Xen devel - Apr 2006 - Writable page tables

I have a question with regards to writable page table support. I am able
to modify a page table entry via the hypercall interface
(xen_l1_entry_update). However, if I enable the writable page table
feature, the hypervisor fails to fix up the write fault on the same page
table. The fixup fails in ptwr_do_page_fault() - the problem appears to
be with the TYPE_COUNT  (it is 0 and all other checks pass). Why is the 
writable code path more restrictive, given that the modifications done
via the writable path will be validated by the hypervisor before hooking
in the page table under question. Since the validation is deferred, I
was expecting this path would have fewer checks.

Regards,

K. Y


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On 6 Apr 2006, at 22:50, Ky Srinivasan wrote:
> I have a question with regards to writable page table support. I am 
> able
> to modify a page table entry via the hypercall interface
> (xen_l1_entry_update). However, if I enable the writable page table
> feature, the hypervisor fails to fix up the write fault on the same 
> page
> table. The fixup fails in ptwr_do_page_fault() - the problem appears to
> be with the TYPE_COUNT  (it is 0 and all other checks pass). Why is the
> writable code path more restrictive, given that the modifications done
> via the writable path will be validated by the hypervisor before 
> hooking
> in the page table under question. Since the validation is deferred, I
> was expecting this path would have fewer checks.
Have you write protected the page but not yet pinned it as a pagetable 
page, nor hooked it into an existing pagetable?

  -- Keir


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