On 05/05/2020 15.50, Cornelia Huck wrote: > Add some hints about "devno" rules.
Thanks a lot for doing this, some documentation in this area is extremely useful! > Signed-off-by: Cornelia Huck <coh...@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``. Maybe mention the QEMU side again, e.g. that when you run "info qtree" in the HMP monitor, you see the device with dev_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``. Does "same" matter here? Otherwise, maybe just say "to a Linux guest" ? > +* 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``. Dito. > +* 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. Ok ... and what happens if you use devno=0.0.1234 ? Will that then show up under fe.0.1234 in the guest?? Thomas
