Re: [Qemu-devel] [PATCH 00/12][RFC] char: add flow control and fix guest_[open|close]

[Amit Shah]

On (Mon) 01 Aug 2011 [09:22:58], Anthony Liguori wrote:
> The char layer has been growing some nasty warts for some time now as we ask 
> it
> to do things it was never intended on doing.  It's been long over due for an
> overhaul and its become evident to me that we need to address this first 
> before
> adding any more features to the char layer.
> 
> This series is the start at sanitizing the char layer.  It effectively turns
> the char layer into an internal pipe.  It supports flow control using an
> intermediate ring queue for each direction.

Let's break down what we have now:

* chardev -> guest (backend writes):
   we have connect/disconnect notifications, we have an ugly
   can_read() implementation that is executed each time iohandlers are
   run.  However, it gives us reasonable flow control.

* guest -> chardev (frontend writes):
   we don't get chardev connect/disconnect events, neither do we get
   to know if the chardev is overwhelmed with data and to stop sending
   any more till it has some room in its queue.  This is because we
   need poll semantics instead of select to get POLLIN and POLLOUT
   events, using which we can ascertain what state the chardev is in.
   There's no call corresponding to the existing qemu_chr_can_read(),
   which essentially confirms if a chardev is able to handle data for
   output.  This series only adds a qemu_char_fe_can_write(), doesn't
   add callbacks for connect/disconnect events since that depends on
   polling.

The problem I think with adding a buffer is it just solves the flow
control problem without solving the connect/disconnect events issue by
just switching to poll, and solving all the problems at once.  Is
there something that we solve by having a buffer in addition to
poll()?

Also, introducing a buffer doesn't really solve all the problems; just
shifts them into the char layer instead of letting the OS handle the
queues naturally.

                Amit