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+Date : 2004-Nov-26
+Author: Gerald Schaefer (firstname.lastname@example.org)
+ Linux API for read access to z/VM Monitor Records
+This item delivers a new Linux API in the form of a misc char device that is
+useable from user space and allows read access to the z/VM Monitor Records
+collected by the *MONITOR System Service of z/VM.
+The z/VM guest on which you want to access this API needs to be configured in
+order to allow IUCV connections to the *MONITOR service, i.e. it needs the
+IUCV *MONITOR statement in its user entry. If the monitor DCSS to be used is
+restricted (likely), you also need the NAMESAVE <DCSS NAME> statement.
+This item will use the IUCV device driver to access the z/VM services, so you
+need a kernel with IUCV support. You also need z/VM version 4.4 or 5.1.
+There are two options for being able to load the monitor DCSS (examples assume
+that the monitor DCSS begins at 144 MB and ends at 152 MB). You can query the
+location of the monitor DCSS with the Class E privileged CP command Q NSS MAP
+(the values BEGPAG and ENDPAG are given in units of 4K pages).
+See also "CP Command and Utility Reference" (SC24-6081-00) for more information
+on the DEF STOR and Q NSS MAP commands, as well as "Saved Segments Planning
+and Administration" (SC24-6116-00) for more information on DCSSes.
+You can use the CP command DEF STOR CONFIG to define a "memory hole" in your
+guest virtual storage around the address range of the DCSS.
+Example: DEF STOR CONFIG 0.140M 200M.200M
+This defines two blocks of storage, the first is 140MB in size an begins at
+address 0MB, the second is 200MB in size and begins at address 200MB,
+resulting in a total storage of 340MB. Note that the first block should
+always start at 0 and be at least 64MB in size.
+Your guest virtual storage has to end below the starting address of the DCSS
+and you have to specify the "mem=" kernel parameter in your parmfile with a
+value greater than the ending address of the DCSS.
+Example: DEF STOR 140M
+This defines 140MB storage size for your guest, the parameter "mem=160M" is
+added to the parmfile.
+The char device is implemented as a kernel module named "monreader",
+which can be loaded via the modprobe command, or it can be compiled into the
+kernel instead. There is one optional module (or kernel) parameter, "mondcss",
+to specify the name of the monitor DCSS. If the module is compiled into the
+kernel, the kernel parameter "monreader.mondcss=<DCSS NAME>" can be specified
+in the parmfile.
+The default name for the DCSS is "MONDCSS" if none is specified. In case that
+there are other users already connected to the *MONITOR service (e.g.
+Performance Toolkit), the monitor DCSS is already defined and you have to use
+the same DCSS. The CP command Q MONITOR (Class E privileged) shows the name
+of the monitor DCSS, if already defined, and the users connected to the
+Refer to the "z/VM Performance" book (SC24-6109-00) on how to create a monitor
+DCSS if your z/VM doesn't have one already, you need Class E privileges to
+define and save a DCSS.
+modprobe monreader mondcss=MYDCSS
+This loads the module and sets the DCSS name to "MYDCSS".
+This API provides no interface to control the *MONITOR service, e.g. specifiy
+which data should be collected. This can be done by the CP command MONITOR
+(Class E privileged), see "CP Command and Utility Reference".
+Device nodes with udev:
+After loading the module, a char device will be created along with the device
+node /<udev directory>/monreader.
+Device nodes without udev:
+If your distribution does not support udev, a device node will not be created
+automatically and you have to create it manually after loading the module.
+Therefore you need to know the major and minor numbers of the device. These
+numbers can be found in /sys/class/misc/monreader/dev.
+Typing cat /sys/class/misc/monreader/dev will give an output of the form
+<major>:<minor>. The device node can be created via the mknod command, enter
+mknod <name> c <major> <minor>, where <name> is the name of the device node
+to be created.
+# modprobe monreader
+# cat /sys/class/misc/monreader/dev
+# mknod /dev/monreader c 10 63
+This loads the module with the default monitor DCSS (MONDCSS) and creates a
+The following file operations are supported: open, release, read, poll.
+There are two alternative methods for reading: either non-blocking read in
+conjunction with polling, or blocking read without polling. IOCTLs are not
+Reading from the device provides a 12 Byte monitor control element (MCE),
+followed by a set of one or more contiguous monitor records (similar to the
+output of the CMS utility MONWRITE without the 4K control blocks). The MCE
+contains information on the type of the following record set (sample/event
+data), the monitor domains contained within it and the start and end address
+of the record set in the monitor DCSS. The start and end address can be used
+to determine the size of the record set, the end address is the address of the
+last byte of data. The start address is needed to handle "end-of-frame" records
+correctly (domain 1, record 13), i.e. it can be used to determine the record
+start offset relative to a 4K page (frame) boundary.
+See "Appendix A: *MONITOR" in the "z/VM Performance" document for a description
+of the monitor control element layout. The layout of the monitor records can
+be found here (z/VM 5.1): http://www.vm.ibm.com/pubs/mon510/index.html
+The layout of the data stream provided by the monreader device is as follows:
+<0 byte read>
+<first MCE> \
+<first set of records> |
+... |- data set
+<last MCE> |
+<last set of records> /
+<0 byte read>
+There may be more than one combination of MCE and corresponding record set
+within one data set and the end of each data set is indicated by a successful
+read with a return value of 0 (0 byte read).
+Any received data must be considered invalid until a complete set was
+read successfully, including the closing 0 byte read. Therefore you should
+always read the complete set into a buffer before processing the data.
+The maximum size of a data set can be as large as the size of the
+monitor DCSS, so design the buffer adequately or use dynamic memory allocation.
+The size of the monitor DCSS will be printed into syslog after loading the
+module. You can also use the (Class E privileged) CP command Q NSS MAP to
+list all available segments and information about them.
+As with most char devices, error conditions are indicated by returning a
+negative value for the number of bytes read. In this case, the errno variable
+indicates the error condition:
+EIO: reply failed, read data is invalid and the application
+ should discard the data read since the last successful read with 0 size.
+EFAULT: copy_to_user failed, read data is invalid and the application should
+ discard the data read since the last successful read with 0 size.
+EAGAIN: occurs on a non-blocking read if there is no data available at the
+ moment. There is no data missing or corrupted, just try again or rather
+ use polling for non-blocking reads.
+EOVERFLOW: message limit reached, the data read since the last successful
+ read with 0 size is valid but subsequent records may be missing.
+In the last case (EOVERFLOW) there may be missing data, in the first two cases
+(EIO, EFAULT) there will be missing data. It's up to the application if it will
+continue reading subsequent data or rather exit.
+Only one user is allowed to open the char device. If it is already in use, the
+open function will fail (return a negative value) and set errno to EBUSY.
+The open function may also fail if an IUCV connection to the *MONITOR service
+cannot be established. In this case errno will be set to EIO and an error
+message with an IPUSER SEVER code will be printed into syslog. The IPUSER SEVER
+codes are described in the "z/VM Performance" book, Appendix A.
+As soon as the device is opened, incoming messages will be accepted and they
+will account for the message limit, i.e. opening the device without reading
+from it will provoke the "message limit reached" error (EOVERFLOW error code)