docs/s390x: document the virtual css

The virtual channel subsystem

=============================

QEMU implements a virtual channel subsystem with subchannels, (mostly

functionless) channel paths, and channel devices (virtio-ccw, 3270, and

devices passed via vfio-ccw). It supports multiple subchannel sets (MSS) and

multiple channel subsystems extended (MCSS-E).

All channel devices support the ``devno`` property, which takes a parameter

in the form ``<cssid>.<ssid>.<device number>``.

The default channel subsystem image id (``<cssid>``) is ``0xfe``. Devices in

there will show up in channel subsystem image ``0`` to guests that do not

enable MCSS-E. Note that devices with a different cssid will not be visible

if the guest OS does not enable MCSS-E (which is true for all supported guest

operating systems today).

Supported values for the subchannel set id (``<ssid>``) range from ``0-3``.

Devices with a ssid that is not ``0`` will not be visible if the guest OS

does not enable MSS (any Linux version that supports virtio also enables MSS).

Any device may be put into any subchannel set, there is no restriction by

device type.

The device number can range from ``0-0xffff``.

If the ``devno`` property is not specified for a device, QEMU will choose the

next free device number in subchannel set 0, skipping to the next subchannel

set if no more device numbers are free.

QEMU places a device at the first free subchannel in the specified subchannel

set. If a device is hotunplugged and later replugged, it may appear at a

different subchannel. (This is similar to how z/VM works.)

Examples

--------

* a virtio-net device, cssid/ssid/devno automatically assigned::

    -device virtio-net-ccw

  In a Linux guest (without default devices and no other devices specified

  prior to this one), this will show up as ``0.0.0000`` under subchannel

  ``0.0.0000``.

  The auto-assigned-properties in QEMU (as seen via e.g. ``info qtree``)

  would be ``dev_id = "fe.0.0000"`` and ``subch_id = "fe.0.0000"``.

* a virtio-rng device in subchannel set ``0``::

    -device virtio-rng-ccw,devno=fe.0.0042

  If added to the same Linux guest as above, it would show up as ``0.0.0042``

  under subchannel ``0.0.0001``.

  The properties for the device would be ``dev_id = "fe.0.0042"`` and

  ``subch_id = "fe.0.0001"``.

* a virtio-gpu device in subchannel set ``2``::

    -device virtio-gpu-ccw,devno=fe.2.1111

  If added to the same Linux guest as above, it would show up as ``0.2.1111``

  under subchannel ``0.2.0000``.

  The properties for the device would be ``dev_id = "fe.2.1111"`` and

  ``subch_id = "fe.2.0000"``.

* a virtio-mouse device in a non-standard channel subsystem image::

    -device virtio-mouse-ccw,devno=2.0.2222

  This would not show up in a standard Linux guest.

  The properties for the device would be ``dev_id = "2.0.2222"`` and

  ``subch_id = "2.0.0000"``.

* a virtio-keyboard device in another non-standard channel subsystem image::

    -device virtio-keyboard-ccw,devno=0.0.1234

  This would not show up in a standard Linux guest, either, as ``0`` is not

  the standard channel subsystem image id.

  The properties for the device would be ``dev_id = "0.0.1234"`` and

  ``subch_id = "0.0.0000"``.

.. toctree::

   s390x/vfio-ap

   s390x/css

Architectural features

======================