Re: [Qemu-devel] Re: Two QMP events issues

[Daniel P. Berrange]

On Mon, Feb 08, 2010 at 08:49:20AM -0600, Anthony Liguori wrote:
> On 02/08/2010 08:12 AM, Daniel P. Berrange wrote:
> >
> >For further backgrou, the key end goal here is that in a QMP client, upon
> >receipt of the  'RESET' event, we need to reliably&  immediately determine
> >why it  occurred. eg, triggered by watchdog, or by guest OS request. There
> >are actually 3 possible sequences
> >
> >  - WATCHDOG + action=reset, followed by RESET.  Assuming no intervening
> >    event can occurr, the client can merely record 'WATCHDOG' and interpret
> >    it when it gets the immediately following 'RESET' event
> >
> >  - RESET, followed by WATCHDOG + action=reset. The client doesn't know
> >    the reason for the RESET and can't wait arbitrarily for WATCHDOG since
> >    there might never be one arriving.
> >
> >  - RESET + source=watchdog. Client directly sees the reason
> >
> >The second scenario is the one I'd like us to avoid at all costs, since it
> >will require the client to introduce arbitrary delays in processing events
> >to determine cause. The first is slightly inconvenient, but doable if we
> >can assume no intervening events will occur, between WATCHDOG and the
> >RESET events. The last is obviously simplest for the clients.
> >   
> 
> I really prefer the third option but I'm a little concerned that we're 
> throwing events around somewhat haphazardly.
> 
> So let me ask, why does a client need to determine when a guest reset 
> and why it reset?

If a guest OS is repeatedly hanging/crashing resulting in the watchdog 
device firing, management software for the host really wants to know about
that (so that appropriate alerts/action can be taken) and thus needs to 
be able to distinguish this from a "normal"  guest OS initiated reboot.

Regards,
Daniel
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