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

[George Dunlap <George.Dunlap@xxxxxxxxxx>]

> 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?

 -George