Re: How does Xen implement writable page table ?

Xinxin Jin
2012-10-04T18:08:14Z

Thanks, Jan!

On Thu, Oct 4, 2012 at 12:49 AM, Jan Beulich <JBeulich@suse.com> wrote:

> >>> On 03.10.12 at 22:28, Xinxin Jin <xinxinjin89@gmail.com>
wrote:
> > 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?
>
> This allows balancing between the amount of changes in the guest
> side code - basic functionality would only require higher level page
> table updates to a para-virtualized, while performance considerations
> would call for all of the updates to avoid emulation.
>
> (You also didn''t mention the third method through the
> mmu_update hypercall.)
>
> >  Why cannot apply writable page table to PDT entries?
>
> It would at least be troublesome in the hypervisor.
>
> Jan
>
>


--
Xinxin


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