Re: [Qemu-devel] [PATCH 06/15] memory: use refcnt to manage MemoryRegion

[Avi Kivity <avi@redhat.com>]

On 08/09/2012 10:27 AM, liu ping fan wrote:
> On Wed, Aug 8, 2012 at 5:20 PM, Avi Kivity <avi@redhat.com> wrote:
>> On 08/08/2012 09:25 AM, Liu Ping Fan wrote:
>>> From: Liu Ping Fan <pingfank@linux.vnet.ibm.com>
>>>
>>> Using refcnt for mr, so we can separate mr's life cycle management
>>> from refered object.
>>>   When mr->ref 0->1, inc the refered object.
>>>   When mr->ref 1->0, dec the refered object.
>>>
>>> The refered object can be DeviceStae, another mr, or other opaque.
>>
>> Please explain the motivation more fully.
>>
> Actually, the aim is to mange the reference of an object, used by mem view.
> DeviceState can be referred by different system, when it comes to the
> view of subsystem, we hold dev's ref. And any indirect reference will
> just mr->ref++, not dev's.
> This can help us avoid the down-walk through the referred chain, like
> alias----> mr ---> DeviceState.

That is a lot of complexity, for no gain.  Manipulating memory regions
is a slow path, and can be done under the bit qemu lock without any
complications.

> 
> In the previous discussion, you have suggest add dev->ref++ in
> core_region_add.  But I think, if we can move it to higher layer --
> memory_region_{add,del}_subregion, so we can avoid to duplicate do
> this in other xx_region_add.

Why would other memory listeners be impacted?  They all operate under
the big qemu lock.  If they start using devices outside the lock, then
they need to take a reference.

> As a payment for this, we need to handle alias which can be avoid at
> core_region_add().  And mr's ref can help to avoid
>  the down-walk.

The payment is two systems of reference counts.

-- 
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