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Re: [Xen-devel] More page-table questions.

To: Keir Fraser <keir@xxxxxxxxxxxxx>
Subject: Re: [Xen-devel] More page-table questions.
From: PUCCETTI Armand <armand.puccetti@xxxxxx>
Date: Mon, 12 Mar 2007 18:27:25 +0100
Cc: "Petersson, Mats" <Mats.Petersson@xxxxxxx>, xen-devel@xxxxxxxxxxxxxxxxxxx
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Keir Fraser a écrit :
On 12/3/07 16:26, "Petersson, Mats" <Mats.Petersson@xxxxxxx> wrote:

Are the page-tables ever updated directly by the guest, or is it all
done via hyper-calls?

Leaf PTEs (i.e., really just PTEs, not PDEs) can be directly written from
the point-of-view of the guest. In fact they are trapped and emulated by
Xen. The guest is somewhat aware of this because it has explicitly
write-protected all its pagetables, so if it were to attempt the direct
write on native hardware in these circumstances it would receive a page

 -- Keir

This is unclear to me: "a guest believes he can write PTEs" means that
his source code to access the page tables is left unchanged between legacy and PV version?

Merely, the hypervisor traps the guest's accesses to the page tables, to control what he is doing (e.g. not overlapping any other domain's pages) and allowing or denying any writes. This should apply to any page table level, so why only blocking writes to PTEs?

This is for 4K pages, but how are 2M pages mixed? or do we assume that every domain pages
are 4K?


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