qemu

FORK: QEMU emulator
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      1 The virtual channel subsystem
      2 =============================
      3 
      4 QEMU implements a virtual channel subsystem with subchannels, (mostly
      5 functionless) channel paths, and channel devices (virtio-ccw, 3270, and
      6 devices passed via vfio-ccw). It supports multiple subchannel sets (MSS) and
      7 multiple channel subsystems extended (MCSS-E).
      8 
      9 All channel devices support the ``devno`` property, which takes a parameter
     10 in the form ``<cssid>.<ssid>.<device number>``.
     11 
     12 The default channel subsystem image id (``<cssid>``) is ``0xfe``. Devices in
     13 there will show up in channel subsystem image ``0`` to guests that do not
     14 enable MCSS-E. Note that devices with a different cssid will not be visible
     15 if the guest OS does not enable MCSS-E (which is true for all supported guest
     16 operating systems today).
     17 
     18 Supported values for the subchannel set id (``<ssid>``) range from ``0-3``.
     19 Devices with a ssid that is not ``0`` will not be visible if the guest OS
     20 does not enable MSS (any Linux version that supports virtio also enables MSS).
     21 Any device may be put into any subchannel set, there is no restriction by
     22 device type.
     23 
     24 The device number can range from ``0-0xffff``.
     25 
     26 If the ``devno`` property is not specified for a device, QEMU will choose the
     27 next free device number in subchannel set 0, skipping to the next subchannel
     28 set if no more device numbers are free.
     29 
     30 QEMU places a device at the first free subchannel in the specified subchannel
     31 set. If a device is hotunplugged and later replugged, it may appear at a
     32 different subchannel. (This is similar to how z/VM works.)
     33 
     34 
     35 Examples
     36 --------
     37 
     38 * a virtio-net device, cssid/ssid/devno automatically assigned::
     39 
     40     -device virtio-net-ccw
     41 
     42   In a Linux guest (without default devices and no other devices specified
     43   prior to this one), this will show up as ``0.0.0000`` under subchannel
     44   ``0.0.0000``.
     45 
     46   The auto-assigned-properties in QEMU (as seen via e.g. ``info qtree``)
     47   would be ``dev_id = "fe.0.0000"`` and ``subch_id = "fe.0.0000"``.
     48 
     49 * a virtio-rng device in subchannel set ``0``::
     50 
     51     -device virtio-rng-ccw,devno=fe.0.0042
     52 
     53   If added to the same Linux guest as above, it would show up as ``0.0.0042``
     54   under subchannel ``0.0.0001``.
     55 
     56   The properties for the device would be ``dev_id = "fe.0.0042"`` and
     57   ``subch_id = "fe.0.0001"``.
     58 
     59 * a virtio-gpu device in subchannel set ``2``::
     60 
     61     -device virtio-gpu-ccw,devno=fe.2.1111
     62 
     63   If added to the same Linux guest as above, it would show up as ``0.2.1111``
     64   under subchannel ``0.2.0000``.
     65 
     66   The properties for the device would be ``dev_id = "fe.2.1111"`` and
     67   ``subch_id = "fe.2.0000"``.
     68 
     69 * a virtio-mouse device in a non-standard channel subsystem image::
     70 
     71     -device virtio-mouse-ccw,devno=2.0.2222
     72 
     73   This would not show up in a standard Linux guest.
     74 
     75   The properties for the device would be ``dev_id = "2.0.2222"`` and
     76   ``subch_id = "2.0.0000"``.
     77 
     78 * a virtio-keyboard device in another non-standard channel subsystem image::
     79 
     80     -device virtio-keyboard-ccw,devno=0.0.1234
     81 
     82   This would not show up in a standard Linux guest, either, as ``0`` is not
     83   the standard channel subsystem image id.
     84 
     85   The properties for the device would be ``dev_id = "0.0.1234"`` and
     86   ``subch_id = "0.0.0000"``.