[v7,0/4] chardev: implement backend chardev multiplexing

Message

Roman Penyaev Jan. 18, 2025, 4:40 p.m. UTC

Mux is a character backend (host side) device, which multiplexes
multiple frontends with one backend device. The following is a
few lines from the QEMU manpage [1]:

  A multiplexer is a "1:N" device, and here the "1" end is your
  specified chardev backend, and the "N" end is the various parts
  of QEMU that can talk to a chardev.

But sadly multiple backends are not supported.

This work implements a new chardev backend `hub` device, which
aggregates input from multiple backend devices and forwards it to a
single frontend device. Additionally, `hub` device takes the output
from the frontend device and sends it back to all the connected
backend devices. This allows for seamless interaction between
different backend devices and a single frontend interface.

The motivation is the EVE project [2], where it would be very
convenient to have a virtio console frontend device on the guest that
can be controlled from multiple backend devices, namely VNC and local
TTY emulator. The following is an example of the QEMU command line:

    -chardev pty,path=/tmp/pty,id=pty0 \
    -chardev vc,id=vc0 \
    -chardev hub,id=hub0,chardevs.0=pty0,chardevs.1=vc0 \
    -device virtconsole,chardev=hub0 \
    -vnc 0.0.0.0:0

Which creates two backend devices:

* Text virtual console (`vc0`)
* A pseudo TTY (`pty0`) connected to the single virtio hvc console with the
  help of a new backend aggregator (`hub0`)

`vc0` renders text to an image, which can be shared over the VNC
protocol.  `pty0` is a pseudo TTY backend which provides bidirectional
communication to the virtio hvc console.

Once QEMU starts, the VNC client and any TTY emulator can be used to
control a single hvc console. For example, these two different
consoles should have similar input and output due to the buffer
aggregation:

    # Start TTY emulator
    tio /tmp/pty

    # Start VNC client and switch to virtual console Ctrl-Alt-2
    vncviewer :0

'chardevs.N' list syntax is used for the sake of compatibility with
the representation of JSON lists in 'key=val' pairs format of the
util/keyval.c, despite the fact that modern QAPI way of parsing,
namely qobject_input_visitor_new_str(), is not used. Choice of keeping
QAPI list sytax may help to smoothly switch to modern parsing in the
future.

v6 .. v7:

After discussing v6 it was decided to:

* Rename "multiplexer" to "aggregator"
* Rename "mux-be" device type to "hub"
* Drop all changes related to the original multiplexer implementation

Code changes:

* Added counting of CHR_EVENT_OPENED and CHR_EVENT_CLOSED events
coming from backend devices. This prevents frontend devices from
closing if one of the backend devices has been disconnected. The
logic is simple: "the last one turns off the light".

v5 .. v6:

* Rebased on latest master
* Changed how chardev is attached to a multiplexer: with version 6
mux should specify list elements with ID of chardevs:

    chardevs.0=ID[,chardevs.N=ID]

'chardevs.N' list syntax is used for the sake of compatibility with
the representation of JSON lists in 'key=val' pairs format of the
util/keyval.c, despite the fact that modern QAPI way of parsing,
namely qobject_input_visitor_new_str(), is not used. Choice of keeping
QAPI list sytax may help to smoothly switch to modern parsing in the
future.

v4 .. v5:

* Spelling fixes in qemu-options description
* Memory leaks fixes in mux-be tests
* Add sanity checks to chardev to avoid stacking of mux devices
* Add corresponding unit test case to cover the creation of stacked
  muxers: `-chardev mux-be,mux-id-be=ID`, which is forbidden
* Reflect the fact that stacking is not supported in the documentation

v3 .. v4:

* Rebase on latest chardev changes
* Add unit tests which test corner cases:
   * Inability to remove mux with active frontend
   * Inability to add more chardevs to a mux than `MUX_MAX`
   * Inability to mix mux-fe and mux-be for the same chardev

v2 .. v3:

* Split frontend and backend multiplexer implementations and
  move them to separate files: char-mux-fe.c and char-mux-be.c

v1 .. v2:

* Separate type for the backend multiplexer `mux-be`
* Handle EAGAIN on write to the backend device
* Support of watch of previously failed backend device
* Proper json support of the `mux-be-id` option
* Unit test for the `mux-be` multiplexer

[1] https://www.qemu.org/docs/master/system/qemu-manpage.html#hxtool-6
[2] https://github.com/lf-edge/eve

Signed-off-by: Roman Penyaev <[e-mail]>
Cc: "Marc-André Lureau" <[e-mail]>
Cc: Markus Armbruster <[e-mail]>
Cc: Kevin Wolf <[e-mail]>
Cc: [e-mail]

Roman Penyaev (4):
  chardev/char-pty: send CHR_EVENT_CLOSED on disconnect
  chardev/char-hub: implement backend chardev aggregator
  tests/unit/test-char: add unit tests for hub chardev backend
  qemu-options.hx: describe hub chardev and aggregation of several
    backends

 chardev/char-fe.c          |   9 +
 chardev/char-hub.c         | 334 +++++++++++++++++++++++++++++++
 chardev/char-pty.c         |   3 +-
 chardev/char.c             |  26 ++-
 chardev/chardev-internal.h |  56 +++++-
 chardev/meson.build        |   1 +
 include/chardev/char.h     |   1 +
 qapi/char.json             |  27 +++
 qemu-options.hx            |  48 ++++-
 tests/unit/test-char.c     | 399 +++++++++++++++++++++++++++++++++++++
 10 files changed, 896 insertions(+), 8 deletions(-)
 create mode 100644 chardev/char-hub.c