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Date: Tue, 23 Jun 2009 10:36:16 -0300
From: Luiz Capitulino <lcapitulino@redhat.com>
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Subject: [Qemu-devel] Re: [PATCH 06/11] QMP: Introduce asynchronous events
	infrastructure
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To: "Daniel P. Berrange" <berrange@redhat.com>
Cc: aliguori@us.ibm.com, ehabkost@redhat.com, jan.kiszka@siemens.com, dlaor@redhat.com, qemu-devel@nongnu.org, avi@redhat.com

On Tue, 23 Jun 2009 11:32:51 +0100
"Daniel P. Berrange" <berrange@redhat.com> wrote:

> On Tue, Jun 23, 2009 at 01:29:11AM -0300, Luiz Capitulino wrote:
> > It is just an exported function that will be used by other subsystems
> > to print specific events to the output buffer.
> > 
> > This is based in ideas by Amit Shah <amit.shah@redhat.com>.
> > 
> > Signed-off-by: Luiz Capitulino <lcapitulino@redhat.com>
> > ---
> >  monitor.c   |   18 ++++++++++++++++++
> >  monitor.h   |    6 ++++++
> >  qemu-tool.c |    4 ++++
> >  3 files changed, 28 insertions(+), 0 deletions(-)
> > 
> > diff --git a/monitor.c b/monitor.c
> > index 462f60b..df58bdd 100644
> > --- a/monitor.c
> > +++ b/monitor.c
> > @@ -266,6 +266,24 @@ void monitor_printf_data(Monitor *mon, const char *fmt, ...)
> >      va_end(ap);
> >  }
> >  
> > +/* Asynchronous events main function */
> > +void monitor_notify_event(MonitorEvent event)
> > +{
> > +    if (!monitor_ctrl_mode(cur_mon))
> > +        return;
> > +
> > +    assert(event < EVENT_MAX);
> > +    monitor_puts(cur_mon, "* EVENT ");
> > +
> > +    switch (event) {
> > +    case EVENT_MAX:
> > +        // Avoid gcc warning, will never get here
> > +        break;
> > +    }
> > +
> > +    monitor_puts(cur_mon, "\n");
> > +}
> > +
> 
> If a client is not reading from the monitor channel quickly enough, then
> would this cause QEMU to block once the FD buffer is full ? A QEMU level
> buffer might give more leyway, but we don't want to grow unbounded, so
> ultimately we'll end up having to drop events. At which point you'd want
> to send an event to the client indicating that the event queue overflowed,
> so it can take remedial action to re-sync its state.

 Wouldn't a buffer flush right at the beginning of this function solve this?
At least for QEMU?

 I'm not sure if it's possible to get QEMU blocked.