Re: [Qemu-devel] Re: [PATCH v3 2/3] qerror: Add a new MACHINE_STOPPED error message

[Luiz Capitulino <lcapitulino@redhat.com>]

On Fri, 27 Aug 2010 14:02:45 -0500
Anthony Liguori <anthony@codemonkey.ws> wrote:

> On 08/27/2010 11:08 AM, Luiz Capitulino wrote:
> >> It's trying to plug a sieve with a band-aid.  It's certainly an
> >> "improvement" but it's of question utility looking at the bigger picture.
> >>      
> > Are you talking about the testing namespace idea? It doesn't have anything
> > to do with balloon or how our interfaces are built. It would be just a way
> > to play with commands that has been converted to QMP but are not available
> > because they're not stable yet (eg. Jan's device_show).
> >    
> 
> My point is that we shouldn't build any QMP APIs and we definitely 
> shouldn't try to QMP-ize monitor commands.
> 
> Instead, we should design logical C APIs that we could consume within 
> QEMU that we think we can support long term and then expose those over QMP.

Comments on the Python comment below.

> Having a sandbox doesn't really solve the fundamental problem of making 
> sure the interface is consumable.

It doesn't and it shouldn't. It's just a way to test commands that might
not be stable yet. A very minor feature, anyway.

> >> Balloon is a perfect example of where what we really need to do is build
> >> interface interfaces that make sense, and then expose them in QMP.
> >>      
> > Main question is: can we drop the stats the way they are today to do the
> > Right Thing for 0.14 or not?
> >    
> 
> I don't see how 0.13.0 is going to get releases with anything but the 
> current behavior.  It's unfortunate but we're too delayed and can't 
> afford a change like this this late in the game.
> 
> In terms of the stable branch, the least disruptive thing would be a 
> timeout.

Okay.

> > I think we have agreed on the internal interfaces approach. My only
> > concern is whether this will conflict when extending the wire protocol
> > (eg. adding new arguments to existing commands). Not a problem if the
> > C API is not stable, of course.
> >    
> 
> We don't do that.  It's a recipe for disaster.  QEMU isn't written in 
> Python and if we try to module our interfaces are if we were a Python 
> library, we're destined to fail.

You mean we don't do simple protocol extensions?

So, if we need an new argument to an existing command, we add a new
command instead? Just because QEMU is not written in Python?

> >> What's a reasonable C-level API to query statistics that potentially may
> >> never return?  Building in a timeout is something of a crappy API
> >> because it puts policy deep in the API that is trivial to implement
> >> elsewhere.  What you'd probably do is something like:
> >>
> >> BalloonStatsRequest *query_guest_balloon_stats(CompletionCallback *cb,
> >> void *opaque);
> >> int cancel_guest_balloon_stats(BalloonStatsRequest *req);
> >>      
> > Shouldn't the API provide a general cancel method? All functions that
> > talk to the guest will need one.
> >    
> 
> See next proposal.  There's no cancel but I'd argue it's not needed.  
> You don't care if the request succeeds or fails so there's no point in 
> cancelling it.  Cancellation only works best when a request has a 
> discrete life cycle but in the case of requesting a guest to update 
> stats, there is not really a well define dstart and end.
> 
> Regards,
> 
> Anthony Liguori
> 
> >> void release_guest_balloon_stats(BalloonStatsRequest *req);
> >>
> >> Regards,
> >>
> >> Anthony Liguori
> >>
> >>      
> >>>>
> >>>>          
> >>>>> Beyond fixing that regression, I agree that this command is terminally
> >>>>> flawed&   we need to deprecate it&   provide better specified new
> >>>>> replacement(s). This seems like 0.14 work to me though.
> >>>>>
> >>>>>            
> >>>> Yup.
> >>>>
> >>>>
> >>>>          
> >>>>> Regards,
> >>>>> Daniel
> >>>>>
> >>>>> [1] I know that they could already suffer if there was a bug in qemu
> >>>>>       that prevented it responding, even if the guest was not being
> >>>>>       malicious/crashed.
> >>>>>
> >>>>>            
> >>>>
> >>>>          
> >>>
> >>>        
> >>      
> >    
>