Re: When to emulate writing page table page and when to do_update_va_mapping()?

[Jeremy Fitzhardinge <jeremy@goop.org>]

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)     What’s 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