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

To: "Tim Deegan" <Tim.Deegan@xxxxxxxxxx>, "Keir Fraser" <Keir.Fraser@xxxxxxxxxxxx>
Subject: RE: [Xen-devel] write page table in user mode
From: "Tian, Kevin" <kevin.tian@xxxxxxxxx>
Date: Mon, 4 Feb 2008 22:13:13 +0800
Cc: "Su, Disheng" <disheng.su@xxxxxxxxx>, xen-devel@xxxxxxxxxxxxxxxxxxx
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Thread-topic: [Xen-devel] write page table in user mode
>From: Tim Deegan [mailto:Tim.Deegan@xxxxxxxxxx] 
>Sent: 2008年2月4日 19:38
>At 11:06 +0000 on 04 Feb (1202123216), Keir Fraser wrote:
>> Well, this isn't just a matter of degree of aggressiveness. 
>We currently
>> don't allow updates from user space at all, ever. The motivation for
>> changing that is not to do with aggressiveness of 
>unshadowing, but with
>> increased compatibility.
>If the problem is really one of unshadowing and reshadowing, then I'm
>not sure how to fix this without significantly blunting the teeth of
>this heuristic -- even replacing this with an attempted "fast" unshadow
>could fail to solve the problem for VMware.

IMO, to avoid shadow-unshadow-reshadow loop, we can simply add 
a check whether L1e for faulting IP falls into same page it tries to 
update. This shouldn't interfere this heuristic much, since it only resides
in emulation logic and above check is easy to make.

>> OTOH it's arguable whether we care about running
>> VMware Workstation inside an HVM guest. :-)
>We could always fix the VPC performance instead and run VMware inside
>VPC inside Xen.

Wow, double nest virtualization. :-) One point I can come up is to keep
user experience if he is used to use VMware or VirtualPC. Whatever the
underlying 'box' is physical or not, that guy may just want to create multiple
nested VMs for his own usage, like test infrastrucure, etc. It would be
annoying for him to ask lower layer administrator for help, if this VM is
loaned. I bet usage on nested virtualization is unlikely to be performance 
centric though...


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