For the record, we went with "either dotted keys or JSON" for -blockdev in 2.9.
Certain uses of QAPI become problematic with dotted keys, because (1) it's untyped except for inner tree nodes, and therefore can't express numbers, boolean, null, empty list, empty object, and (2) The keyval variant of the QObject input visitor (the thing returned by qobject_input_visitor_new_keyval()) can't fully hide this. I don't like this at all, but the whole non-flat command line argument design has been an exercise in picking what I dislike least. Example: object-add's use of 'any' for properties QOM properties aren't specified in the QAPI schema (let's ignore the question whether they should be here). object-add uses type 'any' to take a dictionary of them. object-add obviously needs to do the type checking itself. In C, type 'any' is QObject *. If object-add examines the QObject directly, it now needs to know whether it was made from dotted keys, so it can cope with dotted keys' "everything's a string". Probably the only sane way to do that is to use the appropriate visitor. Perhaps we should represent 'any' as QObjectInputVisitor * instead of QObject *. Example: TCP service name / port number as alternate of string, number We use string, just like getaddrinfo(): if the string is a decimal number, it's a port number, else a service name. An alternate of string and number would be more idiomatic QAPI (let's ignore compatibility here, it's just an example). Works fine with JSON. Breaks with dotted keys, because you always get the alternate's string variant there. Possible ways forward: * When it breaks, fall back to JSON Tolerable if it breaks pretty much only in obscure corner cases. * More syntax Add syntax to disambiguate the type (key sigils?). Has to be optional for backward compatibility reasons. When omitting type information breaks, you have to supply it, or fall back to JSON. More syntax for use in obscure cases is about the last thing the QEMU command line needs. * More magic Make the keyval variant of the QObject input visitor shift the breakage to hopefully less common cases. - Empty list magic When the visitor is asked for a list, and the list's key wasn't specified, return an empty list instead of failure. Unbreaks empty list, breaks absent optional list. - Alternate magic When the visitor is asked for an alternate, pick the alternate's variant based on the value rather than the type for scalar values (the type is always 'str' then). Unbreaks alternates when this picks the variant you want, breaks them when you really want 'str'. - Possibly more When the magic breaks, fall back to JSON. More magic in the QEMU command line feels even worse to me than more syntax.