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 14:37:54 -0500
Anthony Liguori <anthony@codemonkey.ws> wrote:

> On 08/27/2010 02:24 PM, Luiz Capitulino wrote:
> >
> >> 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?
> >    
> 
> Because it's too easy to get it wrong in QEMU.  Here's the rationale.
> 
> If I can't trivially call a QMP function in C, then I'm not going to use 
> QMP functions within QEMU.  I'm not going to create an embedded JSON 
> string just to call a function with three integer arguments.

... And as a QMP client I don't see the point in having to use a new
command just because a new argument was needed. The language QEMU is
written is also unimportant. The wire format is all I see and it's
language independent.

> Yes, if we need to do that, we can create a C API that both the QMP 
> interface uses and we also use internally but why?  All that does is 
> introduce the chance that the C API will have more features than the QMP 
> interface.

Why? I mean, what would stop us on extending QMP when the C interface is
also extended? Examples?

At first, this seems a good middle ground to me: QMP is implemented on
top of the C API and the JSON stuff is limited to QMP.

Of course that C API functions returning structured data will have to
create and return qobjects. But that's needed even with the direct
mapping.

> If we don't use these functions in QEMU, then how do we know that these 
> functions have reasonable semantics?  This is exactly the problem we 
> suffer today.  We have internal APIs that do reasonable things but 
> everything that deals with QMP is a special case.  That creates too many 
> opportunities to get things wrong.
> 
> I think it's a vitally important requirement that all future QMP 
> functions have direct mappings to a C interface.  The long term goal 
> should be for that interface to be used by all of the command line 
> arguments, SDL, and the human monitor.  If those things only relied on a 
> single API and we exposed that API via QMP, than we would have an 
> extremely useful interface.

I agree, I just think an indirect mapping would be more beneficial to
QMP clients.