Kevin Wolf <[email protected]> writes: > Am 05.11.2021 um 11:08 hat Markus Armbruster geschrieben: >> Kevin Wolf <[email protected]> writes: >> >> > Am 04.11.2021 um 13:13 hat Markus Armbruster geschrieben: >> >> The old syntax almost always has its quirks. Ideally, we'd somehow get >> >> from quirky old to boring new in an orderly manner. Sadly, we still >> >> don't have good solutions for that. To make progress, we commonly >> >> combine JSON new with quirky old. >> >> >> >> qemu-system-FOO -object works that way. object_option_parse() parses >> >> either JSON or QemuOpts. It wraps the former in a QObject visitor, and >> >> the latter in an opts visitor. >> >> >> >> QemuOpts is flat by design[*], so the opts visitor parses flat QemuOpts >> >> from a (possibly non-flat) QAPI type. How exactly it flattens, and how >> >> it handles clashes I don't remember. >> >> >> >> Sadly, this means that we get quirky old even for new object types. >> > >> > For -object in the system emulator (the tools all use the keyval >> > visitor, so there it would work as expected), the only reason that we >> > need to keep the quirky old code path around is the list handling in >> > memory-backend.host-nodes. >> > >> > The main difficulty there is that the old QemuOpts based code path >> > allows specifying the option twice and both of them would effectively be >> > combined. Do we have any idea how to replicate this in a keyval parser >> > based world? >> >> I can see just two clean solutions, but both involve upending a lot of >> code. >> >> We can fuse keyval parser and visitor to get a schema-directed parser. >> >> We can change the abstract keyval syntax to permit repeated keys. This >> means replacing QDict in in the abstract syntax tree, with fallout in >> the visitor. >> >> Even if we find a practical solution, I don't like the combination of >> "you may give the same parameter multiple times, and the last one wins" >> and "for a list-valued parameter, the values of repeated parameters are >> collected into a list". Each makes sense on its own. The combination >> not so much. Inheriting "last one wins" from QemuOpts may have been a >> mistake. >> >> The keyval way of doing lists (inherited from the block layer's usage of >> dotted keys? I don't remember) requires the user to count, which isn't >> exactly nice, either. > > Yes. If we didn't have to maintain compatibility (or actually as soon as > we degrade non-JSON option lists to HMP-level support), I would > introduce [] and {} syntax for lists and dicts, even if that means that > use of these characters in strings doesn't work any more or only in a > limited way. I think this would be the best compromise for usability. > > Anyway, this doesn't help us with the compatibility problem we're > discussing here. > >> > If not, do we want to use the remaining time until 6.2 to deprecate >> > this? The nasty part is that the only syntax that works both now and in >> > the future is JSON. We can't easily accept the new keyval syntax while >> > still using the QemuOpts based code. >> >> What exactly do you propose to deprecate? > > We can deprecate on two different levels. I think it's useful to do > both: > > 1. Broad deprecation: Stable non-JSON interfaces are degraded to > a HMP-like compatibility promise.
Calling it "deprecation" might be confusing. HMP isn't deprecated, it's merely not a stable interface. That's kind of like "deprecated when you need stable", but saying "not a stable interface" is clearer. When I write "deprecate" below, I mean something like "go use something else (no conditions)". When I mean "use something else when you need stable", I write "degrade" (short for "degrade to an HMP-like compatibility promise"). > Obviously, this can only be done > for options that support JSON. We can also degrade or even deprecate sugar options in favor of the real ones. Case by case, I guess. > Peter Maydell also wants to do this > only after a big user (read: libvirt) has implemented and is > using JSON, basically as a proof that the alternative is working. > > So this can certainly be done for -object. I believe libvirt also > uses JSON for -device now, so this should be fine now, too. The non-sugar options supporting JSON are -audiodev, -blockdev, -compat, -display (partially), -machine (I think), -object. -netdev is QAPIfied, but still uses QemuOpts. Too late for 6.2, I'm afraid. > Possibly > -drive (in favour of -blockdev), though I'm not completely sure if we > have gotten rid of the final users of -drive. (CCing Peter Krempa for > details.) The problem with deprecating -drive is configuring onboard block devices. We need a stable interface for that, and it must be usable together with -blockdev. We provided such an interface (machine properties) for some onboard block devices starting with commit ebc29e1bea "pc: Support firmware configuration with -blockdev". Many more remain, I believe. > This degradation of the compatibility promise doesn't tell users what > exactly is going to change, which is why doing the second one, too, > might be nice. > > 2. Narrow deprecation: We can just deprecate the non-JSON form, or > certain aspects of it, of memory-backend.host-nodes. This is the > specific things that stops us from switching -object to keyval. > > a. Deprecate the whole option. If you want to use it and need a > stable interface, you have to use JSON. We'll just switch the > non-JSON form on a flag day. Before it, you need to use QemuOpts + > OptsVisitor syntax for the list; after it, you need to use keyval > syntax. I parse "the whole option" as "-object with dotted keys argument". Correct? > b. Deprecate only repeating the option. memory-backend is changed to > first try visiting a list, and if that fails, it visits a string > and goes through a string visitor locally to keep supporting the > integer range syntax. Possible problem: integer range syntax must not leak into the JSON form. > c. Deprecate all list values, but keep supporting a single integer > value by using an alternate between list and int. Single int should also not leak into JSON. > Picking one of these four options is enough to convert -object to > keyval. I would suggest doing both 1. and one of the options in 2. I'm grateful for your analysis.
