Re: [Qemu-devel] [PATCH 0/3] Fix confused output for alias properties

[Paolo Bonzini]

Il 22/09/2014 14:34, Michael S. Tsirkin ha scritto:
> On Mon, Sep 22, 2014 at 02:06:38PM +0200, Paolo Bonzini wrote:
>> Il 22/09/2014 13:22, Gonglei (Arei) ha scritto:
>>>> This doesn't change the fact that ObjectProperty is a generic struct,
>>>> and adding alias-specific fields there is wrong.
>>>
>>> OK, Maybe I should find other ways to attach this purpose and
>>> avoid layering violation. Thanks!
>>
>> Unfortunately I cannot think of any.
>>
>> We could add a description field to ObjectProperty, and replace
>> legacy_name with a description.  The output then would be
>>
>> virtio-blk.drive=str (drive)
>> That's a bit hackish, but perhaps it would be good enough for mst and
>> Markus?
> 
> I would just drop "str" and replace it with drive.

In the above example, the format would be

   CLASS.PROPERTY=TYPE (DESCRIPTION)

where I just put "drive" as the description to resemble the old output.

You cannot just have

   foo.drive=drive

because the type is *not* "drive", it's "str" (if anything, we could
make it link<BlockBackend> which does not really show the connection
between BlockBackend and -drive).

The problem is that "drive" as the type (the legacy_name) is simply not
available for the virtio-blk-pci.drive property.  It's hidden beneath an
opaque pointer.  Gonglei's patches were adding fields to ObjectProperty
just to avoid visiting that opaque pointer, which I consider a layering
violation.

> I seems to convey zero information.
> Description should be something more informative, e.g.
> (ID of a drive to use as a backend)

That would of course be fine too.  The problem is then that we have 644
properties (DEFINE_PROP_* macro invocations) that someone must add a
description to.

> Help for -device could add "Must match ID of a -drive option".
> Help for device HMP command could add "Must match ID of a drive command."

You are turning a bug that nobody noticed until now, and that really
doesn't break anything, into a month or more worth of work.

Paolo