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

[Anthony Liguori <anthony@codemonkey.ws>]

On 09/15/2011 01:31 AM, Gleb Natapov wrote:
> On Wed, Sep 14, 2011 at 01:04:00PM -0500, Anthony Liguori wrote:
>> All device relationships are identified as named properties.  A QOM
>> path name
>> consists of a named device, followed by a series of properties which
>> may or may
>> not refer to other devices.  For instance, all of the following are
>> valid paths:
>>
>>   /i440fx/piix3/i8042/aux
>>   /i440fx/slot[1.0]/i8042/aux
>>   /i440fx/slot[1.0]/bus/piix3/i8042/aux
>>
> Have you looked at device paths generated by get_fw_dev_path() in qdev?

get_fw_dev_path() won't exist in QOM.  The fact that it exists in qdev is a 
problem with qdev.

> This function generates Open Firmware device path.

The function generates *a* OF device path.  OF is not a canonical representation 
of arbitrary hardware.  It's a representation chosen (usually by a human) of 
what information about the hardware is needed by the OS-level software.

If you look at what other folks have done with OF integration in QEMU, you'll 
see a recurring theme of two OF trees, one used to create the hardware and the 
other that is actually exposed to the guest.  The reason you need two is because 
guests sometimes expect very specific things that you really can't generate 
programmatically in every circumstance.

> The difference
> between OF device path and the examples above is that OF device path has
> a meaning outside of QEMU and can be used by firmware to find a device
> a path refers too. Will QOM be able to generate them?

All of the information needed to generate an OF tree is available as device 
properties.  To the extent that you need to knowledge of each bus to generate a 
OF path component, you'll need some extra knowledge of each bus to do that (just 
like with qdev today).  But that knowledge will definitely not be part of QOM.

Paths are not part of QOM.  They're representations used by client software to 
navigate the QOM graph.  There is no real need to make paths part of QOM explicitly.

Regards,

Anthony Liguori