Paolo Bonzini <pbonz...@redhat.com> writes: > On 11/13/21 08:52, Markus Armbruster wrote: >> I'm not asking what to do "if it hurts", or "if you want a cold-plugged >> device". I'm asking whether there's a reason for ever wanting hot plug >> instead of cold plug. Or in other words, what can hot plug possibly >> gain us over cold plug? >> >> As far as I know, the answer is "nothing but trouble". > > Yes, I agree. > >> If that's true, then what we should tell users is to stick to -device >> for initial configuration, and stay away from device_add. > > Yes, which is one issue with stabilizing -preconfig. It's not clear > what exactly it is solving. > >>>>> The boat for this has sailed. The only sane way to do this is a new >>>>> binary. >>>> >>>> "Ideally" still applies to any new binary. >>> >>> Well, "ideally" any new binary would only have a few command line >>> options, and ordering would be mostly irrelevant. For example I'd >>> expect a QMP binary to only have a few options, mostly for >>> debugging/development (-L, -trace) and for process-wide settings (such >>> as -name). >> >> This is where we disagree. For me, a new, alternative qemu-system-FOO binary >> should be able to replace the warty one we have. >> >> One important kind of user is management applications. Libvirt >> developers tell us that they'd like to configure as much as possible via >> QMP. Another kind of user dear to me is me^H^Hdevelopers. For ad hoc >> testing, having to configure via QMP is a pain we'd rathe do without. I >> don't want to remain stuck on the traditional binary, I want to do this >> with the new one. > > Why do you care? For another example, you can use "reboot" or > "systemctl isolate reboot.target" and they end up doing the same thing. > > As long as qemu_init invokes qmp_machine_set, qmp_accel_set, > qmp_device_add, qmp_plugin_add, qmp_cont, etc. to do its job, the > difference between qemu-system-* and qemu-qmp-* is a couple thousands > lines of boring code that all but disappears once the VM is up and > running. IOW, with the right design (e.g. shortcut options for QOM > properties good; dozens of global variables bad), there's absolutely no > issue with some people using qemu-system-* and some using qemu-qmp-*.
I think maintaining two binaries forever is madness. I want the old one to wither away. Making the new binary capable of serving all use cases should not be hard, just work (see my design sketch). I expect the result to serve *better* than the mess we have now. >>>>>> Likewise, we'd fail QMP commands that are "out of phase". >>>>>> @allow-preconfig is a crutch that only exists because we're afraid (with >>>>>> reason) of hidden assumptions in QMP commands. >>>>> >>>>> At this point, it's not even like that anymore (except for block devices >>>>> because my patches haven't been applied). >>>> >>>> My point is that we still have quite a few commands without >>>> 'allow-preconfig' mostly because we are afraid of allowing them in >>>> preconfig state, not because of true phase dependencies. >>> >>> I think there's very few of them, if any (outside the block layer for >>> which patches exist), and those are due to distraction more than fear. >> >> qapi/*.json has 216 commands, of which 26 carry 'allow-preconfig'. > > Well, > https://lists.gnu.org/archive/html/qemu-devel/2021-06/msg01597.html > alone would more than double that. :) > > Places like machine.json, machine-target.json, migration.json, > replay.json have a lot of commands that are, obviously, almost entirely > not suitable for preconfig. I don't think there are many commands left, > I'd guess maybe 30 (meaning that ~60% are done). My point is that "very few" is not literally true, and I think you just confirmed it ;)
