Browse Prior Art Database

VM/XA I/O Subsystem Special Interception Condition

IP.com Disclosure Number: IPCOM000100346D
Original Publication Date: 1990-Apr-01
Included in the Prior Art Database: 2005-Mar-15
Document File: 5 page(s) / 248K

Publishing Venue

IBM

Related People

Beardsley, BC: AUTHOR [+6]

Abstract

The VM/XA* system product is a virtual machine operating system that supports the IBM 3990 Storage Control. This article describes VM/XA's requirement for and use of the 3990's new special interception condition (see the preceding article).

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This is the abbreviated version, containing approximately 25% of the total text.

VM/XA I/O Subsystem Special Interception Condition

       The VM/XA* system product is a virtual machine operating
system that supports the IBM 3990 Storage Control.  This article
describes VM/XA's requirement for and use of the 3990's new special
interception condition (see the preceding article).

      The special interception condition and associated host
processing allows VM/XA to
o    control guest operating system usage of subsystem commands and
multi-device commands, and
o    receive all asynchronous subsystem or device status while
achieving the performance benefits of unmonitored guest use of the
devices connected to a 3990.

      Host operating systems, particularly a virtual machine system
like IBM's VM/XA System Product, often permit user programs to have
temporary, exclusive control of one or more I/O devices.  In an
environment with "global" device commands (see the preceding
article), the host operating system must be able to examine each
user's device commands prior to allowing the commands to be executed
by the device or subsystem.  The reason for such examination is to
retain control of the I/O subsystem by the host operating system
(rather than a user who has been allowed temporary use of a device),
to preserve system integrity, and to preserve the system's and other
users' data integrity (as in, for example, the case of the IBM 3990's
dual-copy function, in which a command to one device can over-write
data on another device).

      The individual examination of each user device command is very
costly from a system performance standpoint.  IBM's large-system
processors, for example, provide a hardware assist specifically to
avoid such examination and certain other operating system processing.
 (This IBM hardware assist is defined in the 370-XA and ESA/370*
architectures as the Interpretive Execution I/O Assist facility.)
This allows a subset of the operating system's users to submit I/O
requests to the device without the involvement of the operating
system.  (In VM/XA, these users are called "preferred guests".)
However, when the effect of a device command can reach beyond the
device being addressed (as with global commands), the bypassing of
device command examination for performance reasons becomes
unacceptable due to the matters of subsystem control, system
integrity, and data integrity mentioned above.

      In an environment with global status presentations, the
operating system must ensure that it receives all such status
presentations and is not preempted from receiving them by a processor
assist as described above, in which such status might be presented
directly to a user as the result of that user's unmonitored use of
the device.  If the operating system fails to receive global status
presentations, the operating system can lose track of the subsystem
status, be unable to respond to reconfiguration and other requests
from the subsystem, and be unable to inform other users of...