[vbox-dev] Nested virtualization problem

[Ruben Van den Bossche]

  Dear VBox developers,

We are currently extending work performed by Olivier Berghmans regarding 
nested virtualization (running a virtual machine inside another virtual 
machine). Because none of us has experience with hypervisor development, 
we would very much appreciate it if you could confirm or correct one of 
our assumptions.

In our experiments, nesting DBT based hypervisors appears impossible 
when using a base hypervisor based on DBT. Using first generation 
hardware support as the base hypervisor results in partial success 
depending on the hypervisor used ( VMWare, Xen, KVM, Virtualbox ). When 
we move the base hypervisor to a machine with second generation hardware 
support most problems disappear.

We think the most likely reason for the problems with DBT or first gen. 
hardware support as base hypervisor technique are the results of shadow 
paging, and more specifically the nested shadow paging which occurs when 
nesting DBT on DBT or first gen HWS. Perhaps some functionality is not 
replicated exactly as one would expect on physical hardware?  Maybe some 
shortcuts result in corrupt memory if they are used in nested 
hypervisors? If you could confirm these assumptions or perhaps even give 
some greater insight in these problems, we would be extremely grateful.

Kind Regards,

Sam Verboven and Ruben Van den Bossche
PhD Students
University of Antwerp

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