RE: [Xen-devel] write page table in user mode

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

Just now I made a conclusion that no such unshadow heuristic is
hit after checking my archived perfc counter logs, since there's no 
item like "unshadow by user-level access". But immediately I
found this is mixed into "emulator fails", and there does be dozens
of count for Linux KB and several for one Windows benchmark (not
sure the detail now).

Whatever the count may be, even one user page slipped out of
unshadow may hurt performance a lot if access to that page is
frequent...

Maybe we can disable such heuristics only when observing guest 
clear CR0.wp actively? Or later out-of-sync can also help here.

BTW, it's worthy of adding a seperate perfc counter for user-level 
access caused unshadow. :-)

Thanks,
kevin

>From: Keir Fraser
>Sent: 2008年2月2日 18:31
>
>It should be safe, but I don't know how much it would affect 
>performance (it
>depends on how much we are actually relying on it as an unshadowing
>heuristic).
>
> -- Keir
>
>On 1/2/08 18:28, "Su, Disheng" <disheng.su@xxxxxxxxx> wrote:
>
>> Hi,
>> I found vmware workstation 5 in xen will write page table in
>> user mode. I don't know the rational behind this...
>> XEN doesn't emulate writing page table in user mode. If I get
>> rid of this constraint, vmware will go a little bit further.
>> Is it safe to remove this constraint?
>> 
>> Best Regards,
>> Disheng, Su
>> 
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>
>
>
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