Xen devel - Apr 2009 - [Shadow Paging] Clarification Guest Paging Table Write Protection

Hi,
    I am new to Xen and trying to understand how Shadow Paging works. I require
clarification on following point:

How does Xen write protects guest page table? 
What I could understand reading code & other sources is that, when ever cr3
gets updated, control gets transferred to the hypervisor which then does the
emulation for the instruction and write protects the guest page table. But the
confusion is, cr3 contains Pseuso-physical address of the guest page table, but
to protect this page protection bit is required to be set in its page table
entry. How does Xen get page table entry for the address in cr3?

Thanks in advance,
Dang

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