On 06/01/2012 05:25 PM, Markus Armbruster wrote:
Anthony Liguori<anth...@codemonkey.ws> writes:
On 06/01/2012 12:48 AM, Markus Armbruster wrote:
Anthony Liguori<anth...@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
