Re: [Qemu-devel] [patch v4 05/16] memory: introduce ref, unref interface for MemoryRegionOps

[Paolo Bonzini]

Il 23/10/2012 14:15, Avi Kivity ha scritto:
> On 10/23/2012 02:06 PM, Paolo Bonzini wrote:
>> Il 23/10/2012 14:02, Avi Kivity ha scritto:
>>> On 10/23/2012 01:57 PM, Paolo Bonzini wrote:
>>>> Il 23/10/2012 13:55, Avi Kivity ha scritto:
>>>>>>> So the stop_machine idea is thrown away?  
>>>>> IIRC I convinced myself that it's just as bad.
>>>>
>>>> It may be just as bad, but it is less code (and less pervasive), which
>>>> makes it less painful.
>>>
>>> It saves you the ->ref() and ->unref() calls, which are boilerplate, but
>>> not too onerous. All of the device model and subsystem threading work
>>> still needs to be done.
>>
>> I'm not worried about saving the ->ref() and ->unref() calls in the
>> devices.  I'm worried about saving it in timers, bottom halves and
>> whatnot.  And also I'm not sure whether all callbacks would have
>> something to ref/unref as they are implemented now.
> 
> Hard to say without examples.
> 
> Something that bothers be with stop_machine is the reliance on
> cancellation.  With timers it's easy, stop_machine, remove the timer,
> resume.  But if you have an aio operation in progress that is not
> cancellable, you have to wait for that operation to complete.  Refcounts
> handle that well, the object stays until completion, then disappears.

Yes, that's the point of doing things asynchronously---you do not need
to do everything within stop_machine, you can start canceling AIO as
soon as the OS sends the hot-unplug request.  Then you only proceed with
stop_machine and freeing device memory when the first part.

In other words, isolate can complete asynchronously.

The good thing is that this is an improvement that can be applied on top
of the current code, which avoids doing too many things at once...

Paolo