Browse Prior Art Database

I/O Subsystem Special Interception Condition

IP.com Disclosure Number: IPCOM000100345D
Original Publication Date: 1990-Apr-01
Included in the Prior Art Database: 2005-Mar-15
Document File: 6 page(s) / 280K

Publishing Venue

IBM

Related People

Beardsley, BC: AUTHOR [+6]

Abstract

The IBM 3990 Storage Control implements device commands that have an effect on the entire 3990 subsystem or on multiple devices within the subsystem. The 3990 also reports asynchronous status from device addresses to request subsystem functions or report subsystem status.

This text was extracted from an ASCII text file.
This is the abbreviated version, containing approximately 22% of the total text.

I/O Subsystem Special Interception Condition

       The IBM 3990 Storage Control implements device commands
that have an effect on the entire 3990 subsystem or on multiple
devices within the subsystem.  The 3990 also reports asynchronous
status from device addresses to request subsystem functions or report
subsystem status.

      This article describes a new special interception condition
implemented in the 3990 Storage Control to permit virtual machine
(VM) operating systems 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.

      Modern computer I/O subsystems continue to advance, with
increasing "intelligence" outboard of the central host processor.
Included in the IBM 3990's new features and functions are commands
that have effects beyond a single I/O device (herein called "global"
commands).  Global commands affect the operation of the entire
subsystem or more than a single device.

      Also included in advanced I/O subsystems are asynchronous
status presentations (herein called "global" status presentations)
that signify events having a larger scope than the single device the
address of which is used to report the status.

      Examples of global commands in the IBM 3990 Model 3 Storage
Control include commands that turn the subsystem cache on or off,
enable or disable the subsystem-wide cache fast write function, and
create a dual-copy pair of devices.

      Examples of global status presentations in the 3990 model 3
include certain attention and state-change status presentations to
the host to signal reconfiguration requests and subsystem
state-changes that encompass more than the single device presenting
the status.

      Global commands and status presentations can present a problem
for host operating systems, and in particular for virtual machine
operating systems.  Operating systems typically allocate system I/O
resources on a device or portion-of-a-device basis.  In an
environment of global commands and status presentations, such an
allocation scheme can allow a user to exercise system control beyond
the boundaries of the user's allocated resources.  At the same time,
changing the allocation scheme to give an entire I/O subsystem to a
single user would usually result in an extreme over-allocation of
resources to one user and defeat a primary purpose (i.e.,
resource-sharing) of the operating system.

      The mechanism herein disclosed enlists the assistance of the
I/O subsystem in the host operating system's monitoring of the use of
I/O devices.  A "privileged communication mechanism" is created
between the host operating system and the I/O subsystem.

      Such a communication mechanism can comprise a physically or
logically separate communication path used only by th...