[PATCH 1/3] docs/s390x: document the virtual css

[Cornelia Huck <cohuck@redhat.com>]

Add some hints about "devno" rules.

Signed-off-by: Cornelia Huck <cohuck@redhat.com>
---
 docs/system/s390x/css.rst    | 64 ++++++++++++++++++++++++++++++++++++
 docs/system/target-s390x.rst |  1 +
 2 files changed, 65 insertions(+)
 create mode 100644 docs/system/s390x/css.rst

diff --git a/docs/system/s390x/css.rst b/docs/system/s390x/css.rst
new file mode 100644
index 000000000000..8e18194a2f0a
--- /dev/null
+++ b/docs/system/s390x/css.rst
@@ -0,0 +1,64 @@
+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 of 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``.
+
+* 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``.
+
+* 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``.
+
+* a virtio-mouse device in a non-standard channel subsystem::
+
+    -device virtio-mouse-ccw,devno=2.0.2222
+
+  This would not show up in a standard Linux guest.
diff --git a/docs/system/target-s390x.rst b/docs/system/target-s390x.rst
index 7d76ae97b401..37ca032d98ef 100644
--- a/docs/system/target-s390x.rst
+++ b/docs/system/target-s390x.rst
@@ -23,6 +23,7 @@ or vfio-ap is also available.
 
 .. toctree::
    s390x/vfio-ap
+   s390x/css
 
 Architectural features
 ======================
-- 
2.21.3