Re: [Qemu-devel] Re: KVM call minutes for June 8

[Daniel P. Berrange]

On Wed, Jun 09, 2010 at 12:18:20PM -0300, Luiz Capitulino wrote:
> On Tue, 08 Jun 2010 16:13:37 -0500
> Anthony Liguori <address@hidden> wrote:
> 
> > On 06/08/2010 03:59 PM, Luiz Capitulino wrote:
> > >   Now, QError.
> > >
> > >   This is something I think we should fix for 0.13. However, I still don't
> > > know how to get it right: most of what you say in the wiki page has 
> > > already
> > > been discussed before.
> > >
> > >   For example, you suggest the error object should be returned, this is
> > > probably a requirement to have async commands working, but this thread
> > > explains the problems we had with that:
> > >
> > >   http://lists.gnu.org/archive/html/qemu-devel/2010-02/msg00818.html
> > >    
> > 
> > Sometimes we have to make difficult changes and touch a lot of code.  
> > There's really no other way and we're just going to have to grit our 
> > teeth and do it.  Honestly, it's not that bad though having gone through 
> > much of the code at this point.
> 
>  Let's see, we should start working on this as soon as 0.13 is out.
> 
> > >   Another issue is that QErrors are getting too specific. I see two 
> > > problems
> > > here, first how errors should be done and how to map errno properly, also
> > > already discussed:
> > >
> > >   http://lists.gnu.org/archive/html/qemu-devel/2010-05/msg00314.html
> > >    
> > 
> > We're going to have to basically drop most of the QErrors we have today 
> > for 0.14 after we straighten out the error mess.
> > 
> > Errors are pretty easy to deprecate and replace.  All clients have to 
> > handle new types of errors so the worst thing that will happen is that 
> > they have less specific errors.
> 
>  Another problem is that a client might be waiting for a specific error
> that would never arrive. For example, say you wrote a shell and the
> CommandNotFound error is something expected to happen, this will break.
> 
>  Yes, well written clients should check the 'error' key first and won't
> panic if what they expect is not there. Even then, it's an annoying
> issue.

libvirt is even more paranoid than this. It checks for 'error' key
being present, and if that's not present, it then also for the 'return'
key not being present.

>  What we can do to minimize the damage is:
> 
>  1. Clearly state somewhere that errors are going to be removed
>  2. Keep errors undocumented (this turned out to be a feature!)
> 
> > The big problem is things like the block io error events.  I don't see 
> > how we can support that given what we have in 0.13.  Putting errno on 
> > the wire is a bad idea.  We need to marshal a QError object using the 
> > __class__ memory to identify it as QError and pass that with the event.  
> > It could potentially be made to work for 0.13 but it'll be ugly.
> 
>  Yes, that's the main problem of the whole thing I think.
> 
>  From QEMU pov, the best thing to do would be to let this event as it's
> today and wait for the new QError (it's a compatible change, anyway).

FYI, in terms of errors, libvirt currently relies on very little.

When dealing with 'balloon' and 'query-balloon' we check for
two explicit errors:

   class=DeviceNotActive 
   class=KVMMissingCap

in order to detect whether ballooning is supported in this guest.

In all other cases, we simply pass the 'class' and 'desc' field back up to
the libvirt error reporting code, and don't care what strings are in those
fields. So if we add a whole lot more QErrors, or  deprecate error codes the
impact won't be serious apart from those two cases I mention.

>  However, libvirt was counting on this feature and on the snapshot commands
> to switch from the text Monitor. We have two options:
> 
>  1. Ask them to wait one more release (not so good for us)
>  2. Try to find a way to have those features in for 0.13
> 
>  Daniel has commented to me that making the snapshot commands synchronous
> for 0.13 wouldn't be that bad, what do you think?

The thought is that changing a command from synchronous to asynchronous is
not an ABI incompatible change. An existing app simply won't know to take
advantage of the new possibilities that async commands offer.

Regards,
Daniel
-- 
|: Red Hat, Engineering, London    -o-   http://people.redhat.com/berrange/ :|
|: http://libvirt.org -o- http://virt-manager.org -o- http://deltacloud.org :|
|: http://autobuild.org        -o-         http://search.cpan.org/~danberr/ :|
|: GnuPG: 7D3B9505  -o-   F3C9 553F A1DA 4AC2 5648 23C1 B3DF F742 7D3B 9505 :|