3. The virtual aachine with the virtual=real option operates in the
preallocated storage area with normal CCW translation in effect
until the CP SET NOTRAIS ON coamand is issued. At that tiae, with
several exceptions, all subsequent I/O operations are perforaed froa the virtual CCWs in the virtual=real space without
translation. The exceptions occur under any of the following
conditions: • SIO tracing active • First cew not in the V=R region • I/O operation is a sense co.mand • I/O device is a dial-up terminal • I/O is for a nondedicated device
(spooled unit record console virtual CTCA
or ainidisks that are less than a full volume) • Pending device status • I/O device has an alternate path
Any of the above conditions will force CCW translation. Since minidisks are nondedicated devices, they aay be used by prograas
running in the V=R region even though CP SET 10TRAIS 01 is in
effect. 4. If the virtual=real machine perforas a virtual reset or IPL, then
the normal CCW translation goes into effect until the CP SET ROTRANS ON coamand is again issued. This Feraits siaulation of an
IPL sequence by CP. Only the virtual=real virtual aachine can
issue the command. A message is issued if normal translation mode is entered.
5. A virtual=real machine is not allowed to IPL a naaed or shared system. It must IPL by device address.
6. When NOTRAIS is in effect for a virtual=real aachine, no aeaningful SEEK data is collected by !ONITOR operations. AFFINITY This virtual machine option allows virtual machines that operate on
attached processor systems to select, if desired, the processor of their
choice for program execution. The selection can be aade by the V8/370 directory OPTION statement, or it can be made dynaaically by an operand
of the CP SET coa.and:
For class G users SET AFFINITY For class A users SET AFFINITY use rid
where nn is the processor address of the aain or the attached processor.
In application, the
preference of operation
affinity setting of a virtual aachine implies a
to either (or neither) processor. Affinity of
1-34 IBM VM/370 System Logic and Problem Deteraination--Voluae 1

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