Re: [Qemu-devel] [PATCH V2] balloon: Don't try fetching info if machine is stopped

[Daniel P. Berrange]

On Sun, Aug 22, 2010 at 04:54:06PM -0500, Anthony Liguori wrote:
> On 08/19/2010 07:48 PM, Amit Shah wrote:
> >If the machine is stopped and 'info balloon' is invoked, the monitor
> >process just hangs waiting for info from the guest. Return the most
> >recent balloon data in that case.
> >
> >See https://bugzilla.redhat.com/show_bug.cgi?id=623903
> >
> >Reported-by:<address@hidden>
> >Signed-off-by: Amit Shah<address@hidden>
> >   
> 
> !vm_running is just a special case of an unresponsive guest.  Even if 
> the guest was running, if it was oops'd and the administrator didn't 
> know, you would have the same issue.
> 
> I'd suggest using a timeout based on rt_clock.  If the stats request 
> times out, print an appropriate message to the user.

By all means add a timeout to cope with a unresponsive guest,
but in this case where we can be sure the guest can't respond
there's no point waiting on a timeout.

On a tangent, a future todo item I mentioned to some people at
the summit, is that I'd like to see the interaction with the
balloon driver change/enhanced. It is currently hard/impossible
to use this in an efficient manner. We should never have overloaded
the existing balloon command with guest memstats refresh + data,
because it has made 'query balloon' a significantly slower command
than it was, in particular you can no longer rely on it being a
'fast' response, or even responding at all. This is bad when all
you care about is the balloon actual level, and not the guest 
memstats. I'd like to see something along the lines of:

 - 'query balloon' to return the balloon + memstats values that
   are *currently* known to QEMU. Probably need a new name
   to avoid changing semantics of the existing command

 - 'balloon changed' event to indicate when the balloon actual
   level has changed (so we never need to poll on 'query balloon')

 - 'update memstats' to request an guest kernel mem stats update

 - 'memstats updated' event (or update memstats async reply) to
   indicate when the guest kernel has refreshed the mem stats.

Primarily I'd like to avoid having to invoke anything at all
to get the main balloon driver actual memory level. Being able
to fetch the memstats without also triggering an update is 
fairly useful too though.

Daniel
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