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

From: Anthony Liguori
Subject: Re: [Qemu-devel] Can we improve virtio data structures with QOM?
Date: Fri, 01 Jun 2012 17:32:24 +0800
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:11.0) Gecko/20120329 Thunderbird/11.0.1

On 06/01/2012 05:25 PM, Markus Armbruster wrote:
Anthony Liguori<address@hidden>  writes:

On 06/01/2012 12:48 AM, Markus Armbruster wrote:
Anthony Liguori<address@hidden>   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.


Anthony Liguori

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