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Re: [Xen-devel] Shadow's Dirty Bit Problem
- To: zhujun <zhujun@xxxxxxxxxxxxxx>
- From: Gianluca Guida <gianluca.guida@xxxxxxxxxxxxx>
- Date: Sun, 28 Jun 2009 14:02:25 +0200
- Cc: xen-devel@xxxxxxxxxxxxxxxxxxx
- Delivery-date: Sun, 28 Jun 2009 05:02:53 -0700
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On Sun, Jun 28, 2009 at 8:04 AM, zhujun<zhujun@xxxxxxxxxxxxxx> wrote:
> In the function _sh_propagate, the A&D bits of all the shadow page
> tables entries above level 1 will be set. (// Set the A&D bits for higher
> level shadows). I am very confused about it. From Intel manual, the Dirty
> bit above level 1 is reserved and must be set 0. Am I right? If yes, how
> does the shadow page table deal with this problem?
Actually, it is an available bit in higher level. So there's no
problem in putting it to 1. Also, note that there are level 2 entries
with dirty bits: superpages ones.
> Another question. In Xen’s current version, does it use the Dirty
> bit to do something?
It's used only for tracking VRAM, to know whether the guest wrote to
the VGA buffer or not. Otherwhise, we set it to one.
It was a type of people I did not know, I found them very strange and
they did not inspire confidence at all. Later I learned that I had been
introduced to electronic engineers.
E. W. Dijkstra
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