Anthony Liguori <anth...@codemonkey.ws> writes: > 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
No, a user should use what solves his problem nicely. Most of the time, the problem is simple. I quite agree we should provide simple ways to solve the known simple problems. Occasionally, you run into a non-simple problem, and then you'll really appreciate a discoverable bridge from the simple way to the general way. You'll also appreciate when the general way still satisfies basic usability criteria. A mandatory parameter that must have the one right value, or else things break, doesn't satisfy. "Experts/tools only" is no excuse for shoddy interfaces. -drive isn't such a good example for "simple"; it's a bloody mess, in my opinion. > 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. Real tools aren't built around ideas on what users shouldn't be doing with them. >> 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. Still got that bogus "bus" in there :) >>> 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. That should be really useful. > 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. Heh.
