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[Xen-devel] How does Xen implement writable page table ?

Hi everyone,
If I follow the source code correctly,  fixup_page_fault() -> ptwr_do_page_fault handles the write attempts to a page table. In my understanding, guests update page table through two ways:
1) Issue hypercall update_va_mapping to update PDT; 
2) Modify writable bottom-level ptes and then trap to Xen's ptwr_do_page_fualt 
Here is my confusion: Why Xen supports two page table updating mechanisms?  Why cannot apply writable page table to PDT entries?
Thanks a lot !!

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