[PATCH 2/9] qapi: Mark unstable QMP parts with feature 'unstable'

Tue Oct 26 16:34:26 UTC 2021

On Tue, Oct 26, 2021 at 3:56 AM Markus Armbruster <armbru at redhat.com> wrote:

> John Snow <jsnow at redhat.com> writes:
>
> > On Mon, Oct 25, 2021 at 1:25 AM Markus Armbruster <armbru at redhat.com>
> wrote:
> >
> >> Add special feature 'unstable' everywhere the name starts with 'x-',
> >> except for InputBarrierProperties member x-origin and
> >> MemoryBackendProperties member x-use-canonical-path-for-ramblock-id,
> >> because these two are actually stable.
> >>
> >> Signed-off-by: Markus Armbruster <armbru at redhat.com>
> >> ---
> >>  qapi/block-core.json | 123 +++++++++++++++++++++++++++++++------------
> >>  qapi/migration.json  |  35 +++++++++---
> >>  qapi/misc.json       |   6 ++-
> >>  qapi/qom.json        |  11 ++--
> >>  4 files changed, 130 insertions(+), 45 deletions(-)
> >>
> >> diff --git a/qapi/block-core.json b/qapi/block-core.json
> >> index 6d3217abb6..ce2c1352cb 100644
> >> --- a/qapi/block-core.json
> >> +++ b/qapi/block-core.json
> >> @@ -1438,6 +1438,9 @@
> >>  #
> >>  # @x-perf: Performance options. (Since 6.0)
> >>  #
> >> +# Features:
> >> +# @unstable: Member @x-perf is experimental.
> >> +#
> >>
> >
> > It'd be a lot cooler if we could annotate the unstable member directly
> > instead of confusing it with the syntax that might describe the entire
> > struct/union/command/etc, but ... eh, it's just a doc field, so I'm not
> > gonna press on this. I don't have the energy to get into a doc formatting
> > standard discussion right now, so: sure, why not?
>
> By design, we have a single doc comment block for the entire definition.
>
> When Kevin invented feature flags (merge commit 4747524f9f2), he added
> them just to struct types.  He added "feature sections" for documenting
> features.  It mirrors the "argument sections" for documenting members.
> Makes sense.
>
> Aside: he neglected to update qapi-code-gen.rst section "Definition
> documentation", and I failed to catch it.  I'll make up for it.
>
> Peter and I then added feature flags to the remaining definitions
> (commit 23394b4c39 and 013b4efc9b).  Feature sections make sense there,
> too.
>
> I then added them to struct members (commit 84ab008687).  Instead of
> doing something fancy for documenting feature flags of members, I simply
> used the existing feature sections.  This conflates member features with
> struct features.
>
>
Yeah, that's the part I don't like. If this isn't the first instance of
breaking the seal, though, this is the wrong time for me to comment on it.
Don't worry about it for this series.

> What could "something fancy" be?  All we have for members is "argument
> sections", which are basically a name plus descriptive text.  We'd have
> to add structure to that, say nest feature sections into argument
> sections.  I have no appetite for that right now.
>
>
(Tangent below, unrelated to acceptance of this series)

Yeah, I don't have an appetite for it at the moment either. I'll have to
read Marc-Andre's recent sphinx patches and see if I want to do more work
-- I had a bigger prototype a few months back I didn't bring all the way
home, but I am still thinking about reworking our QAPIDoc format. It's ...
well. I don't really want to, but I am not sure how else to bring some of
the features I want home, and I think I need some more time to think
carefully through exactly what I want to do and why.

I wouldn't mind chatting about it with you sometime soon -- there's a few
things to balance here, such as:

(1) Reworking the doc format would be an obnoxious amount of churn, ...
(2) ...but the Sphinx internals are really not meant to be used the way
we're using them right now, ...
(3) ...but I also don't want to write our QAPI docstrings in a way that
*targets* Sphinx. Not that I think we'll be dropping it any time soon, but
it still feels like the wrong idea to tie them so closely together.

I'm hoping there's an opportunity to make the parsing nicer with minimal
changes to the parsing and format, though. It just might require enforcing
a *pinch* more structure. I could see how I feel about per-field
annotations at that point. I consider them interesting for things like the
Python SDK where I may want to enable/disable warnings for using deprecated
stuff at the client-level. (e.g., let's say you're using Python SDK 6.2 to
talk to a 6.1 client. Nothing stops you from doing this, but some commands
will simply be rejected by QEMU as it won't know what you're talking about.
Using deprecated fields or commands to talk to an older client will produce
no visible warning from QEMU either, as it wasn't deprecated at that point
in time. Still, the client may wish to know that they're asking for future
trouble -- so it's a thought that client-side warnings have some play here.)

Ehhhhhhhhhhhhhhhh don't worry about it for now, I guess I'll plow forward a
bit with the Sphinx stuff I'm thinking of at some point Soon:tm: and worry
about where the immovable objects are when I get there. This is foot-based
landmine-detection, and it works 100% of the time.

>
> >
> >>  # Note: @on-source-error and @on-target-error only affect background
> >>  #       I/O.  If an error occurs during a guest write request, the
> >> device's
> >>  #       rerror/werror actions will be used.
> >> @@ -1452,7 +1455,9 @@
> >>              '*on-source-error': 'BlockdevOnError',
> >>              '*on-target-error': 'BlockdevOnError',
> >>              '*auto-finalize': 'bool', '*auto-dismiss': 'bool',
> >> -            '*filter-node-name': 'str', '*x-perf': 'BackupPerf'  } }
> >> +            '*filter-node-name': 'str',
> >> +            '*x-perf': { 'type': 'BackupPerf',
> >> +                         'features': [ 'unstable' ] } } }
> >>
> >>  ##
> >>  # @DriveBackup:
>
> [...]
>
> > Seems OK, but I didn't audit for false positives/negatives. Trusting your
> > judgment here. (It looks like Phil started to audit this in his reply to
> > your previous commit, so I'll trust that.)
>
> I'm pretty sure the x- names that don't get feature 'unstable' are
> actually stable; see my reply to Kashyap.
>
> I did check git history for each that does get feature 'unstable'.
> Double-checking is of course welcome.
>
>
Yeh, just explaining why it's not an R-B. I'm trying to be a bit better
about reviews by not forcing myself to do "all or nothing". I trust your
work, of course -- I just also did not double check it :)

I need to change the way in which I read and discuss code so that I can be
more responsive.

> > Acked-by: John Snow <jsnow at redhat.com>
>
> Thanks!
>
>
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