Re: [Qemu-devel] [PATCH 2/3] Always use nonblocking mode for qemu_chr_open_fd.

[Gerd Hoffmann <kraxel@redhat.com>]

Anthony Liguori wrote:
> I really don't like introducing polling.  I think we should be trying to
> reduce the overall number of wake ups in QEMU, not add more.

Right now qemu wakes up quite often anyway, so it doesn't become worse
(bad excuse, I know).

> I'm inclined to say that we simply declare that reconnecting to a PTY is
> broken/unsupported and strongly encourage the use of something else
> (like unix sockets).  That's always been my understanding FWIW.

It's not only reconnects, its also the initial connect.

Right now qemu simply blocks once the pty output buffer is full.  With
nobody reading this effectively means qemu blocks forever and doesn't
responds any more to anything.  It can't stay that way IMO.

I think we have three options here:

  (1) Detect whenever someone is connected.  Drawback: needs polling
      (unless someone comes up with a better patch).
  (2) Run ptys in non-blocking mode.  Drawback: we risk loosing data
      in case the pty reader is slow (also when nobody is connected of
      course, but that is intentional ...).
  (3) Drop pty support.  Drawback: we break existing setups.

Comments?  Other suggestions?

cheers,
  Gerd

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