Re: [PATCH 11/11] qdev: Rework array properties based on list visitor

[Kevin Wolf]

Am 08.09.2023 um 17:18 hat Peter Maydell geschrieben:
> On Fri, 8 Sept 2023 at 15:37, Kevin Wolf <kwolf@redhat.com> wrote:
> >
> > Until now, array properties are actually implemented with a hack that
> > uses multiple properties on the QOM level: a static "foo-len" property
> > and after it is set, dynamically created "foo[i]" properties.
> >
> > In external interfaces (-device on the command line and device_add in
> > QMP), this interface was broken by commit f3558b1b ('qdev: Base object
> > creation on QDict rather than QemuOpts') because QDicts are unordered
> > and therefore it could happen that QEMU tried to set the indexed
> > properties before setting the length, which fails and effectively makes
> > array properties inaccessible. In particular, this affects the 'ports'
> > property of the 'rocker' device.
> >
> > This patch reworks the external interface so that instead of using a
> > separate top-level property for the length and for each element, we use
> > a single true array property that accepts a list value. In the external
> > interfaces, this is naturally expressed as a JSON list and makes array
> > properties accessible again.
> >
> > Creating an array property on the command line without using JSON format
> > is currently not possible. This could be fixed by switching from
> > QemuOpts to a keyval parser, which however requires consideration of the
> > compatibility implications.
> 
> Could we have a specific example in the commit message of:
> 
> The old (currently broken) syntax for setting the ports
> property on the rocker device is:
>  -device rocker,len-ports=2,ports[0]=dev0,ports[1]=dev1
> The new syntax that works as of this commit is:
>  [whatever]
> 
> ?

Sure, that's a good idea.

> I would expect most users have no idea what the JSON list
> syntax is.

To fill in your "[whatever]", it's something like this:

-device '{"driver":"rocker","ports":["dev0","dev1"]}'

If we can eventually get -device converted to the keyval parser instead
of QemuOpts, the non-JSON syntax will look like this:

-device rocker,ports.0=dev0,ports.1=dev1

But I assume we'll have to solve some compatibility problems with other
devices before this can be done.

Kevin