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chardev/char-hub: implement backend chardev aggregator

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This patch 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 idea of the change is trivial: keep list of backend devices
(up to 4), init them on demand and forward data buffer back and
forth.

The following is QEMU command line example:

   -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 2 backend devices: text virtual console (`vc0`) and a
pseudo TTY (`pty0`) connected to the single virtio hvc console with
the backend aggregator (`hub0`) help. `vc0` renders text to an image,
which can be shared over the VNC protocol.  `pty0` is a pseudo TTY
backend which provides biderectional communication to the virtio hvc
console.

'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 syntax may help to smoothly switch to modern parsing in the
future.

Signed-off-by: Roman Penyaev <r.peniaev@gmail.com>
Reviewed-by: "Marc-André Lureau" <marcandre.lureau@redhat.com>
Cc: qemu-devel@nongnu.org
Message-ID: <20250123085327.965501-3-r.peniaev@gmail.com>
This commit is contained in:
Roman Penyaev 2025-01-23 09:53:22 +01:00 • committed by Marc-André Lureau
parent 4e059a9d96
commit b66ed23238
6 changed files with 401 additions and 3 deletions
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27
qapi/char.json
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@ -332,6 +332,19 @@
'data': { 'chardev': 'str' },
'base': 'ChardevCommon' }
##
# @ChardevHub:
#
# Configuration info for hub chardevs.
#
# @chardevs: List of chardev IDs, which should be added to this hub
#
# Since: 10.0
##
{ 'struct': 'ChardevHub',
'data': { 'chardevs': ['str'] },
'base': 'ChardevCommon' }
##
# @ChardevStdio:
#
@ -479,6 +492,8 @@
#
# @mux: (since 1.5)
#
# @hub: (since 10.0)
#
# @msmouse: emulated Microsoft serial mouse (since 1.5)
#
# @wctablet: emulated Wacom Penpartner serial tablet (since 2.9)
@ -521,6 +536,7 @@
'pty',
'null',
'mux',
'hub',
'msmouse',
'wctablet',
{ 'name': 'braille', 'if': 'CONFIG_BRLAPI' },
@ -595,6 +611,16 @@
{ 'struct': 'ChardevMuxWrapper',
'data': { 'data': 'ChardevMux' } }
##
# @ChardevHubWrapper:
#
# @data: Configuration info for hub chardevs
#
# Since: 10.0
##
{ 'struct': 'ChardevHubWrapper',
'data': { 'data': 'ChardevHub' } }
##
# @ChardevStdioWrapper:
#
@ -703,6 +729,7 @@
'pty': 'ChardevPtyWrapper',
'null': 'ChardevCommonWrapper',
'mux': 'ChardevMuxWrapper',
'hub': 'ChardevHubWrapper',
'msmouse': 'ChardevCommonWrapper',
'wctablet': 'ChardevCommonWrapper',
'braille': { 'type': 'ChardevCommonWrapper',