Re: [libvirt] [PATCH 2/2] qapi: deprecate implicit filters

28.08.2019 20:48, John Snow wrote: > (Peter: search for "pkrempa" down below.) > > On 8/28/19 5:20 AM, Vladimir Sementsov-Ogievskiy wrote: >> 27.08.2019 23:12, John Snow wrote: >>> >>> >>> On 8/23/19 5:22 AM, Vladimir Sementsov-Ogievskiy wrote: >>>> 14.08.2019 13:07, Vladimir Sementsov-Ogievskiy wrote: >>>>> To get rid of implicit filters related workarounds in future let's >>>>> deprecate them now. >>>> >>>> Interesting, could we deprecate implicit filter without deprecation of unnecessity of >>>> parameter? As actually, it's good when this parameter is not necessary, in most cases >>>> user is not interested in node-name. >>>> >>> >>> https://en.wiktionary.org/wiki/unnecessity -- I am surprised to learn >>> that this a real word in the language I speak. :) >>> >>> I assume you're referring to making the optional argument mandatory. >> >> exactly, it's my a bit "google-translate-driven" English) >> > > It teaches me some fun words! > >>> >>>> Obviously we can do the following: >>>> >>>> 1. In 4.2 we deprecate unnecessity, which implies deprecation of implicit filters >>>> 2. After some releases in 4.x we can drop deprecated functionality, so we drop it together with >>>> implicit filters. And, in same release 4.x we return it back (as it's compatible change :) >>>> but without implicit filters (so, if filter-node-name not specified, we just create >>>> explicit filter with autogenerated node-name) >>>> >>>> So, effectively we just drop "deprecation mark" together with implicit filters, which is nice >>>> but actually confusing. >>>> >>>> Instead, we may do >>>> 1. In 4.2 deprecate >>>> 2. In 4.x drop optionality together with implicit filters >>>> 3. In 4.y (y > x of course) return optionality back >>>> >>> >>> Ah, I see what you're digging at here now... >>> >>>> It's a bit safer, but for users who miss releases [4.x, 4.y) it's no difference.. >>>> >>>> Or we just write in spec, that implicit filters are deprecated? But we have nothing about implicit >>>> filters in spec. More over, we directly write that we have filter, and if parameter is omitted >>>> it's node-name is autogenerated. So actually, the fact the filter is hidden when filter-node-name is >>>> unspecified is _undocumented_. >>>> >>>> So, finally, it looks like nothing to deprecated in specification, we can just drop implicit filters :) >>>> >>>> What do you think? >>>> >>> >>> What exactly _IS_ an implicit filter? How does it differ today from an >>> explicit filter? I assumed the only difference was if it was named or >>> not; but I think I must be mistaken now if you're proposing leaving the >>> interface alone entirely. >>> >>> Are they instantiated differently? >>> >> >> As I understand, the only difference is their BlockDriverState.impicit field, and several places in code >> where we skip implicit filter when trying to find something in a chain starting from a device. >> > > Oh, oh, yes. I see. > >> Hmm, OK, let's see: >> >> 1. the only implicit filters are commit_top and mirror_top if user don't specify filter-node-name. >> >> Where it make sense, i.e., where implicit field used? >> > > `git grep -E '(->|\.)implicit'` is what I used to find usages. > >> 2. bdrv_query_info, bdrv_query_bds_stats, bdrv_block_device_info(only when called from bdrv_query_info), they'll >> report filter as top node if we don't mark it implicit. >> > > So that's a bit of a change, but only visually. The "reality" is still > the same, we just report it more "accurately." libvirt MIGHT need a > heads up here. I'm looping pkrempa back in for comment. > > <pkrempa> > Would libvirt be negatively impacted by the revelation of formerly > internal ("implicit") nodes created by mirror and commit via query block > commands? At the moment, QEMU hides them from you if you do not name them. > </pkrempa> > >> 3. bdrv_refresh_filename, bdrv_reopen_parse_backing, bdrv_drop_intermediate: >>     I think it's not a problem, just drop special case for implicit fitlers >> > > I'm much less certain about what the impact of this would be and would > need to audit it (and don't have the time to, personally.) > > Do you have a POC or RFC patch that demonstrates dropping these special > cases? It might be nice to see as proof that it's safe to deprecate.

> >> So, seems the only real change is query-block and query-blockstats output when mirror or commit is started >> without specifying filter-node-name (filter would be on top) >> >> So, how should we deprecate this, or can we just change it? >> > > I'm not sure if it's worth it yet, what does dropping the implicit field > buy us? Conceptually I understand that it's simpler without the notion > of implicit fields, but I imagine there's some cleanup in particular > that motivated this. Reviewing Max's "block: Deal with filters" series motivated me.

> > I'd say to just change the behavior, we should: > > - Give a standard three-release warning that the behavior will change in > an incompatible way > - Demonstrate with an RFC patch that special cases around ->implicit in > block.c can be removed and do not make the code more complex, > - Get blessings from Peter Krempa. > > As always: Libvirt is not the end-all be-all of QEMU management, but if > libvirt is capable of working around design changes then I believe any > project out there today also could, so it's a good litmus test. > > --js >