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[Xen-devel] Consult some concepts about shadow paging mechanism

To: xen-devel@xxxxxxxxxxxxxxxxxxx
Subject: [Xen-devel] Consult some concepts about shadow paging mechanism
From: Jui-Hao Chiang <windtracekimo@xxxxxxxxx>
Date: Wed, 22 Apr 2009 09:14:02 -0400
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Dear All:

I am pretty new to xen-devel, please correct me in the following.

Assume we have the following terms
GPT: guest page table
SPT: shadow page table

(Question a) When guest OS is running, is it always using SPT for
address translation? If it is the case, how does guest OS refer and
modify its own GPT content? It seems that there is a page table entry
in SPT for the GPT page.

(Question b) The hypervisor is performing synchronization between GPT
and SPT. When guest OS increase access to some page (call it
Normal_Page) by marking 'read only' GPT entries as 'read write',
what's the read-write mode of the GPT page in the beginning?
(1) If it's read-only in SPT, the this modification will trigger a
page fault for GPT page, so that hypervisor can synchronize those two
tables at this moment.
(2) If it's read-write in SPT, then the page fault will only occur
when guest first write the Normal_Page content since the SPT entry has
read-only for the Normal_Page. Then hypervisor change the SPT entry to
read-write and to make sure it's the same as GPT, and return to guest
OS to run.

Thanks for your patience to look at these questions.

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