On 06/21/2009 09:46 PM, Alexander Graf wrote:
You can use the dirty memory logging API. vga uses this to track
which regions of the screen have changed, and live migration uses it
to allow the guest to proceed while copying its memory to the other
node. It works exactly by write protecting guest memory and trapping
the resultant fault.
I stumbled across this on my ppc implementation: Is there an obvious
reason we don't use the pte's dirty bit?
Yes:
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.
--
I have a truly marvellous patch that fixes the bug which this
signature is too narrow to contain.
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