Re: [Qemu-devel] Can we improve virtio data structures with QOM?

[Anthony Liguori <anthony@codemonkey.ws>]

On 06/01/2012 05:25 PM, Markus Armbruster wrote:
> Anthony Liguori<anthony@codemonkey.ws>  writes:
>
>> On 06/01/2012 12:48 AM, Markus Armbruster wrote:
>>> Anthony Liguori<anthony@codemonkey.ws>   writes:
>>>
>>> [On how to model virtio devices in QOM:]
>>>> Basically, it should look like:
>>>>
>>>> VirtioPCIDevice is-a PCIDevice
>>>>
>>>> VirtioPCIDevice has-a link<VirtioDevice>
>>>
>>> Could you explain why this is link<>   and not child<>?
>>
>> So you can do:
>>
>> qemu -device virtio-pci,id=foo,vdev=bar -device virtio-blk,id=bar,bus=foo
>
> This lets folks specify both directions of the virtio-pci<->  virtio-blk
> connection independently.  What if $dev->bus->vdev != $dev, i.e. the
> backlink doesn't point back?

Then the user did something silly.

If you're really asking, should we expect a user to use a command line like 
this?  Absolutely not!

A user should use something like -drive file=foo.img,if=virtio

Heck, I still think we should do -vda foo.img.

> Easiest way to avoid that is to deny access to the backlink, and set it
> automatically instead.  Wouldn't be surprised if such bi-directional
> links turned out to be a pretty common pattern.

Most likely they will.  But I don't think users should be setting any links in 
the first place.

> In qdev, we've always called the property naming the parent "bus",
> because it's always been a qbus.  Doesn't make sense here: the property
> refers to a device, not a bus.  Should we call it something else?

Actually, in qdev it's called parent_bus.

>> The alternative would be:
>>
>> qemu -device virtio-pci,id=foo,child_type=virtio-blk
>>
>> But that feels ugly to me.  If you want to have a variable type of
>> device, a link is the right tool.
>
> Okay, that's a general rule (I was hoping you'd state one).  Do we have
> a place for such rules, or do we expect people to learn them by osmosis?

I actually just did a tutorial session out here walking through how to write 
device emulation from scratch.  I would thinking of making a QOM Style Guide 
based on that.  I'm pretty short on free time for the next week but I'll try to 
queue it up.  If anyone wants to take a first pass, I'd happily review it and 
contribute.

>> BTW, I make no mention of BusState here.  That's intentional.  There's
>> no need to involve buses IMHO.
>
> I never liked qbus anyway.

qbus never liked any of us too FWIW.  It was a real jerk that way.

Regards,

Anthony Liguori