Re: [Qemu-devel] [PATCH 10/10] qdev: fix create in place obj's life cycle problem

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

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On 2012-08-27 10:17, liu ping fan wrote:
> On Mon, Aug 27, 2012 at 3:47 PM, Jan Kiszka <jan.kiszka@web.de> wrote:
>> On 2012-08-27 09:01, Paolo Bonzini wrote:
>>> Il 25/08/2012 09:42, liu ping fan ha scritto:
>>>>>>
>>>>>> I don't see why MMIO dispatch should hold the IDEBus ref rather than the
>>>>>> PCIIDEState.
>>>>>>
>>>> When transfer memory_region_init_io()  3rd para from void* opaque to
>>>> Object* obj,  the obj : opaque is not neccessary 1:1 map. For such
>>>> situation, in order to let MemoryRegionOps tell between them, we
>>>> should pass PCIIDEState->bus[0], bus[1] separately.
>>>
>>> The rule should be that the obj is the object that you want referenced,
>>> and that should be the PCIIDEState.
>>>
>>> But this is anyway moot because it only applies to objects that are
>>> converted to use unlocked dispatch.  This likely will not be the case
>>> for IDE.
>>
>> BTW, I'm pretty sure - after implementing the basics for BQL-free PIO
>> dispatching - that device objects are the wrong target for reference
> 
> Hi Jan, thanks for reminder, but could you explain it more detail?
> mmio dispatch table holds 1 ref for device, before releasing this
> ref,( When unplugging, we detach all the device's mr from memory, then
> drop the ref. So I think that no leak will be exposed by mr  and it is
> safe to use device as target for reference.

It would be a mistake to assume that memory regions can only be embedded
in device objects. Memory regions can be reconfigured or dynamically
added/removed (see e.g. portio lists) - there is no "device" in this
sentence. Regions are stored in the dispatching table, they will first
of all be touched without holding the BQL. So their content has to be
stable in that period, and it is the proper abstraction, IMHO, to focus
on their life cycle management and attach all the rest to them.

> 
>> counting. We keep memory regions in our dispatching tables (PIO
>> dispatching needs some refactoring for this), and those regions need
>> protection for BQL-free use. Devices can't pass away as long as the have
> Yes, it is right. Device can pass away only after mr removed from
> dispatching tables

Great, then you don't have to worry about device objects in the context
of dispatching.

Jan



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