Re: [PATCH 0/4] KVM: Dirty logging optimization using rmap

[Xiao Guangrong <xiaoguangrong@linux.vnet.ibm.com>]

On 11/29/2011 10:03 PM, Avi Kivity wrote:

> On 11/29/2011 02:01 PM, Avi Kivity wrote:
>> On 11/29/2011 01:56 PM, Xiao Guangrong wrote:
>>> On 11/29/2011 07:20 PM, Avi Kivity wrote:
>>>
>>>
>>>> We used to have a bitmap in a shadow page with a bit set for every slot
>>>> pointed to by the page.  If we extend this to non-leaf pages (so, when
>>>> we set a bit, we propagate it through its parent_ptes list), then we do
>>>> the following on write fault:
>>>>
>>>
>>>
>>> Thanks for the detail.
>>>
>>> Um, propagating slot bit to parent ptes is little slow, especially, it
>>> is the overload for no Xwindow guests which is dirty logged only in the
>>> migration(i guess most linux guests are running on this mode and migration
>>> is not frequent). No?
>>
>> You need to propagate very infrequently.  The first pte added to a page
>> will need to propagate, but the second (if from the same slot, which is
>> likely) will already have the bit set in the page, so we're assured it's
>> set in all its parents.
> 
> btw, if you plan to work on this, let's agree on pseudocode/data
> structures first to minimize churn.  I'll also want this documented in
> mmu.txt.  Of course we can still end up with something different than
> planned, but let's at least try to think of the issues in advance.
> 


Yeap, this work is interesting, i will keep researching it. ;)