[Qemu-devel] Re: [PATCH 26/35] kvm: Eliminate KVMState arguments

[Jan Kiszka <jan.kiszka@web.de>]

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Am 10.01.2011 20:59, Anthony Liguori wrote:
> On 01/08/2011 02:47 AM, Jan Kiszka wrote:
>> Am 08.01.2011 00:27, Anthony Liguori wrote:
>>   
>>> On 01/07/2011 03:03 AM, Jan Kiszka wrote:
>>>     
>>>> Am 06.01.2011 20:24, Anthony Liguori wrote:
>>>>
>>>>       
>>>>> On 01/06/2011 11:56 AM, Marcelo Tosatti wrote:
>>>>>
>>>>>         
>>>>>> From: Jan Kiszka<jan.kiszka@siemens.com>
>>>>>>
>>>>>> QEMU supports only one VM, so there is only one kvm_state per
>>>>>> process,
>>>>>> and we gain nothing passing a reference to it around. Eliminate any
>>>>>> need
>>>>>> to refer to it outside of kvm-all.c.
>>>>>>
>>>>>> Signed-off-by: Jan Kiszka<jan.kiszka@siemens.com>
>>>>>> CC: Alexander Graf<agraf@suse.de>
>>>>>> Signed-off-by: Marcelo Tosatti<mtosatti@redhat.com>
>>>>>>
>>>>>>
>>>>>>            
>>>>> I think this is a big mistake.
>>>>>
>>>>>          
>>>> Obviously, I don't share your concerns. :)
>>>>
>>>>
>>>>       
>>>>> Having to manage kvm_state keeps the abstraction lines well defined.
>>>>>
>>>>>          
>>>> How does it help?
>>>>
>>>>
>>>>       
>>>>> Otherwise, it's far too easy for portions of code to call into KVM
>>>>> functions that really shouldn't.
>>>>>
>>>>>          
>>>> I can't imagine we gain anything from requiring kvm_check_extension
>>>> callers to hold a kvm_state "capability". Yes, it's now much easier to
>>>> call kvm_[vm_]ioctl, but that's the key point of this change:
>>>>
>>>> So far we primarily complicated the internal interface between generic
>>>> and arch-dependent kvm parts by requiring kvm_state joggling. But
>>>> external users already find interfaces without this restriction
>>>> (kvm_log_*, kvm_ioeventfd_*, ...). That's because it's at least
>>>> complicated to _cleanly_ pass kvm_state references to all users that
>>>> need it - e.g. sysbus devices like kvmclock or upcoming in-kernel
>>>> irqchips.
>>>>
>>>>        
>>> I think you're basically making my point for me.
>>>
>>> ioeventfd is a broken interface.  It shouldn't be a VM ioctl but rather
>>> a VCPU ioctl because PIO events are dispatched on a per-VCPU basis.
>>>      
>> OK, but I don't want to argue about the ioeventfd API. So let's put this
>> case aside. :)
>>
>>   
>>> kvm_state is available as part of CPU state so it's quite easy to get at
>>> if these interfaces just took a CPUState argument (and they should).
>>>      
>> My point is definitely NOT about cpu-bound devices. That case is clear
>> and is not touched at all by this patch.
>>
>> My point is about devices that have clear system scope like kvmclock,
>> ioapic, pit, pic,
> 
> I don't see how ioapic, pit, or pic have a system scope.

They are not bound to any CPU like the APIC which you may have in mind.

> 
> I don't know enough about kvmclock.

It's just the same.

> 
>>   whatever-the-future-will-bring. And about KVM services
>> that have global scope like capability checks and other feature
>> explorations or VM configurations done by the KVM arch code. You still
>> didn't explain what we gain in these concrete scenarios by handing the
>> technically redundant abstraction kvm_state around, especially _inside_
>> the KVM core.
>>    
> 
> If you have to pass around a KVMState pointer, you establish an explicit
> relationship and communication between subsystems.  Any place where the
> global KVMState is used is a red flag that something is wrong.

It is and will be _only_ used inside kvm-all.c. Again: What is the
benefit of restricting access to kvm_check_extension this way?

> 
> I don't see what the advantage to making all of the KVMState global and
> implicit.  It seems like a big step backwards to me.  Can you give a
> very concrete example of where you think it results in easier to
> understand code as I don't see how making relationships implicit ever
> makes code easier to understand?

The best example does not yet exist (fortunately): Just look at patch 28
and then try to pass some kvm_state reference to the kvmclock device. Is
this handle worth changing the sysbus API?

Jan


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