chardev/char-hub: implement backend chardev aggregator

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>

1 files changed, 27 insertions, 0 deletions

diff --git a/qapi/char.json b/qapi/char.json
index e045354350..f02b66c06b 100644
--- a/qapi/char.json
+++ b/qapi/char.json
@@ -333,6 +333,19 @@
   '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:
 #
 # Configuration info for stdio chardevs.
@@ -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' },
@@ -596,6 +612,16 @@
   'data': { 'data': 'ChardevMux' } }
 
 ##
+# @ChardevHubWrapper:
+#
+# @data: Configuration info for hub chardevs
+#
+# Since: 10.0
+##
+{ 'struct': 'ChardevHubWrapper',
+  'data': { 'data': 'ChardevHub' } }
+
+##
 # @ChardevStdioWrapper:
 #
 # @data: Configuration info for stdio chardevs
@@ -703,6 +729,7 @@
             'pty': 'ChardevPtyWrapper',
             'null': 'ChardevCommonWrapper',
             'mux': 'ChardevMuxWrapper',
+            'hub': 'ChardevHubWrapper',
             'msmouse': 'ChardevCommonWrapper',
             'wctablet': 'ChardevCommonWrapper',
             'braille': { 'type': 'ChardevCommonWrapper',