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

[Avi Kivity <avi@redhat.com>]

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.

-- 
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