[Xen-devel] Two shadow page tables for HVM

[Emre Can Sezer <ecsezer@xxxxxxxx>]

Hi,

As part of my research I am going to play around with page permissions.  
However, I might have to do this pretty often, so I'm thinking about 
having two page tables that are synchronized and only differ in their 
permissions.  The idea is to have a set of permissions when code block A 
is being executed and another set when block B is being executed.  I 
plan to capture execution jumps by specifying the inactive block as 
non-executable.
I am running a HVM guest on a x86_64 machine.  I'm only interested in 
kernel pages, in that I don't have to have a second page table for user 
level pages as their permissions will be the same.
So far I can think of only two ways of doing this.  First, I can have 
two top level shadow page tables and use one of the unused slots in 
struct arch_domain to store this page.  Then I modify 
propagate_l*e_from_guest functions to ensure that they create and 
synchronize the second page table.  Second, I can have pages that are 
twice as large as original page tables.  I'm not sure what the 
implications are concerning shadow cache and the linear page table 
mappings. 

Which one of these methods would be easier to implement?  Is there an 
easier way of having two sets of page tables?  If I had the means, would 
it be worth switching to AMD for the NPT?
Thanks in advance,

John


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