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

[Peter Maydell]

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]

?

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

thanks
-- PMM