Real reserve/release is a DASD hardware feature that serializes access among real processors by dedicating a device to one processor at a time. Use real reserve/release processing if two or more real processors are sharing a device and you are running CA MIM on both of these processors.
Real reserve/release dedicates a device by letting only one path group control the device at a time. Because each real processor uses a unique path group to access a device, real reserve/release can prevent integrity exposures among real processors.
When a guest operating system running under z/VM performs I/O to DASD, the CP component of z/VM takes responsibility for issuing a channel program to the real hardware on behalf of the guest. During this process, some portions of the channel program may be modified by z/VM.
If the device is not dedicated to a processor already, then the DASD hardware executes the reserve channel command word (CCW), which dedicates the device to the channel from which the reserve CCW was issued. The device rejects other reserve CCWs until the controlling z/OS guest issues a release CCW to release the device.
z/VM may remove reserve or release CCWs from the channel program of a guest under certain circumstances. If reserve or release CCWs are removed, then an integrity exposure can occur when several real processors are sharing a device.
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