Tuesday, August 8, 2017

Channel I/O: More about channel paths

recent discussion on qemu-devel touched upon some aspects of channel paths and their handling (or not-handling) in QEMU. I will try to summarize and give some further information here.

I previously published some information on channel paths here. This post will concentrate a bit more on aspects that are not yet relevant in QEMU, but may become so in the future.

To recap: Channel paths represent the means by which the mainframe talks to the device - it (somewhat) corresponds to the actual cabling. Let's take a look at the output of lscss on a z/VM guest as an actual example:

Device   Subchan.  DevType CU Type Use  PIM PAM POM  CHPIDs
----------------------------------------------------------------------
0.0.0150 0.0.0000  0000/00 3088/08      80  80  ff   08000000 00000000
0.0.0151 0.0.0001  0000/00 3088/08      80  80  ff   08000000 00000000
0.0.8000 0.0.0002  1732/01 1731/01 yes  80  80  ff   00000000 00000000
0.0.8001 0.0.0003  1732/01 1731/01 yes  80  80  ff   00000000 00000000
0.0.8002 0.0.0004  1732/01 1731/01 yes  80  80  ff   00000000 00000000
0.0.8003 0.0.0005  1732/01 1731/01      80  80  ff   01000000 00000000
0.0.8004 0.0.0006  1732/01 1731/01      80  80  ff   01000000 00000000
0.0.8005 0.0.0007  1732/01 1731/01      80  80  ff   01000000 00000000
0.0.0191 0.0.0008  3390/0a 3990/e9      e0  e0  ff   2a3a0900 00000000
0.0.208f 0.0.0009  3390/0c 3990/e9 yes  e0  e0  ff   3a2a1a00 00000000
0.0.218f 0.0.000a  3390/0c 3990/e9 yes  e0  e0  ff   2a3a0900 00000000
0.0.228f 0.0.000b  3390/0c 3990/e9 yes  e0  e0  ff   2a3a1a00 00000000
0.0.238f 0.0.000c  3390/0c 3990/e9 yes  e0  e0  ff   093a2a00 00000000
0.0.000c 0.0.000d  0000/00 2540/00      80  80  ff   08000000 00000000
0.0.000d 0.0.000e  0000/00 2540/00      80  80  ff   08000000 00000000
0.0.000e 0.0.000f  0000/00 1403/00      80  80  ff   08000000 00000000
0.0.0009 0.0.0010  0000/00 3215/00 yes  80  80  ff   08000000 00000000
0.0.0190 0.0.0011  3390/0a 3990/e9      e0  e0  ff   3a2a1a00 00000000
0.0.019d 0.0.0012  3390/0a 3990/e9      e0  e0  ff   093a2a00 00000000
0.0.019e 0.0.0013  3390/0a 3990/e9      e0  e0  ff   093a2a00 00000000
0.0.0592 0.0.0014  3390/0a 3990/e9      e0  e0  ff   3a2a1a00 00000000
0.0.ffff 0.0.0015  9336/10 6310/80      80  80  ff   08000000 00000000

A couple of interesting observations with regard to channel paths can be made here:
  • Devices 0.0.0150/0.0.0151, 0.0.000c/0.0.000d, 0.0.000e, 0.0.0009, and 0.0.ffff all share the same channel path, 08, despite being of different types (virtual CTC, virtual card punch/card reader/printer, virtual console, and virtual FBA DASD). This is because they are all emulated devices, and z/VM chooses to use the same virtual channel path for them.
  • Devices 0.0.8000 - 0.0.8002 uses channel path 0 as their only channel path, as can be seen by the PIM being 80.
  • Devices 0.0.8000 - 0.0.8002 and 0.0.8003-0.0.8005 use the same channel path, respectively; that is because they make up the device triplet for an OSA device.
  • The remaining devices (all ECKD DASD) use several channel paths (09, 1a, 2a, 3a), but only three at a time (as evidenced by the PIM of 0e), and also in different combination. This is probably a quirk of the individual setup for this guest.
The output of lschp of the same guest looks like this:

CHPID  Vary  Cfg.  Type  Cmg  Shared  PCHID
============================================
0.00   1     1     11    -    -      (ff00)
0.01   1     1     11    -    -      (ff01)
0.08   1     1     1a    -    -       0598 
0.09   1     1     1a    -    -       0599 
0.0a   1     1     1a    -    -       059c 
0.0b   1     1     25    -    -       059d 
0.0c   1     1     1a    -    -       05ac 
0.17   1     1     11    -    -       05b4 
0.18   1     1     1a    -    -       05a0 
0.19   1     1     1a    -    -       05a1 
0.1a   1     1     1a    -    -       05a4 
0.1b   1     1     25    -    -       05a5 
0.1c   1     1     1a    -    -       05ad 
0.28   1     1     1a    -    -       05d8 
0.29   1     1     1a    -    -       05d9 
0.2a   1     1     1a    -    -       05dc 
0.2b   1     1     25    -    -       05dd 
0.2c   1     1     1a    -    -       05d0 
0.34   1     1     11    -    -       05ec 
0.35   1     1     11    -    -       05a8 
0.38   1     1     1a    -    -       05e0 
0.39   1     1     1a    -    -       05e1 
0.3a   1     1     1a    -    -       05e4 
0.3b   1     1     25    -    -       05e5 
0.3c   1     1     1a    -    -       05d1 
0.60   1     1     24    -    -      (070c)
0.61   1     1     24    -    -      (070d)
0.62   1     1     24    -    -      (070e)
0.63   1     1     24    -    -      (070f)

Here we find the various channel paths again, together with more:
  • There are several channel paths that are available to the guest, but not in use by any device currently available to the guest (and therefore not turning up in the output of lscss).
  • Channel paths 00 and 01 (used by the OSA cards) use an internal channel (the number in the last column are in brackets) - we can therefore conclude that the cards are virtualized by z/VM.
  • The channel path 08 (which is referenced by all virtual devices) is actually backed by a physical path (0598). I frankly have no idea why z/VM is doing that.
  • The channel paths used by the ECKD DASD (09, 1a, 2a, 3a) all are of the same type (1a - FICON, IIRC) and are backed by different physical paths (last column).
Various modifications can be done to the channel paths; under Linux, the chchp tool is useful for that. Let's try to vary off a path:

chchp -v 0 0.3a
Vary offline 0.3a... done.

lschp shows the changed state for the path:

0.3a   0     1     1a    -    -       05e4

The lscss output remains unchanged - which isn't surprising as doing a vary off only affects the state of the channel path within Linux: Linux will no longer use the path for I/O, but the path masks as managed by the hardware and z/VM are not changed.

Let's try to configure off another path:

chchp -c 0 0.2a
Configure standby 0.2a... failed - attribute value not as expected

That did not work as expected. Why? This is supposed to issue a SCLP command to set the channel path to standby - but the my guest apparently does not have the rights or ability to do so. Which is a pity, as I would have liked to show the effects of configuring a channel path to standby:
  • It (unsurprisingly) changes the state in lschp.
  • It also changes the path masks, as shown in lscss.
  • It may generate a machine check with a channel report word (CRW) that informs the OS that something has happened to the channel path - this is dependent upon the environment, however.
So let's stop here. I'll continue with another setup, once I have it.

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