Re: [Qemu-devel] [RFC] Plan for moving forward with QOM

[Anthony Liguori <anthony@codemonkey.ws>]

On 09/15/2011 05:49 AM, Stefan Hajnoczi wrote:
> On Thu, Sep 15, 2011 at 7:31 AM, Gleb Natapov<gleb@redhat.com>  wrote:
>> On Wed, Sep 14, 2011 at 01:04:00PM -0500, Anthony Liguori wrote:
>> For isa machines:
>> /isa/ide@0170/drive@0/disk@0
>> /isa/fdc@03f1/floppy@1
>> /isa/fdc@03f1/floppy@0
>> /isa/ide@0170/drive@0/disk@1
>
> Yes, it's not clear to me whether the OF device path will be
> orthogonal to QOM or not.

It's orthogonal to QOM.

> Another part where I am not sure yet:
> "Device names are completely independent of pathnames.  For devices that are no
> user created, device names should be treated as opaque blobs with absolutely no
> semantic meaning."
>
> What does this mean?  Perhaps you're saying that:
> create_object('rtl8139', 'my-nic')
> pci_bus_name = resolve_path('/i440fx')
> set_object_property(pci_bus_name, 'slot[2.0]', 'my-nic')

More specifically:

def resolve_path(pathname):
     ...

qmp.create_object('rtl8139', 'my-nic')
pci_bus_name = resolve_path('/i440fx')
qmp.set_object_property(pci_bus_name, 'slot[2.0]', 'my-nic')

Paths are a concept of the client.  Maybe we add some path resolution logic to 
QMP as a convenience to help keep clients consistent with each other but from a 
strict perspective, a canonical path representation doesn't exist in QOM.

> The device name is a unique identifier that can be used to select a
> specific device.  The path is a relative way of selecting devices
> given a chain of properties - you only use property names in a path,
> not device names.  Is this correct?

With the exception of the very first component in the path, yes, in the path 
representation I proposed property names are used as the remaining components.

> For convenience I imagine people would prefer this:
> create_object('rtl8139', 'my-nic')
> set_object_property('/i440fx', 'slot[2.0]', 'my-nic')
>
> So here set_object_property() detects that there is a '/' in the
> device name.  Therefore it treats the argument as a path instead of a
> device name.  We'd need to add the constraints that paths always start
> with '/' and device names may not contain '/'.

Yes, this is exactly how my python client code works FWIW.  But 
set_object_property is a local function to the client, not a function of QMP.

Again, we don't need to be this strict in the QMP interface but from a QOM 
design perspective, I think the separation is important.

> Anyway, did I understand the independence of paths and device names correctly?

Yup.

Regards,

Anthony Liguori

> Stefan
>