature in a virtual aachine if it: • Has an instruction address stop set. • Traces SVC and prograa interrupts. Since an address stop is recognized by an SVC interrupt, V8/370 must handle SVC interrupts while address stops are set. Whenever you issue
the ADSTOP coaaand, Y8/370 automatically turns off the SVC handling
portion of the assist feature for your virtual aachine. The assist
feature is turned on again after the instruction is encountered and the
address stop reaoved. If you issue the QUERY SET coamand 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 aachine issues a TRACE co.aand with the SVC, PRIV, BRANCH, INSTRUCT, or ALL operands, the virtual assist feature is
autoaatica11y turned off for that virtual aachine. 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. 1-36 IBM V"/370 System Logic and Problem DeterBination--Voluae 1