Xen devel - Feb 2006 - [PATCH] x86-64 linux: another adjustment to vmalloc fault handling

Changeset 8847 improved things, but doesn''t generally work. We namely
experienced problems (when we originally tried the
same change) with accesses to machine_to_phys_mapping, which (in mfn_to_pfn) is
specifically allowed to fault. Hence
there should not be attempts to fix up faults within the hypervisor range (as
for any page table entries obtained there
no m2p translation exists for the guest, resulting in bogus physical addresses
getting used for further fault
processing, frequently leading to nested faults).

Jan


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On 17 Feb 2006, at 10:38, Jan Beulich wrote:
> Changeset 8847 improved things, but doesn''t generally work. We
namely
> experienced problems (when we originally tried the
> same change) with accesses to machine_to_phys_mapping, which (in 
> mfn_to_pfn) is specifically allowed to fault. Hence
> there should not be attempts to fix up faults within the hypervisor 
> range (as for any page table entries obtained there
> no m2p translation exists for the guest, resulting in bogus physical 
> addresses getting used for further fault
> processing, frequently leading to nested faults).
But machine_to_phys_mapping == 0xffff800000000000 while
PAGE_OFFSET == HYPERVISOR_VIRT_END == 0xffff880000000000.

So the patch doesn''t really change anything, and 
machine_to_phys_mapping lives below both PAGE_OFFSET and 
HYPERVISOR_VIRT_END anyway.

  -- Keir


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>>> Keir Fraser <Keir.Fraser@cl.cam.ac.uk> 17.02.06 12:05:13
>>>
>
>On 17 Feb 2006, at 10:38, Jan Beulich wrote:
>
>> Changeset 8847 improved things, but doesn''t generally work. We
namely
>> experienced problems (when we originally tried the
>> same change) with accesses to machine_to_phys_mapping, which (in 
>> mfn_to_pfn) is specifically allowed to fault. Hence
>> there should not be attempts to fix up faults within the hypervisor 
>> range (as for any page table entries obtained there
>> no m2p translation exists for the guest, resulting in bogus physical 
>> addresses getting used for further fault
>> processing, frequently leading to nested faults).
>
>But machine_to_phys_mapping == 0xffff800000000000 while
>PAGE_OFFSET == HYPERVISOR_VIRT_END == 0xffff880000000000.
>
>So the patch doesn''t really change anything, and 
>machine_to_phys_mapping lives below both PAGE_OFFSET and 
>HYPERVISOR_VIRT_END anyway.
Oh, I''m sorry, I mixed up TASK_SIZE64 and PAGE_OFFSET (we had used the
former initially, because it''s being done
similarly on i386). Between PAGE_OFFSET and HYPERVISOR_VIRT_END I just found
using the latter here more correct, as
PAGE_OFFSET is not required to live at HYPERVISOR_VIRT_END, but that should
always be the boundary for checking.

Jan

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