Re: [Qemu-stable] [Qemu-devel] [PATCH] mux: fix ctrl-a b again

[Paolo Bonzini]

On 18/04/2018 14:22, Marc-André Lureau wrote:
>>> - Chardev: stdio, mux, ringbuf, pty, file, null etc..
>>> - CharBackend: the "user" end
>>> - frontend: the "user"
>> The frontend is the device, the monitor, etc.
> yes, I should have listed it for clarity
> 
>> The backend is how the
>> frontend sees a Chardev, it never talks to it directly.
> 
> Ok, but why not call CharFrontend, since it's the frontend interface
> to the chardev? (which we call the backend, by extension, and because
> many functions are named *_be_* )

The frontend interface is not the same thing as the frontend, though.
The frontend interface is the backend.

> It's less confusing, because that part is mostly internal to char.c
> implementation.
> 
> But rather than:
> 
> static void chr_be_event(Chardev *s, int event)
> {
>     CharBackend *be = s->be;
> ...
>     be->chr_event(be->opaque, event);
> }
> 
> I would rather have:
> 
> static void chr_be_event(Chardev *s, int event)
> {
>     CharFrontend *fe = s->fe;
> ...
>     fe->chr_event(fe->opaque, event);
> }

What I would rather have instead is this:

   static void qemu_chr_be_send_event(CharBackend *be, int event)
   {
       if (be->frontend_ops && be->frontend_ops->chr_event) {
           /* Or: be->frontend->chr_event(be->frontend, event);
           be->frontend_ops->chr_event(be->frontend_ops->opaque, event);
       }
   }

   static void chr_send_event(Chardev *s, int event)
   {
       qemu_chr_be_send_event(s->be, event);
   }

(and likewise qemu_chr_be_can_write asks be->frontend_ops->chr_can_read,
qemu_chr_be_write_impl invokes be->frontend_ops->chr_read).  Then the
tx<->rx switch is clear from accessing frontend_ops.

> This would not only mean that we clearly associate CharBackend with
> the frontend, but also the chardev code becomes clearer, sending an
> event on the chardev (be), forwards it to the frontend side... (that
> is so much clearer to me)

The problem I have with calling it CharFrontend, is that for example
"int tag" has nothing to do with the front end.

Paolo