Eduardo Habkost <ehabk...@redhat.com> writes: > On Thu, May 18, 2017 at 01:59:53PM +0200, Markus Armbruster wrote: >> Eduardo Habkost <ehabk...@redhat.com> writes: >> >> > Currently there's no way for QMP clients to get a list of device types >> > that are really usable with -device. This information would be useful >> > for management software and test scripts (like the device-crash-test >> > script I have submitted recently). Interestingly, the "info qdm" HMP >> > command provides this information, but no QMP command does. >> > >> > Add two new fields to the return value of qom-list-types: >> > "user-creatable-device" and "hotpluggable-device". >> >> Does the combination >> >> "user-creatable-device": false, >> "hotpluggable-device": true >> >> make any sense? > > It doesn't, and the code ensures this won't happen.
Would a single variable with three states be clearer? I don't like entangled booleans much... >> Exposing information on user-creatable/hot-pluggable via QMP makes >> sense. The question is how. This is a design question, and as so often >> with design questions, I need more words to make my case than I'd like >> to. Please bear with me. >> >> > Also, add extra >> > arguments for filtering the list of types based on the new fields. >> >> I consider the case for filtering weak. Let's ignore this part for now. > > I considered sending a version that didn't include filtering. I > will be happy to ignore it. :) > >> >> We have a number of commands to introspect static information, >> e.g. query-version, query-commands, query-qmp-schema, query-target, >> query-machines, query-cpu-definitions, query-chardev-backends, >> device-list-properties, qom-list-types. >> >> Aside: us abandoning the convention to name such commands query-FOO is >> regrettable. >> >> In its basic form, i.e. without arguments, qom-list-types does what its >> name suggests: it lists the QOM types. >> >> It also permits finding out whether a type is abstract, but only in a >> roundabout way: subtract the result of running it without arguments (or >> with 'abstract':false) from the result with 'abstract':true. >> >> It also permits finding the "implements" relation, but only in an even >> more roundabout way: run it with 'implements':T for every abstract T, >> then solve the resulting puzzle. >> >> Unless there's a direct way to find both (and I'm not aware of any), >> this is bad design. The obvious fix is to extend its return type to >> include the information. > > Agreed. > >> >> With qom-list-types fixed that way, there's precedence for exposing >> additional information on QOM types there. >> >> Note that both the "abstract" bit and the "implements directly" list >> apply to any QOM type, not just to certain subtypes. As proposed, >> "user-creatable-device' and "hotpluggable-device" apply only to the >> "device" subtype. There's no precedence for exposing information >> specific to certain subtypes. >> >> If we want to do it anyway, then qom-list-type should perhaps return a >> union. Taken to the logical conclusion, this becomes a nest of unions >> mirroring the "direct subtype of" relation. Hmm. > > I don't think that's the logical conclusion. The differences > between "object" and "device" are hardcoded in our interfaces: we > already have -object and -device. Modelling our data to take > care of thoes differences doesn't mean we will also have to treat > the differences between other QOM types (e.g. between > "pci-device" and "usb-device") the same way. *If* we make qom-list-type return a union to properly reflect which results apply to which devices, *then* the logical conclusion is a nest of unions mirroring the "direct subtype of" relation. "Hmm" expresses my less than enthusiastic reaction to the thought of such a nest. > Now, we could choose to encode that in different ways. We could > have a single command for all QOM types (like qom-list-types or a > new query-qom-type command) that return an union, or have > separate commands for object types and device types. I'm not > sure what's the better option here (see below for additional > comments on that). > >> >> However, we already have a command to introspect device types: >> device-list-properties. Can we expose the information as read-only >> property/ies of type "device" and be done? > > I don't think we should. device-list-properties has a very > specific purpose: listing QOM properties that can be read or > written using qom-get and qom-set, or set in -device. If there's > no use case for qom-get/qom-set on a property like "hotpluggable" > or "user-creatable", we shouldn't make them QOM properties (hence > they shouldn't appear on device-list-properties). I figure your argument boils down to "we should distinguish between information about a device instance and a device type, and properties are for the former, but ... > But that doesn't mean we can't have something like a > "query-device-type" command that returns other information about > a given device type, in addition to the property list. ... we still need something for the latter." > (In my specific use case (the device-crash-test script), I would > prefer to avoid having to fetch the full list of properties of > every single device type, just to find out which ones are > user-creatable. But this is not really performance-critical > code, so I can live with that.) > > So, I see a few options here: > > 1) Including DeviceClass::user_creatable and > DeviceClass::hotpluggable in qom-list-types output. Probably > using an union. > > 2) Adding a new "query-device-type" command, returning > DeviceClass::user_creatable and DeviceClass::hotpluggable, and > possibly other DeviceClass fields (like the list of > properties) > > 3) Adding a new "query-qom-type" command, returning extended > information about a QOM type. This might include the list of > properties supported by the type. This might include > device-specific data if the command return an union. That way's a nest of unions. > Those options are not mutually exclusive. e.g.: we might decide > that a small set of fields is useful if included in > qom-list-types too, even if we implement a query-device-type or > query-qom-type command. Yes. But let's start with just one. >> But perhaps they aren't really specific to devices. There are other QOM >> types that can be created by users, e.g. with -object, so the >> "user-creatable" bit applies more widely. Are any of them only >> creatable during initial startup? If yes, then that applies more >> widely, too. > > I prefer to have very specific semantics like "this type can be > used with -device" than something more generic and prone to > confusion like "this can be user-creatable, but the method used > to create it can vary". Point taken. We have user-creatable objects and user-creatable devices. The latter are objects, but not user-creatable objects. Aesthetically displeasing. >> > I'm not sure about the naming of the new command arguments. Maybe the >> > names are too long, but I believe that "user-creatable-devices-only=false" >> > would have more obvious semantics than "user-creatable-devices=false" >> > (the latter looks ambiguous: it could mean "return only >> > non-user-creatable devices" or "return all devices"). >> >> I consider the filtering feature unnecessary complexity. The filtering >> we have got in against my objections. I won't veto additional filtering >> outright, but I will insist on test coverage. >> >> The unfiltered output of qom-list-types is less than 10 KiB. Even if we >> extend it some, the need to filter it client-side seems dubious. If a >> management application really wants to save resources here, it should >> cache the result and re-get it only when QEMU changes. > > I don't think we really need filtering, and I will be happy to > remove that feature on the next version. We can always add it later if we find a real need.
