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

["Tian, Kevin" <kevin.tian@xxxxxxxxx>]

>From: Xin, Xiaohui
>Sent: 2007年12月10日 10:48
>
>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?

It seems that unshadow has no correctness issue, and only
hurt the performance when unshadowed page is used in page
walk path again next time. At that time, shadow fault path will 
re-walk guest page table to find updated value.

In this context, if windows programs Non-PTE-like values in PTE
like swap usage in Linux, by clearing the present bit, the
performance to inject guest page fault for those special values
are hurt a bit. The difference is on a fast-path N/P check and a
full-path N/P check.

So we need balance the gain and loss here. Tim, how much
difference per your experience for a fast-path N/P? If not much,
maybe it's worthy doing it then. :-)

Thanks,
Kevin

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