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Re: [Xen-devel] When to emulate writing page table page and when to do_update_va_mapping()?



Tom Creck wrote:
Hello, everyone:

I know that x86_emulate_memop() is used to emulate writing to page table pages as Xen intentionally write protect page table pages.
         Also, the hypercall Hypervisor_update_va_mapping() -> 
mod_l2/l1_entry() is also used to modify page table pages.
         So I get confused about their difference. My questions are:

(1) When are the above two routines of modifying page table pages invoked respectively?
(2)     Whatâs the difference in functionality among the two?

Writing directly to pagetables is a good idea if you think it's likely 
that the pagetable is unpinned.  That is, when it's still just an 
ordinary RW page while the pagetable is under construction (fork/exec) 
or destruction (exit).  In this case, a simple memory-write is much more 
efficient than doing a hypercall.
On the other hand, if you're likely to be updating an active pinned 
pagetable, a hypercall is more efficient than trapping and emulating a 
write (especially if you can batch multiple updates together with a 
multicall).
   J


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