Re: [Qemu-devel] [PATCH for-1.5] qmp: add query-drive-mirror-capabilities

[Luiz Capitulino <lcapitulino@redhat.com>]

On Wed, 24 Apr 2013 15:59:15 -0600
Eric Blake <eblake@redhat.com> wrote:

> On 04/24/2013 03:29 PM, Luiz Capitulino wrote:
> >> -> { "execute": "query-command-capabilities" }
> >> <- { [ { "type": "drive-mirror",
> >>          "data": [ "granularity", "buf-size" ] },
> >>        { "type", ... }
> >>      ] }
> >>
> >> And whether a '*command' argument should be optional for filtered
> >> output, vs. always unconditionally dumping all information on all
> >> commands with capabilities, vs. mandatory (can only get capabilities for
> >> one command at a time), all goes back to the larger question of whether
> >> query-* commands should allow filtering.
> > 
> > Not discussing filtering for now, but your proposal would superseded by
> > full introspection, wouldn't it?
> 
> Yeah, the two do seem rather similar, in that they are both providing a
> form of introspection.  As I see it, it boils down to WHAT is being
> introspected:
> 
> With query-command-capabilities, we are asking about capabilities, which
> can always be represented as pure enum values (either the capability is
> present or it is not).  For DriveMirrorCapabilities we happened to map
> two enum values to the name of two optional drive-mirror parameters; but
> where we could introduce other capabilities that are unrelated to an
> optional parameter;

That's a good point, although I wonder if a command could have a new
capability that's not mapped to a new argument. IOW, I'd expect most/all
new capabilities to always be mapped to new arguments.

In any case, I think the way to go is to add full introspection and then
add query-command-capabilities afterwards if it turns out to be necessary.