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Re: [Qemu-block] [Qemu-devel] Non-flat command line option argument synt

From: Daniel P. Berrange
Subject: Re: [Qemu-block] [Qemu-devel] Non-flat command line option argument syntax
Date: Mon, 6 Feb 2017 17:50:55 +0000
User-agent: Mutt/1.7.1 (2016-10-04)

On Mon, Feb 06, 2017 at 06:24:42PM +0100, Markus Armbruster wrote:
> "Daniel P. Berrange" <address@hidden> writes:
> > On Mon, Feb 06, 2017 at 04:36:50PM +0100, Markus Armbruster wrote:
> >> Kevin Wolf <address@hidden> writes:
> >> > Want to make use of the shiny new QemuOpts and get things parsed into
> >> > a nested object? Well, provide a real schema instead of "any" then.
> >> 
> >> Sadly, this is somewhere between impractical and impossible.
> >> 
> >> The QAPI schema is fixed at compile-time.  It needs to be, because its
> >> purpose is to let us generate code we can compile and link into QEMU.
> >> 
> >> We use 'any' basically for things that aren't fixed at compile-time.
> >> 
> >> Example: qdev properties and device_add
> >> 
> >> Even though traditional qdev properties are fixed at compile time, they
> >> are not known until run-time.  That's because they're defined in the
> >> device models, and the registry of device models is only built at
> >> run-time.
> >> 
> >> I believe this would've been fixable with some effort: make the devices
> >> define suitable pieces of schema, and collect them somehow at
> >> compile-time.  "Would've been", because progress!  See next example.
> >> 
> >> Example: QOM properties and object-add, qom-set, qom-get
> >> 
> >> QOM properties are created at run-time.  They cannot be fixed at
> >> compile-time *by design*.  I always hated that part of the design, but I
> >> was assured we absolutely need that much rope^Wflexibility.
> >> 
> >> So, all we know about the "props" argument of object-add is that it's a
> >> JSON object.  The tightest imaginable QAPI schema would be an 'object'
> >> type, except that doesn't exist, so we settle for 'any'.
> >
> > The CLI parser is executing at runtime though, so I would think
> > it should need to care if the schema its using to parse the CLI
> > args was defined at build time or execution time. It merely needs
> > the schema to be present at the time it parses the data.
> Whatever "the schema" is, it can't be the QAPI schema, and it can't be
> used by generating code (which is how the visitors use the QAPI schema).
> Let's assume for the moment that QOM is the only source of schema stuff
> that becomes known only at run-time.  Then "the schema" is an
> amalgamation of the QAPI schema and QOM reflection.  I say "reflection",
> not "schema", because there is no QOM schema, only ways to examine (the
> current structure of) QOM objects.
> > So is there a way we dynamically report the info we need by improving
> > visitor support for QOM somehow.
> To parse the argument of -object, we need to create a QOM object of the
> type given by qom-type, so we can examine it to find its properties and
> their types.
> Consider
>     -object foo=[eins,qom-type=zwei,bar={x=y,qom-type=drei,baz=}]
> What's the value of qom-type?  Remember, -object has "props" unwrapped
> so that everything stays flat.
> If foo is an array, qom-type is missing.
> If foo is a string, then qom-type is zwei.  Except when bar is a string,
> because then it gets overridden to drei.

That's a strange syntax you've used for illustration there - a half
way between json and nested-dotted syntax. For pure json syntax it
would be a clear if qom-type was missing at the top level. for nested
dotted syntax, it again seems clear to me - split on ',' and find the
unqualified qom-type key (or the leading default arg)

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