Re: [libvirt] [Qemu-devel] fix/re-do query-command-line-options

Amos Kong <akong(a)redhat.com&gt; writes: Yes, and that's an issue not just for query-command-line-options, but also for -writeconfig / -readconfig. Code goes into contortions to extend the old option without breaking it. Part of the contortions is your commit 968854c, which adds special-case code to make the -drive's parameters visible in query-command-line-options again, after commit 0006383 made them invisible. More on the invisible parameters problem below. What do you have in mind here? Here's one: QemuOpts *sucks* :) Apart from its general suckage, which has been discussed at some length elsewhere, it sucks as a schema, because it's not nearly expressive enough. With QemuOpts, we can basically enumerate acceptable keys and how their values should be parsed, plus a few extras like an implied key. On value parsing, all we have is "on"/"off", uint64_t in decimal, uint64_t in fancy size syntax, and "anything". This is both more and less than a QAPI schema. It's more, because it specifies both type (e.g. uint64_t) and external syntax (e.g. decimal). It's less, because it has fewer basic types, and no composite types. On acceptable keys, all we can do is a list fixed at compile-time. We can't express discriminated unions, such as when key "type" has value "tap", then keys ... are acceptable, when it has value "bridge", then keys ... are acceptable, and so forth. We routinely do that in QAPI schemata. We can't collect acceptable keys at run-time. For instance, -device accepts general keys "id", "bus", and a discriminated union of device model properties, with discriminator "driver". But the available device models and their properties are only known at runtime. When we need something like that (and we need it often), we give up and specify "any key=value accepted", leaving the actual checking to the code using the option. All command line introspection can see then is "the option exists, and it takes a key=value,... argument". Which borders on useless. Options that have this issue now: -acpitable, -smbios, -netdev, -net, -device, -object. Note that several of these are fairly new, which makes me expect more of them. -drive isn't on this list only because your commit 968854c got it off the list after commit 0006383 got it on it. QAPI schema can't yet solve this problem, either. That's why it doesn't cover -device / device_add. Since all QMP commands need to be covered, and QMP surely needs a command to plug devices, the problem will have to be solved there eventually. I think the data you can usefully collect with this approach is approximately the data getopt_long()[*] gets: list of named command line options, and whether they take an argument. You can use this data to fill in options not covered by QemuOpts. This is a definite improvement. It still falls short of fully solving the command line introspection problem. However, I'm not into rejecting imperfect incremental improvements we can have now in favor of perfect solutions we can maybe have some day. Go right ahead with your incremental improvement! [*] Which we don't use, because we prefer our command line idiosyncratic.