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

["Xin, Xiaohui" <xiaohui.xin@xxxxxxxxx>]

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.

-- 
Tim Deegan <Tim.Deegan@xxxxxxxxxx>
Principal Software Engineer, Citrix Systems.
[Company #5334508: XenSource UK Ltd, reg'd c/o EC2Y 5EB, UK.]

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