Re: Checking guest memory pages changes from host userspace

[Avi Kivity <avi@redhat.com>]

On 06/21/2009 11:01 PM, Avi Kivity wrote:
>> I don't know which operation is more frequent - writing into dirty 
>> mapped memory or reading the dirty map. And I have no idea how long 
>> it would take to find out dirty pages...
>
> The cost of write protection is one fault per dirtied spte.  The cost 
> of looking at the dirty bit is a cache miss per spte (could be reduced 
> by scanning in spte order rather than gfn order).
>
> The problem is when you have a low percentage of memory dirtied.  Then 
> you're scanning a lot of sptes to find a few dirty ones - so the cost 
> per dirty page goes up.
>
> We've talked about write-protecting the upper levels first, but given 
> a random distribution of writes, that doesn't help much.
>

Thinking about it a bit more, when we write-protect pages we're O(spte) 
anyway, so that shouldn't be a barrier.

-- 
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