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Re: [Qemu-devel] [PATCH 28/35] kvm: x86: Introduce kvmclock device to sa


From: Jan Kiszka
Subject: Re: [Qemu-devel] [PATCH 28/35] kvm: x86: Introduce kvmclock device to save/restore its state
Date: Tue, 18 Jan 2011 16:54:46 +0100
User-agent: Mozilla/5.0 (X11; U; Linux i686 (x86_64); de; rv:1.8.1.12) Gecko/20080226 SUSE/2.0.0.12-1.1 Thunderbird/2.0.0.12 Mnenhy/0.7.5.666

On 2011-01-18 16:48, Anthony Liguori wrote:
> On 01/18/2011 09:43 AM, Jan Kiszka wrote:
>> On 2011-01-18 16:04, Anthony Liguori wrote:
>>    
>>> On 01/18/2011 08:28 AM, Jan Kiszka wrote:
>>>      
>>>> On 2011-01-12 11:31, Jan Kiszka wrote:
>>>>
>>>>        
>>>>> Am 12.01.2011 11:22, Avi Kivity wrote:
>>>>>
>>>>>          
>>>>>> On 01/11/2011 03:54 PM, Anthony Liguori wrote:
>>>>>>
>>>>>>            
>>>>>>> Right, we should introduce a KVMBus that KVM devices are created on.
>>>>>>> The devices can get at KVMState through the BusState.
>>>>>>>
>>>>>>>              
>>>>>> There is no kvm bus in a PC (I looked).  We're bending the device model
>>>>>> here because a device is implemented in the kernel and not in
>>>>>> userspace.  An implementation detail is magnified beyond all proportions.
>>>>>>
>>>>>> An ioapic that is implemented by kvm lives in exactly the same place
>>>>>> that the qemu ioapic lives in.  An assigned pci device lives on the PCI
>>>>>> bus, not a KVMBus.  If we need a pointer to KVMState, then we must find
>>>>>> it elsewhere, not through creating imaginary buses that don't exist.
>>>>>>
>>>>>>
>>>>>>            
>>>>> Exactly.
>>>>>
>>>>> So we can either "infect" the whole device tree with kvm (or maybe a
>>>>> more generic accelerator structure that also deals with Xen) or we need
>>>>> to pull the reference inside the device's init function from some global
>>>>> service (kvm_get_state).
>>>>>
>>>>>          
>>>> Note that this topic is still waiting for good suggestions, specifically
>>>> from those who believe in kvm_state references :). This is not only
>>>> blocking kvmstate merge but will affect KVM irqchips as well.
>>>>
>>>> It boils down to how we reasonably pass a kvm_state reference from
>>>> machine init code to a sysbus device. I'm probably biased, but I don't
>>>> see any way that does not work against the idea of confining access to
>>>> kvm_state or breaks device instantiation from the command line or a
>>>> config file.
>>>>
>>>>        
>>> 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.
>>    
> 
> With vfio, would an assigned PCI device even need kvm_state?

IIUC: Yes, for establishing the irqfd link.

Jan

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