[Qemu-devel] Re: [PATCH] Introduce a -libvirt-caps flag as a stop-gap

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

On Tue, Jul 27, 2010 at 10:55:03AM -0500, Anthony Liguori wrote:
> Today libvirt parses -help output to attempt to enumerate capabilities.  This
> is very broken and has led to multiple failures.  Since libvirt is an important
> management interface to QEMU, we need to do a better job giving them the ability
> to detect what a QEMU executable supports.  Right now, we keep fixing up help
> output to appease it's parsing code but this is undesirable.
> 
> The Right Solution is to introduce a robust capabilities advertisement that
> enumerates every feature we have.  As with most Right Solutions, we don't have
> mergable code today and it's unclear that we'll get there by the next release.
> 
> This patch introduces an incremental solution of just spitting out the handful
> of capabilities libvirt is probing for today.  This interface will need to
> remain forever but can stop being updated once we have a Right Solution.

This isn't really workable because it only encodes the subset of things
that libvirt currently looks for. If someone comes along with a libvirt
patch for a new features that is already supported by QEMU, but isn't
in this simple output, we're stuck. Adding a one-off special case for
the 0.13 release that we know will be obsolete in 0.14, and obviously
can't be used in qemu < 0.12 is not really a worthwhile use of time.
libvirt has to keep supporting help parsing indefinitely for <= 0.12
releases & expects to support a new extensible & flexible approach for
qemu >= 0.14. Adding a special case that both libvirt & qemu have to
support indefinitely for 0.13 is not really very nice.

Daniel
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