Am 05.12.2014 um 10:34 hat Markus Armbruster geschrieben: > Eric Blake <ebl...@redhat.com> writes: > > > On 12/04/2014 08:56 AM, Markus Armbruster wrote: > > > >> > >> @device is a sub-optimal name for this single parameter. Either we > >> accept that and move on, or we deprecate it in favor of a new parameter > >> with a better name. I guess the better name isn't worth that much > >> trouble, in particular since the command name is already ugly. > >> > >> When @node-name is given, @device must not be given. So @device is > >> mandatory *except* in the @node-name usage we retain for compatibility. > >> Deprecate the usage. > >> > >> Command documentation could then look like this: > >> > >> ## > >> # @block-resize > >> # > >> # Resize a block image while a guest is running. > >> # > >> # @device: the name of the block backend or node to resize (node names > >> # supported since 2.3) > >> # > >> # @size: new image size in bytes > >> # > >> # Deprecated usage (since 2.3): > >> # > >> # @device: #optional the name of the block backend to resize > >> # > >> # @node-name: #optional name of the node to resize (since 2.0) > >> # > >> # Either @device or @node-name must be set but not both. > > > > But this isn't discoverable. You aren't changing whether any parameters > > are optional, only enhancing the semantics of existing parameters. How > > is libvirt supposed to know if qemu is old (node names have to go > > through node-name) or new (node names are preferred through device)? > > Good point. > > > Just blindly try the 'device' argument, and if it fails, fall back to an > > attempt with node-name? > > Works, but "try interfaces one after the other until you find one that > works" is a rather lame discovery technique.
As long as we don't have introspection, it's the only one we have. > > On the other hand, if 'node-name' becomes the preferred interface, then > > libvirt has a solution: if node-name is present (it is easy during > > up-front QMP probing to determine whether 'node-name' is a recognized > > optional argument or an unknown argument), then always use node-name. > > As long as libvirt always names the nodes of each device (which it > > should be doing, but that's a patch series for another day and another > > list), then a device lookup is never needed. If 'node-name' is not > > present, then only the 'device' form is supported, and libvirt can only > > manage the top image of a backend (but can make that point obvious to > > the end user that they should upgrade qemu for more functionality). > > If we deprecate @device instead of @node-name, we make the appropriate > (non-deprecated) use of backend names rather than node names hard to > probe. > > Command argument introspection could help only if it carried > "deprecated" flags. Might be a good idea anyway, but command > introspection is one of those nice-to-haves we can't seem to deliver. > > A possible alternative is our common way to cheat at probing: when > probing for A is hard, probe for B, and assume support for B implies > support for A. > > My guess that a "better name [than @device for the single parameter] > isn't worth that much trouble" needs to be reevaluated with > discoverability in mind. Yes, it's troublesome, but it's also easily > discoverable. Still requires trying the new argument and then falling back to @device/ @node-name. But as long as libvirt doesn't support the node-name interface yet anyway, I think this discussion is mostly moot. Kevin
