Re: where is writable page table implemented?

[Mark Williamson <mark.williamson@cl.cam.ac.uk>]

> 2) in ptwr_do_page_fault(), it should disconnect the page from page
> directory and then mark the page as writable.  Where are they done? In
> x86_emulate()?

The disconnection was originally implemented so that guests could make further 
writes to that page without incurring any more traps into Xen.  The goal was 
to improve the performance of batch pagetable changes (e.g. when a fork() is 
being performed).

This approach was abandoned in favour of trapping every direct pagetable 
write, and having guests explicitly hypercall into Xen when they really do 
want to modify a batch of pages.  This reduced the overheads in the usual 
case when the guest is only modifying a small number of pagetable entries.

The new approach gives an overall performance benefit but is ABI compatible 
with guests from before this change (they just won't get the batching).

Cheers,
Mark

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