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Date: Sun, 28 Mar 2010 16:01:26 +0100
From: Jamie Lokier <jamie@shareable.org>
Subject: Re: [Qemu-devel] Re: [PATCH 10/15] virtio-serial: Add QMP events for
	failed port/device add
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To: Amit Shah <amit.shah@redhat.com>
Cc: Luiz Capitulino <lcapitulino@redhat.com>, "Michael S. Tsirkin" <mst@redhat.com>, Gerd Hoffmann <kraxel@redhat.com>, qemu list <qemu-devel@nongnu.org>, Juan Quintela <quintela@redhat.com>

Amit Shah wrote:
> > Without this specific thing, which is an indicator that guest has lost
> > state outside its control, the guest<->host communication is
> > unreliable (even for things like "cut and paste"), so every app that
> > cares has to implement a packet framing protocol with no binary data
> > (to reserve an escaping byte), or with CRCs like
> > PPP-over-virtio-serial, which is complicated and silly imho.  If it
> > were a real serial port, not emulated, that's the sort of thing apps
> > would actually do (or use timeouts, which are more dubious in
> > emulator-land).  But I hope we're not that sadistic :-)
> 
> I agree. So: ports have in-qemu users, they get guest_open /
> guest_close callbacks and get data which they can pass on to external
> apps. Looks like we're fine there?

Provided the guest_open / guest_close callbacks are synchronous with
the data - so that data sent/received following guest_open exactly
matches what the guest sends/receives from its beginning, that
internal interface looks fine to me.

We can tidy up the chardev later as needed :-)

-- Jamie

> > *Inband* open/close indication aren't 100% guarantees of reliability,
> > but I think they raise it to the point where an app can usefully count
> > on it.