On 05.02.20 10:35, Wang, Wei W wrote:

On Wednesday, February 5, 2020 5:23 PM, David Hildenbrand wrote:

quoted

So, if you run a TCG guest and use it with free page reporting, the race is
possible? So the correctness depends on two dirty bitmaps in the hypervisor
and how they interact. wow this is fragile.

Not sure how TCG tracks the dirty bits. But In whatever implementation, the hypervisor should have

There is only a single bitmap for that purpose. (well, the one where KVM
syncs to)

already dealt with the race between he current round and the previous round dirty recording.
(the race isn't brought by this feature essentially)

It is guaranteed to work reliably without this feature as you only clear
what *has been migrated*, not what your guest thinks should not been
migrated at one point and decides differently at another point. The race
is bought forwards by this feature.


-- 
Thanks,

David / dhildenb