1The virtual channel subsystem 2============================= 3 4QEMU implements a virtual channel subsystem with subchannels, (mostly 5functionless) channel paths, and channel devices (virtio-ccw, 3270, and 6devices passed via vfio-ccw). It supports multiple subchannel sets (MSS) and 7multiple channel subsystems extended (MCSS-E). 8 9All channel devices support the ``devno`` property, which takes a parameter 10in the form ``<cssid>.<ssid>.<device number>``. 11 12The default channel subsystem image id (``<cssid>``) is ``0xfe``. Devices in 13there will show up in channel subsystem image ``0`` to guests that do not 14enable MCSS-E. Note that devices with a different cssid will not be visible 15if the guest OS does not enable MCSS-E (which is true for all supported guest 16operating systems today). 17 18Supported values for the subchannel set id (``<ssid>``) range from ``0-3``. 19Devices with a ssid that is not ``0`` will not be visible if the guest OS 20does not enable MSS (any Linux version that supports virtio also enables MSS). 21Any device may be put into any subchannel set, there is no restriction by 22device type. 23 24The device number can range from ``0-0xffff``. 25 26If the ``devno`` property is not specified for a device, QEMU will choose the 27next free device number in subchannel set 0, skipping to the next subchannel 28set if no more device numbers are free. 29 30QEMU places a device at the first free subchannel in the specified subchannel 31set. If a device is hotunplugged and later replugged, it may appear at a 32different subchannel. (This is similar to how z/VM works.) 33 34 35Examples 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"``. 87