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

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

Sorry, CC list is lost. :(

On 11/29/2011 06:01 PM, Xiao Guangrong wrote:

> Avi Kivity <avi <at> redhat.com> writes:
> 
>>
>> On 11/16/2011 06:28 AM, Takuya Yoshikawa wrote:
>>> (2011/11/14 21:39), Avi Kivity wrote:
>>>> There was a patchset from Peter Zijlstra that converted mmu notifiers to
>>>> be preemptible, with that, we can convert the mmu spinlock to a mutex,
>>>> I'll see what happened to it.
>>>
>>> Interesting!
>>>
>>>> There is a third method of doing write protection, and that is by
>>>> write-protecting at the higher levels of the paging hierarchy.  The
>>>> advantage there is that write protection is O(1) no matter how large the
>>>> guest is, or the number of dirty pages.
>>>>
>>>> To write protect all guest memory, we just write protect the 512 PTEs at
>>>> the very top, and leave the rest alone.  When the guest writes to a
>>>> page, we allow writes for the top-level PTE that faulted, and
>>>> write-protect all the PTEs that it points to.
>>>
>>> One important point is that the guest, not GET DIRTY LOG caller, will pay
>>> for the write protection at the timing of faults.
>>
>> I don't think there is a significant difference.  The number of write
>> faults does not change.  The amount of work done per fault does, but not
>> by much, thanks to the writeable bitmap.
>>
> 
> Avi,
> 
> I think it needs more thinking if only less page need be write protected.
> 
> For example, framebuffer-based device used by Xwindow, only ~64M pages needs
> to be write protected, but in your way, guest will get write page fault on all
> memory? Hmm?
> 
> It has some tricks but i missed?
> 
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