Re: [Qemu-devel] [PATCH 2/2] qapi: deprecate implicit filters

[Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>]

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
> 


-- 
Best regards,
Vladimir