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3270 devices
============
The 3270 is the classic 'green-screen' console of the mainframes (see the
`IBM 3270 Wikipedia article <https://en.wikipedia.org/wiki/IBM_3270>`__).
The 3270 data stream is not implemented within QEMU; the device only provides
TN3270 (a telnet extension; see `RFC 854 <https://tools.ietf.org/html/rfc854>`__
and `RFC 1576 <https://tools.ietf.org/html/rfc1576>`__) and leaves the heavy
lifting to an external 3270 terminal emulator (such as ``x3270``) to make a
single 3270 device available to a guest. Note that this supports basic
features only.
To provide a 3270 device to a guest, create a ``x-terminal3270`` linked to
a ``tn3270`` chardev. The guest will see a 3270 channel device. In order
to actually be able to use it, attach the ``x3270`` emulator to the chardev.
Example configuration
---------------------
* Make sure that 3270 support is enabled in the guest's Linux kernel. You need
``CONFIG_TN3270`` and at least one of ``CONFIG_TN3270_TTY`` (for additional
ttys) or ``CONFIG_TN3270_CONSOLE`` (for a 3270 console).
* Add a ``tn3270`` chardev and a ``x-terminal3270`` to the QEMU command line::
-chardev socket,id=ch0,host=0.0.0.0,port=2300,wait=off,server=on,tn3270=on
-device x-terminal3270,chardev=ch0,devno=fe.0.000a,id=terminal0
* Start the guest. In the guest, use ``chccwdev -e 0.0.000a`` to enable
the device.
* On the host, start the ``x3270`` emulator::
x3270 <host>:2300
* In the guest, locate the 3270 device node under ``/dev/3270/`` (say,
``tty1``) and start a getty on it::
systemctl start serial-getty@3270-tty1.service
This should get you an additional tty for logging into the guest.
* If you want to use the 3270 device as the Linux kernel console instead of
an additional tty, you can also append ``conmode=3270 condev=000a`` to
the guest's kernel command line. The kernel then should use the 3270 as
console after the next boot.
Restrictions
------------
3270 support is very basic. In particular:
* Only one 3270 device is supported.
* It has only been tested with Linux guests and the x3270 emulator.
* TLS/SSL is not supported.
* Resizing on reattach is not supported.
* Multiple commands in one inbound buffer (for example, when the reset key
is pressed while the network is slow) are not supported.
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