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
2 messages in com.xensource.lists.xen-develRe: [Xen-devel] When to emulate writi...