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

[Anthony Liguori <aliguori@us.ibm.com>]

On 09/14/2011 03:00 PM, Edgar E. Iglesias wrote:
> On Wed, Sep 14, 2011 at 01:04:00PM -0500, Anthony Liguori wrote:
>> Hi,
>>
>> I spent a couple hours today writing up some comparisons and an
>> initial task list for converting qdev to QOM.  The main location of
>> this is the wiki[1] but I thought I would inline it here for easier
>> commenting.
>>
>> I decided to do this because I wanted to avoid a massively long 00
>> patch explaining the rationale for the first batch of changes that
>> I'm going to send out.
>>
>> There is so much complexity in qdev and the device model in general
>> that it's hard to come up with a concise document.  I'd really
>> appreciate suggestions for topics to write up more rationale as that
>> would help me avoid writing a book on the topic :-)
>
> Thanks Anthony,
>
> I'd appreciate if you could elaborate more on the backlinks. Also,
> tiny code examples would help. Maybe you've got a git repo with
> more code to link to?

Message-Id: <1311558293-5855-1-git-send-email-aliguori@us.ibm.com>

Or

http://repo.or.cz/w/qemu/aliguori.git/shortlog/refs/heads/qom

>
> Regarding the composition, a problem I face (which I mostly just
> hack my way around) is that inter device connections may exist
> at various layers at the same time. For example, a device that is
> burried under a couple of busses/bridges may somehow be tighly
> connected to another device at a very different location in
> the bus hierarchy by means of another overlayed interconnect (e.g
> data channels, timing generation, IRQs etc). Maybe you could
> clarify a bit more on how QOM handles these topics.

Links are meant to handle this.  You can have something like:

+ PlatformDevice1
+ PlatformDevice2
+ PciBus
   |
   +- PciDevice1
   +- PciDevice2
      |
      + I2CBus
        |
        +- I2CDevice1
           |- pd2: link<PlatformDevice2>

So you have I2CDevice that sites behind multiple levels of hierarchy, but has a 
direct relationship (via a link) to a platform device much higher in the 
hierarchy.  This is an simple demonstration of how you can create a graph in QOM.

qdev doesn't allow graphs in the object model which is the problem that you're 
having today.  That's because its a tree with alternating levels.  The top level 
is always a bus, the next level is always devices, the next level are always 
busses, etc.

With QOM, there are only devices, and devices can link to 0 or more other devices.

Regards,

Anthony Liguori


>
> Cheers