Xen devel - Mar 2012 - Is shadow 2 mechanism supported in Xen 3.1?

Hi, guys. I want to ask some questions about shadow pagetables mechanism.

Recently I am developing a task manager for Windows XP in HVM DomU. For
convenience, I use some code from ether(http://*ether*.gtisc.*gatech*.edu/)
which based on Xen 3.1. From the xen blog (
http://blog.xen.org/index.php/2008/08/27/xen-33-feature-shadow-3), I find
that there are three different mechanisms for shadow pagetables.



I wonder if shadow 2 is supported in Xen 3.1.



This is important because only shadow 2 emulated every write to guest
pagetables. If so, I can count the exact number of pages belong to a
process and calculate the memory usage of every process.



Besides, I wonder once this counting number drops to zero, can I absolutely
assume that the process exits.



Thanks.


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Hi, 

At 23:35 +0800 on 08 Mar (1331249756), peng yang wrote:
> I wonder if shadow 2 is supported in Xen 3.1.
Yes.
> This is important because only shadow 2 emulated every write to guest
> pagetables. If so, I can count the exact number of pages belong to a
> process and calculate the memory usage of every process.
Sadly, not.  Shadow 2 emulates all writes to _shadowed_ pagetables, but
there''s no guarantee that all pagetables will be shadowed.  Pagetables
that haven''t been used by the guest yet won''t be shadowed yet.
Also,
there''s only a finite amount of shadow memory, so less-recently-used
pagetables get un-shadowed to make room.  (A related problem is that Xen
can''t always tell when a page has stopped being a pagetable, so just
because it''s shadowed doesn''t meen it''s definitely a
pagetable).
> Besides, I wonder once this counting number drops to zero, can I absolutely
> assume that the process exits.
No, for the same reason.

Cheers,

Tim.