Re: [Qemu-devel] [PATCH 28/35] kvm: x86: Introduce kvmclock device to save/restore its state

[Jan Kiszka]

On 2011-01-18 17:04, Anthony Liguori wrote:
>>>>> A KVM device should sit on a KVM specific bus that hangs off of
>>>>> sysbus.
>>>>> It can get to kvm_state through that bus.
>>>>>
>>>>> That bus doesn't get instantiated through qdev so requiring a pointer
>>>>> argument should not be an issue.
>>>>>
>>>>>
>>>>>          
>>>> This design is in conflict with the requirement to attach KVM-assisted
>>>> devices also to their home bus, e.g. an assigned PCI device to the PCI
>>>> bus. We don't support multi-homed qdev devices.
>>>>
>>>>        
>>> The bus topology reflects how I/O flows in and out of a device.  We do
>>> not model a perfect PC bus architecture and I don't think we ever intend
>>> to.  Instead, we model a functional architecture.
>>>
>>> I/O from an assigned device does not flow through the emulated PCI bus.
>>> Therefore, it does not belong on the emulated PCI bus.
>>>
>>> Assigned devices need to interact with the emulated PCI bus, but they
>>> shouldn't be children of it.
>>>      
>> You should be able to find assigned devices on some PCI bus, so you
>> either have to hack up the existing bus to host devices that are, on the
>> other side, not part of it or branch off a pci-kvm sub-bus, just like
>> you would have to create a sysbus-kvm.
> 
> Management tools should never transverse the device tree to find
> devices.  This is a recipe for disaster in the long term because the
> device tree will not remain stable.
> 
> So yes, a management tool should be able to enumerate assigned devices
> as they would enumerate any other PCI device but that has almost nothing
> to do with what the tree layout is.

I'm probably misunderstanding you, but if the bus topology as the guest
sees it is not properly reflected in an object tree on the qemu side, we
are creating hacks again.

Management and analysis tools must be able to traverse the system buses
and find guest devices this way. If they create a device on bus X, it
must never end up on bus Y just because it happens to be KVM-assisted or
has some other property. On the other hand, trying to hide this
dependency will likely cause severe damage to the qdev design.

Jan

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