Re: [PATCH v2] docs: specify stability of hypfs path documentation

[George Dunlap <George.Dunlap@xxxxxxxxxx>]

> On Jul 16, 2020, at 3:34 PM, Jürgen Groß <jgross@xxxxxxxx> wrote:
> 
> On 16.07.20 16:20, George Dunlap wrote:
>>> On Jul 16, 2020, at 12:34 PM, Jürgen Groß <jgross@xxxxxxxx> wrote:
>>> 
>>> On 16.07.20 13:24, Jan Beulich wrote:
>>>> On 16.07.2020 12:31, Jürgen Groß wrote:
>>>>> On 16.07.20 12:11, Jan Beulich wrote:
>>>>>> On 15.07.2020 16:37, George Dunlap wrote:
>>>>>>> IT sounds like you’re saying:
>>>>>>> 
>>>>>>> 1. Paths listed without conditions will always be available
>>>>>>> 
>>>>>>> 2. Paths listed with conditions may be extended: i.e., a node currently 
>>>>>>> listed as PV might also become available for HVM guests
>>>>>>> 
>>>>>>> 3. Paths listed with conditions might have those conditions reduced, 
>>>>>>> but will never entirely disappear.  So something currently listed as PV 
>>>>>>> might be reduced to CONFIG_HAS_FOO, but won’t be completely removed.
>>>>>>> 
>>>>>>> Is that what you meant?
>>>>>> 
>>>>>> I see Jürgen replied "yes" to this, but I'm not sure about 1.
>>>>>> above: I think it's quite reasonable to expect that paths without
>>>>>> condition may gain a condition. Just like paths now having a
>>>>>> condition and (perhaps temporarily) losing it shouldn't all of
>>>>>> the sudden become "always available" when they weren't meant to
>>>>>> be.
>>>>>> 
>>>>>> As far a 3. goes, I'm also unsure in how far this is any better
>>>>>> stability wise (from a consumer pov) than allowing paths to
>>>>>> entirely disappear.
>>>>> 
>>>>> The idea is that any user tool using hypfs can rely on paths under 1 to
>>>>> exist, while the ones under 3 might not be there due to the hypervisor
>>>>> config or the used system.
>>>>> 
>>>>> A path not being allowed to entirely disappear ensures that it remains
>>>>> in the documentation, so the same path can't be reused for something
>>>>> different in future.
>>>> And then how do you deal with a condition getting dropped, and
>>>> later wanting to get re-added? Do we need a placeholder condition
>>>> like [ALWAYS] or [TRUE]?
>>> 
>>> Dropping a condition has to be considered very carefully, same as
>>> introducing a new path without any condition.
>>> 
>>> In worst case you can still go with [CONFIG_HYPFS].
>> Couldn’t we just have a section of the document for dead paths that aren’t 
>> allowed to be used?
>> Alternately, we could have a tag for entries we don’t want used anymore; 
>> [DEAD] or [OBSOLETE] maybe? [DEFUNCT]? [REMOVED]?
>> So I think I’d write a separate section, like this:
>> ~~
>> # Stability
>> Path *presence* is not stable, but path *meaning* is always stable: if a 
>> tool you write finds a path present, it can rely on behavior in future 
>> versions of the hypervisors, and in different configurations.  Specifically:
>> 1. Conditions under which paths are used may be extended, restricted, or 
>> removed.  For example, a path that’s always available only on ARM systems 
>> may become available on x86; or a path available on both systems may be 
>> restricted to only appearing on ARM systems.  Paths may also disappear 
>> entirely.
>> 2. However, the meaning of a path will never change.  If a path is present, 
>> it will always have exactly the meaning that it always had.  In order to 
>> maintain this, removed paths should be retained with the tag [REMOVED].  The 
>> path may be restored *only* if the restored version of the path is 
>> compatible with the previous functionality.
>> ~~~
>> Thoughts?
> 
> Would work for me, too.

So whose court is the ball in now?  Are you going to send another patch, or 
would you prefer I do it?

 -George