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Re: [PATCH v3 03/12] vfio-user: define vfio-user-server object
From: |
Markus Armbruster |
Subject: |
Re: [PATCH v3 03/12] vfio-user: define vfio-user-server object |
Date: |
Sat, 06 Nov 2021 07:34:25 +0100 |
User-agent: |
Gnus/5.13 (Gnus v5.13) Emacs/27.2 (gnu/linux) |
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.
> 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.
> 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.
> 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?
> 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.
> Picking one of these four options is enough to convert -object to
> keyval. I would suggest doing both 1. and one of the options in 2.
I'm grateful for your analysis.
- Re: [PATCH v3 03/12] vfio-user: define vfio-user-server object, Stefan Hajnoczi, 2021/11/01
- Re: [PATCH v3 03/12] vfio-user: define vfio-user-server object, Markus Armbruster, 2021/11/04
- Re: [PATCH v3 03/12] vfio-user: define vfio-user-server object, Kevin Wolf, 2021/11/04
- Re: [PATCH v3 03/12] vfio-user: define vfio-user-server object, Markus Armbruster, 2021/11/05
- Re: [PATCH v3 03/12] vfio-user: define vfio-user-server object, Kevin Wolf, 2021/11/05
- Re: [PATCH v3 03/12] vfio-user: define vfio-user-server object, Peter Krempa, 2021/11/05
- Re: [PATCH v3 03/12] vfio-user: define vfio-user-server object,
Markus Armbruster <=
- Re: [PATCH v3 03/12] vfio-user: define vfio-user-server object, Kevin Wolf, 2021/11/08
- Re: [PATCH v3 03/12] vfio-user: define vfio-user-server object, Peter Krempa, 2021/11/08
Re: [PATCH v3 03/12] vfio-user: define vfio-user-server object, Stefan Hajnoczi, 2021/11/04