Re: [PATCH v7 2/4] chardev/char-hub: implement backend chardev aggregator

["Alex Bennée" <alex.bennee@linaro.org>]

Roman Penyaev <r.peniaev@gmail.com> writes:

> 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.
>
<snip>
> +static void qemu_chr_open_hub(Chardev *chr,
> +                                 ChardevBackend *backend,
> +                                 bool *be_opened,
> +                                 Error **errp)
> +{
> +    ChardevHub *hub = backend->u.hub.data;
> +    HubChardev *d = HUB_CHARDEV(chr);
> +    strList *list = hub->chardevs;
> +
> +    d->be_eagain_ind = -1;
> +
> +    if (list == NULL) {
> +        error_setg(errp, "hub: 'chardevs' list is not defined");
> +        return;
> +    }
> +
> +    while (list) {
> +        Chardev *s;
> +
> +        s = qemu_chr_find(list->value);
> +        if (s == NULL) {
> +            error_setg(errp, "hub: chardev can't be found by id '%s'",
> +                       list->value);
> +            return;
> +        }
> +        if (CHARDEV_IS_HUB(s) || CHARDEV_IS_MUX(s)) {
> +            error_setg(errp, "hub: multiplexers and hub devices can't be "
> +                       "stacked, check chardev '%s', chardev should not "
> +                       "be a hub device or have 'mux=on' enabled",
> +                       list->value);
> +            return;

So I was looking at this to see if I could implement what I wanted which
was a tee-like copy of a serial port output while maintaining the C-a
support of the mux.

Normally I just use the shortcut -serial mon:stdio

However that form is a special case so I tried the following and ran
into the above:

          -chardev stdio,mux=on,id=char0 \
          -chardev file,path=console.log,id=clog  \
          -mon chardev=char0,mode=readline \
          -chardev hub,id=hub0,chardevs.0=char0,chardevs.1=clog

Giving:
  qemu-system-aarch64: -chardev -hub,id=hub0,chardevs.0=char0,chardevs.1=clog: hub: -multiplexers and hub devices can't be stacked, check chardev
            -'char0', chardev should not be a hub device or have 'mux=on' -enabled
          
So what stops this sort of chain?

-- 
Alex Bennée
Virtualisation Tech Lead @ Linaro