Re: [Qemu-devel] Re: [PATCH 08/11] QMP: Port balloon command

["Daniel P. Berrange" <berrange@redhat.com>]

On Fri, Jun 26, 2009 at 12:21:01PM +0300, Avi Kivity wrote:
> On 06/25/2009 10:11 PM, Luiz Capitulino wrote:
> >
> >  Yes, having a library was suggested by Amit some months ago. The
> >problem is that it has various issues wrt maintainability.
> >
> >  For example, libvirt is able to run two instances of different
> >versions of qemu at the same time. How to handle this if you
> >update libmonitor.so?
> >   

The sane way is to *NOT* break ABI of libmonitor.so, and not change
the wire protocol in a non backwards compatible way. This is entirely
doable, it just the maintainer of libmonitor/qemu to decide that  ABI
stability is important. So if you find an existing API  / command
needs to gain an extra argument, you don't change the existing API,
you add a new one. Or ideally design the API upfront so that it 
can be extended without breaking back compatability.

> You could use dlopen().  The problem with a library is that it assumes 
> everyone is still trapped in C, which is not the case.

dlopen() of a different libmonitor.so for every version of QEMU is just
crazy, whether C or not. Maintain ABI & protocol back compatability 
with new releases and avoid the issue in the first place.

Daniel
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