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Re: [Qemu-devel] [PATCH v2 0/7] APIC/IOAPIC cleanup

From: Avi Kivity
Subject: Re: [Qemu-devel] [PATCH v2 0/7] APIC/IOAPIC cleanup
Date: Sun, 22 Aug 2010 22:44:05 +0300
User-agent: Mozilla/5.0 (X11; U; Linux x86_64; en-US; rv: Gecko/20100806 Fedora/3.1.2-1.fc13 Thunderbird/3.1.2

 On 08/22/2010 09:52 PM, Anthony Liguori wrote:
So really, I think this suggests that some devices shouldn't have any requirement to sit on a bus. A UART16650A does not sit on bus. It sits on a card and is wired to the ISA bus or is sometimes wired directly to pins on a CPU on a SoC.

I don't think we want to model individual resistors on a serial card as separate qdev objects. We want the serial card itself to be a qdev (as it is a hotpluggable entity) and the individual serial interfaces on that card (as they are duplicates of each other and of interest to the user).

You're missing the fundamental problem which arises because we've introduced an object model without thinking through how devices ought to be modelled.

All devices should have a DeviceState associated with them. Otherwise, there's really no point in having qdev at all.

We have lots of devices today that don't have DeviceState's associated with them because the have a separate qdev representation with a reference to the non-DeviceState object.

We have non-DeviceState objects because otherwise we end up with an inheritance diamond. We have this problem because we want to have relationships like: DeviceState <- SystemDeviceState <- ISADevice <- ISASerialDevice.

But ISASerialDevice is not the only type of serial device. You can also have a SystemSerialDevice that's directly attached to the System bus. That means you'd have to have:

SerialDevice -> ISASerialDevice -> SystemDeviceState -> DeviceState
-> SystemSerialDevice -> SystemDeviceState -> DeviceState

Which is a classic MI diamond. The only way to resolve this modelling problem is to split out the common code and rely on a has-a relationship instead of an is-a. That gives you:


ISASerialDevice has-a SerialDevice
SystemSerialDevice has-a SerialDevice

And since we want SerialDevice inherit from a DeviceState (recall, all devices should have DeviceStates):


No more MI diamond and all devices have DeviceStates. Coincidentally, it matches more closely how hardware works..

Well, I agree, but I honestly lost the context. How does this relate to the APIC and cpu hotplug?

I'll take the opportunity to say that we should be using a language that has first-class (...) support for these concepts instead of having to divine them from the code.

Generally speaking, any time we have one device that needs to sit on multiple busses, we're going to have to model it in this fashion.

We'll just have to address them one by one then. Perhaps if many come up we can try a generic solution.

I have a truly marvellous patch that fixes the bug which this
signature is too narrow to contain.

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