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Re: [Qemu-devel] Can we improve virtio data structures with QOM?
From: |
Markus Armbruster |
Subject: |
Re: [Qemu-devel] Can we improve virtio data structures with QOM? |
Date: |
Fri, 01 Jun 2012 13:43:30 +0200 |
User-agent: |
Gnus/5.13 (Gnus v5.13) Emacs/23.3 (gnu/linux) |
Anthony Liguori <address@hidden> writes:
> 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
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.