Doubts regarding Shadow Page Table Management in KVM

[Sukanto Ghosh <sukanto@cse.iitb.ac.in>]

Hi all,

I was reading the "The shadowy depths of the KVM MMU" presentation by 
Avi Kivity.

 From my understanding of shadow page-table,
Guest Page Table :     maps GV -> GP 
Shadow Page Table :  maps GV -> HP

What is meant by shadowed guest page ? (Does it refer to : the guest PT 
pages that have a corresponding shadow PT page)


I has been mentioned that: "The same page may already be mapped by the 
guest in multiple locations. So, we track writable mappings of every 
guest page ".
I can't understand why do we have to keep track of these reverse 
mappings ? And what are these reverse mappings ? (from HP -> GP or HP -> 
GV or GP -> GV)

Let's consider the following scenario:
GVA1 -> GPA1 -> HPA1
GVA2 -> GPA1 -> HPA1

Also, let GPA1 be part of a guest PT page.

i)  The guest modifies a PTE in GPA1 by doing "mov GVA2"
ii) The host traps this as the page HPA1 is write-protected, and does 
the corresponding modification to the guest PT page and its 
corresponding shadow PT page.
(where do we need to use a reverse mapping)


I would be glad if you can point out where I am making the mistake.


--
Thanks & Regards,
Sukanto Ghosh



NB:    GVA = guest virtual address      GPA = guest physical address   
   HPA = host physical address