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, 21 insertions, 2 deletions

diff --git a/chardev/char.c b/chardev/char.c
index 7705da5ad0..5a9e9762ad 100644
--- a/chardev/char.c
+++ b/chardev/char.c
@@ -943,7 +943,26 @@ QemuOptsList qemu_chardev_opts = {
         },{
             .name = "chardev",
             .type = QEMU_OPT_STRING,
+        },
+        /*
+         * Multiplexer options. Follows QAPI array syntax.
+         * See MAX_HUB macro to obtain array capacity.
+         */
+        {
+            .name = "chardevs.0",
+            .type = QEMU_OPT_STRING,
+        },{
+            .name = "chardevs.1",
+            .type = QEMU_OPT_STRING,
         },{
+            .name = "chardevs.2",
+            .type = QEMU_OPT_STRING,
+        },{
+            .name = "chardevs.3",
+            .type = QEMU_OPT_STRING,
+        },
+
+        {
             .name = "append",
             .type = QEMU_OPT_BOOL,
         },{
@@ -1106,8 +1125,8 @@ ChardevReturn *qmp_chardev_change(const char *id, ChardevBackend *backend,
         return NULL;
     }
 
-    if (CHARDEV_IS_MUX(chr)) {
-        error_setg(errp, "Mux device hotswap not supported yet");
+    if (CHARDEV_IS_MUX(chr) || CHARDEV_IS_HUB(chr)) {
+        error_setg(errp, "For mux or hub device hotswap is not supported yet");
         return NULL;
     }