Re: [Xen-devel][PATCH] unshadow the page table page which are used as data page

[Keir Fraser <Keir.Fraser@xxxxxxxxxxxx>]

It's a very common thing to do with unused ptes. You can't really infer
anything from writes where _PAGE_PRESENT is clear.

 -- Keir

On 10/12/07 02:48, "Xin, Xiaohui" <xiaohui.xin@xxxxxxxxx> wrote:

> Hi, Tim
> Heard from Kevin that the Linux kernel writes swap cache entries in swap cache
> pages. And the swap cache entries contains only type and offset which seems
> not contains valid mfn at all. Does the patch will hurt this? Is there any
> other situations that guest write NON-PTE entries in the page tables?
> 
> Thanks
> Xiaohui
> 
> -----Original Message-----
> From: Tim Deegan [mailto:Tim.Deegan@xxxxxxxxxx]
> Sent: 2007年12月7日 22:40
> To: Xin, Xiaohui
> Cc: xen-devel@xxxxxxxxxxxxxxxxxxx; Kay, Allen M
> Subject: Re: [Xen-devel][PATCH] unshadow the page table page which are used as
> data page
> 
> Hi, 
> 
> At 21:12 +0800 on 07 Dec (1197061925), Xin, Xiaohui wrote:
>> Tim,
>>  Attached is the updated patch which based on some part of your
>> suggestion and some part of our new thoughts about it. We have
>> re-checked the code path of the guest write emulate, found that in
>> some extent(not all) the code checks the valid mfn for the guest
>> written data. But maybe for the optimization, the code just check
>> valide mfn when PRESENT bit exists. Maybe it can cover most of the
>> cases, but not all, that's what we have found in the vt-d iperf test.
> 
> Hmmm.  The new behaviour is slightly nonintuitive, as it lets the guest
> write non-present entries without unshadowing only if the bits that
> would have been the GFN are in fact a valid GFN (which happens to
> include zero).  I think it's OK, but needs a comment.
> 
> Please don't change validate_gl1e or include level-1 shadow types in
> check_for_data_page_unshadow().  Windows, in particular, keeps all sorts
> of non-PTE-like values in PTE slots, and we can't treat those as a
> reason to unshadow.
> 
>>  To minimize the hurt to other performance of shadow, the patch tries
>> to use the valid mfn check in the original code, please have a
>> review. I'm not sure about the cost of the gfn_to_mfn(), and not sure
>> whether we may get some trade-off. If you have good ideas, please let
>> us know.
> 
> gfn_to_mfn() is very cheap when shadow mode is being used.
> 
> Cheers,
> 
> Tim.



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