Re: [Qemu-devel] KVM call minutes for Feb 8

Anthony Liguori
2011-02-13T16:56:36Z

On 02/13/2011 09:56 AM, Avi Kivity wrote:
> On 02/13/2011 05:38 PM, Anthony Liguori wrote:
>>>
>>>>
>>>> 2) get rid of the entire concept of machines. Creating a i440fx is
>>>> essentially equivalent to creating a bare machine.
>>>
>>> No, it's not. The 440fx does not include an IOAPIC, for example.
>>> There may be other optional components, or differences in wiring,
>>> that make two machines with i440fx not identical.
>>
>>
>> The IOAPIC is basically the only other component and I view it as
>> part of the CPU interface to the chipset.
>
> But it isn't. The IOAPIC is not per-core or per-socket.

Yeah, I wasn't implying that it was.

> It's strictly board level. It's a board interface to the cpu.
>
>>
>> But still, if we're creating a machine from scratch:
>>
>> qemu -device i440fx,id=nb -device piix3,id=sb,chipset=nb -device
>> ioapic,id=ioapic,chipset=sb -device cpu,ioapic=ioapic,northbridge=nb
>>
>> Is not all that unreasonable and presents a fully functioning PC.
>
> Sure. And -M blah is a shortcut.

Exactly. Or better yet, blah is a config file that contains

[device "nb"]
driver=i440fx

[device "sb"]
driver=piix3
chipset=nb

[device "ioapic"]
driver=ioapic
chipset=sb

[device "cpu"]
driver=cpu
ioapic=ioapic
northbridge=nb

>>
>>>>
>>>> 4) model the CPUs as devices that take a pointer to a host
>>>> controller, for x86, the normal case would be giving it a pointer
>>>> to i440fx.
>>>>
>>>
>>> Surely the connection is via a bus? An x86 cpu talks to the bus,
>>> and there happens to be an 440fx north bridge at the end of it. It
>>> could also be a Q35 or something else.
>>
>> I see being on a bus as really just taking a pointer to an
>> interface. So yes, the i440fx would implement a PentiumCpuInterface
>> or something like that and the CPU would take a pointer to a
>> PentiumCpuInterface[1].
>>
>> This is part of why having proper polymorphism is important. We need
>> it in order to be able to express concepts like interfaces.
>
> The CPUs and northbridge are peers on the system bus. However, this
> isn't something good to model, since it keeps changing without any
> guest software impact.

If we can move away from Bus abstraction and to a simpler interface
mechanism, then we can express peer relationships by just having
bidirection references. IOW:

-device cpus,northbridge=nb,id=cpus,count=16 -device i440fx,cpus=cpus

I don't think modelling each CPU makes sense. We should probably just
model all cpus in a single device for the sake of simplicity.

Regards,

Anthony Liguori