Re: [PATCH v3 03/12] vfio-user: define vfio-user-server object

[Kevin Wolf]

Am 06.11.2021 um 07:34 hat Markus Armbruster geschrieben:
> Kevin Wolf <kwolf@redhat.com> writes:
> 
> > Am 05.11.2021 um 11:08 hat Markus Armbruster geschrieben:
> >> Kevin Wolf <kwolf@redhat.com> writes:
> >> 
> >> > Am 04.11.2021 um 13:13 hat Markus Armbruster geschrieben:
> >> >> The old syntax almost always has its quirks.  Ideally, we'd somehow get
> >> >> from quirky old to boring new in an orderly manner.  Sadly, we still
> >> >> don't have good solutions for that.  To make progress, we commonly
> >> >> combine JSON new with quirky old.
> >> >> 
> >> >> qemu-system-FOO -object works that way.  object_option_parse() parses
> >> >> either JSON or QemuOpts.  It wraps the former in a QObject visitor, and
> >> >> the latter in an opts visitor.
> >> >> 
> >> >> QemuOpts is flat by design[*], so the opts visitor parses flat QemuOpts
> >> >> from a (possibly non-flat) QAPI type.  How exactly it flattens, and how
> >> >> it handles clashes I don't remember.
> >> >> 
> >> >> Sadly, this means that we get quirky old even for new object types.
> >> >
> >> > For -object in the system emulator (the tools all use the keyval
> >> > visitor, so there it would work as expected), the only reason that we
> >> > need to keep the quirky old code path around is the list handling in
> >> > memory-backend.host-nodes.
> >> >
> >> > The main difficulty there is that the old QemuOpts based code path
> >> > allows specifying the option twice and both of them would effectively be
> >> > combined. Do we have any idea how to replicate this in a keyval parser
> >> > based world?
> >> 
> >> I can see just two clean solutions, but both involve upending a lot of
> >> code.
> >> 
> >> We can fuse keyval parser and visitor to get a schema-directed parser.
> >> 
> >> We can change the abstract keyval syntax to permit repeated keys.  This
> >> means replacing QDict in in the abstract syntax tree, with fallout in
> >> the visitor.
> >> 
> >> Even if we find a practical solution, I don't like the combination of
> >> "you may give the same parameter multiple times, and the last one wins"
> >> and "for a list-valued parameter, the values of repeated parameters are
> >> collected into a list".  Each makes sense on its own.  The combination
> >> not so much.  Inheriting "last one wins" from QemuOpts may have been a
> >> mistake.
> >> 
> >> The keyval way of doing lists (inherited from the block layer's usage of
> >> dotted keys?  I don't remember) requires the user to count, which isn't
> >> exactly nice, either.
> >
> > Yes. If we didn't have to maintain compatibility (or actually as soon as
> > we degrade non-JSON option lists to HMP-level support), I would
> > introduce [] and {} syntax for lists and dicts, even if that means that
> > use of these characters in strings doesn't work any more or only in a
> > limited way. I think this would be the best compromise for usability.
> >
> > Anyway, this doesn't help us with the compatibility problem we're
> > discussing here.
> >
> >> > If not, do we want to use the remaining time until 6.2 to deprecate
> >> > this? The nasty part is that the only syntax that works both now and in
> >> > the future is JSON. We can't easily accept the new keyval syntax while
> >> > still using the QemuOpts based code.
> >> 
> >> What exactly do you propose to deprecate?
> >
> > We can deprecate on two different levels. I think it's useful to do
> > both:
> >
> > 1. Broad deprecation: Stable non-JSON interfaces are degraded to
> >    a HMP-like compatibility promise.
> 
> Calling it "deprecation" might be confusing.  HMP isn't deprecated, it's
> merely not a stable interface.  That's kind of like "deprecated when you
> need stable", but saying "not a stable interface" is clearer.
> 
> When I write "deprecate" below, I mean something like "go use something
> else (no conditions)".  When I mean "use something else when you need
> stable", I write "degrade" (short for "degrade to an HMP-like
> compatibility promise").
> 
> >                                      Obviously, this can only be done
> >    for options that support JSON.
> 
> We can also degrade or even deprecate sugar options in favor of the real
> ones.  Case by case, I guess.

Right. And essentially, the non-JSON form would be considered a sugar
option, even if the option string is the same.

> >                                   Peter Maydell also wants to do this
> >    only after a big user (read: libvirt) has implemented and is
> >    using JSON, basically as a proof that the alternative is working.
> >
> >    So this can certainly be done for -object. I believe libvirt also
> >    uses JSON for -device now, so this should be fine now, too.
> 
> The non-sugar options supporting JSON are -audiodev, -blockdev, -compat,
> -display (partially), -machine (I think), -object.
> 
> -netdev is QAPIfied, but still uses QemuOpts.  Too late for 6.2, I'm
> afraid.

Ok. Not sure about the libvirt status for some of these, but -object and
-device are the ones that I know are going to be in the way in the
future, so degrading their non-JSON form would already be helpful.

> >                                                                Possibly
> >    -drive (in favour of -blockdev), though I'm not completely sure if we
> >    have gotten rid of the final users of -drive. (CCing Peter Krempa for
> >    details.)
> 
> The problem with deprecating -drive is configuring onboard block
> devices.  We need a stable interface for that, and it must be usable
> together with -blockdev.
> 
> We provided such an interface (machine properties) for some onboard
> block devices starting with commit ebc29e1bea "pc: Support firmware
> configuration with -blockdev".  Many more remain, I believe.

So maybe we need to define a form of -drive (or a new option) that would
stay stable and can just take a block node and create a DriveInfo for
it.

Anyway, not for 6.2, so let's ignore this for now.

> >    This degradation of the compatibility promise doesn't tell users what
> >    exactly is going to change, which is why doing the second one, too,
> >    might be nice.
> >
> > 2. Narrow deprecation: We can just deprecate the non-JSON form, or
> >    certain aspects of it, of memory-backend.host-nodes. This is the
> >    specific things that stops us from switching -object to keyval.
> >
> >    a. Deprecate the whole option. If you want to use it and need a
> >       stable interface, you have to use JSON. We'll just switch the
> >       non-JSON form on a flag day. Before it, you need to use QemuOpts +
> >       OptsVisitor syntax for the list; after it, you need to use keyval
> >       syntax.
> 
> I parse "the whole option" as "-object with dotted keys argument".
> Correct?

No, degrading non-JSON -object (it's still QemuOpts, so "dotted keys"
aren't even supported) is already option 1.

This one is specifically "memory-backend.host-nodes on the CLI".

> >    b. Deprecate only repeating the option. memory-backend is changed to
> >       first try visiting a list, and if that fails, it visits a string
> >       and goes through a string visitor locally to keep supporting the
> >       integer range syntax.
> 
> Possible problem: integer range syntax must not leak into the JSON form.
> 
> >    c. Deprecate all list values, but keep supporting a single integer
> >       value by using an alternate between list and int.
> 
> Single int should also not leak into JSON.

Honestly, I don't care about them leaking into JSON and QMP, and I don't
think you should either.

If we insist on a perfectly idiomatic QAPI schema as if we were writing
the objects today, we won't have made any progress even in 10 years.
Many QOM objects have properties that are a mess and it will show in the
schema. Strings that are parsed, alternates to provide different syntax
for the same thing, etc. I don't like any of this, but we're not
designing new interfaces here, but describing existing ones.

I do understand and agree that you want to keep the core infrastructure
reasonably clean from hacks, because it affects everything and we're
touching it a lot. But if an individual property in some QOM object is
in the way, we should just hack around it. Spending a lot of thought on
how to get it cleaned up would have a much higher cost than maintaining
a small hack.

Kevin