87 lines
3.1 KiB
ReStructuredText
87 lines
3.1 KiB
ReStructuredText
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The virtual channel subsystem
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=============================
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QEMU implements a virtual channel subsystem with subchannels, (mostly
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functionless) channel paths, and channel devices (virtio-ccw, 3270, and
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devices passed via vfio-ccw). It supports multiple subchannel sets (MSS) and
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multiple channel subsystems extended (MCSS-E).
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All channel devices support the ``devno`` property, which takes a parameter
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in the form ``<cssid>.<ssid>.<device number>``.
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The default channel subsystem image id (``<cssid>``) is ``0xfe``. Devices in
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there will show up in channel subsystem image ``0`` to guests that do not
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enable MCSS-E. Note that devices with a different cssid will not be visible
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if the guest OS does not enable MCSS-E (which is true for all supported guest
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operating systems today).
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Supported values for the subchannel set id (``<ssid>``) range from ``0-3``.
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Devices with a ssid that is not ``0`` will not be visible if the guest OS
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does not enable MSS (any Linux version that supports virtio also enables MSS).
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Any device may be put into any subchannel set, there is no restriction by
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device type.
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The device number can range from ``0-0xffff``.
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If the ``devno`` property is not specified for a device, QEMU will choose the
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next free device number in subchannel set 0, skipping to the next subchannel
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set if no more device numbers are free.
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QEMU places a device at the first free subchannel in the specified subchannel
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set. If a device is hotunplugged and later replugged, it may appear at a
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different subchannel. (This is similar to how z/VM works.)
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Examples
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--------
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* a virtio-net device, cssid/ssid/devno automatically assigned::
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-device virtio-net-ccw
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In a Linux guest (without default devices and no other devices specified
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prior to this one), this will show up as ``0.0.0000`` under subchannel
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``0.0.0000``.
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The auto-assigned-properties in QEMU (as seen via e.g. ``info qtree``)
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would be ``dev_id = "fe.0.0000"`` and ``subch_id = "fe.0.0000"``.
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* a virtio-rng device in subchannel set ``0``::
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-device virtio-rng-ccw,devno=fe.0.0042
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If added to the same Linux guest as above, it would show up as ``0.0.0042``
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under subchannel ``0.0.0001``.
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The properties for the device would be ``dev_id = "fe.0.0042"`` and
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``subch_id = "fe.0.0001"``.
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* a virtio-gpu device in subchannel set ``2``::
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-device virtio-gpu-ccw,devno=fe.2.1111
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If added to the same Linux guest as above, it would show up as ``0.2.1111``
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under subchannel ``0.2.0000``.
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The properties for the device would be ``dev_id = "fe.2.1111"`` and
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``subch_id = "fe.2.0000"``.
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* a virtio-mouse device in a non-standard channel subsystem image::
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-device virtio-mouse-ccw,devno=2.0.2222
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This would not show up in a standard Linux guest.
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The properties for the device would be ``dev_id = "2.0.2222"`` and
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``subch_id = "2.0.0000"``.
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* a virtio-keyboard device in another non-standard channel subsystem image::
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-device virtio-keyboard-ccw,devno=0.0.1234
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This would not show up in a standard Linux guest, either, as ``0`` is not
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the standard channel subsystem image id.
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The properties for the device would be ``dev_id = "0.0.1234"`` and
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``subch_id = "0.0.0000"``.
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