within a single processor, reservejrelease CCW commands permit several operating systems running as virtual machines under one VM/370 system to use both virtual and real reserve/release to protect
data from cne another. Real reserve/release support is for dedicated
devices.
In this example, 230 and 330 are alternate device addresses for a particular DASD to be shared by USERA and USERB (two virtual machines
running on the same real computing system). share this device:
1. Generate the virtual machine operating system for USERA to support
both the device at 230 (two-channel switch) and the reserve/release
software.
2. Generate the virtual machine operating system for USERB to support
both the device at 330 (two-channel switch) and the reserve/release
software.
3. Generate VM/370 as though 230 and 330 were different devices (with
different control units and channels).
4. Issue the CP ATTACH command to attach device 230 to USERA and
device 330 to USERB. If the system generated for USERB is to run in a real machine,
rather than a virtual machine: • Generate the VM/370 system with device 230 but not 330. • Issue the CP ATTACH command to attach device 230 to USER!. In both cases: • The device addresses generated for systems to run in a virtual machine need not be the same as on the real machine. • The devices used by virtual machines must be dedicated (attached or
defined with a DEDICATE statement in the VM/370 directory). While theoretically possible, do not share the CP SYSRES and any
other CP-owned disk between two processors. VM/370 checks all CCW commands passed by operating systems running in
virtual machines. It bases reserve/release CCW command processing on:
the type of device, the presence or lack of alternate path support, and
whether the MDISK statement in the VM/370 directory contains a "V" on
the mode operand. For the hardware to execute the reserve/release CCi commands, the two-channel switch special feature must have been
installed. Depending upon the various combinations of these items, IM/370 either permits the reserve CCW command to execute on the hardware
or changes the reserve CCW command to a sense CCi command. To determine
the conditicns when a "reserve" is changed to a "sense" CCW command,
refer to Figure 2.
22 IBM VM/370 Operating Systems in a Virtual Machine

Iii 11 I Reserve/Release 1 Virtual Reserve/ ICCW Comndl Type Alternate IExecutes in thelRelease RequestedlSent by 1 of Path IHardware (2-4 I (V Added to IVM/310 tol i
Device Support IChannel Switch) I Mode in MDISK) I Device fNotel I 1 1 1 I I" IDedicatedlNot defined 1 Not applicable I Not applicable I Reserve 1 1 1 IDASD or 1-----------1---------------1-----------------1---------1----1 ITape IDefined 1 Not applicable 1 Not applicable 1 Sense I 2 I I 1 I I 1 I I IMinidiskslNot defined 1 Yes No I Reserve I 1 I I 1-----------1--------------- -----------------1---------1----1 I INot defined I Yes Yes I Reserve I 1 I 1 1-----------1--------------- -----------------1---------1----1 1 INot defined 1 No No 1 Reserve 1 3 1 I 1-----------1--------------- -----------------1---------1----1 I INot defined 1 No Yes 1 Sense I 4 1 1 1-----------1--------------- -----------------1---------1----1 I IDefined 1 Not applicable Not applicable 1 Sense 1 5 1 I 1 lNormal Operation --The command is passed unChanged to the hardware. 1 I 2When the VM/370 system has been generated with alternate path support 1 for those devices, it prevents the devices from being reserved. This 1 action causes VM/370 to avoid a possible channel lockout. VM/370 does not return any indication of this action to the operating system
issuing the ccw command that the device was not reserved.
3without the two-channel switch special feature, VM/370 sends the
reserve/release CCW command unchanged to the hardware. However, the
hardware rejects the command and does not reserve the device. ·Before sending the command to the hardWare, VM/370 changes the
reserve ccw command to a sense CCW command and places a virtual
reserve on the minidisk. The real device is not reserved. The
virtual reserve prevents other operating systems running under the same VM/370 system from accessing the minidisk; however, these same virtual systems may reserve other minidisks located on the same real volu.e. Because the two-channel switch feature is not
installed on the channels, only one address path goes to the device
from the V8/370 processor. This path allows V8/370 virtual
reserve/release processing to send a sense CCW to the device,
although the reserve CCW command would be rejected by the hardware. 5When alternate paths to a device have been defined (by the ALTCU operand on the RDEVICE macro instruction and the ALTCB operand on the RCTUNIT macro instruction), VM/370 changes reserve/release CCW commands to CCW commands to prevent a possible channel lockout.
Figure 2. Summary of VM/370 Reserve/Release Support
section 1. General Considerations 23