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

To: Tim Deegan <Tim.Deegan@xxxxxxxxxx>
Subject: Re: [Xen-devel] write page table in user mode
From: Keir Fraser <Keir.Fraser@xxxxxxxxxxxx>
Date: Mon, 04 Feb 2008 11:06:56 +0000
Cc: "Tian, Kevin" <kevin.tian@xxxxxxxxx>, "Su, Disheng" <disheng.su@xxxxxxxxx>, xen-devel@xxxxxxxxxxxxxxxxxxx
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On 4/2/08 11:00, "Tim Deegan" <Tim.Deegan@xxxxxxxxxx> wrote:

>> Ugh, it would be much nicer to remove the unshadow-on-user-access heuristic
>> entirely.  We just need to do enough testing to ensure it doesn't harm
>> performance in other cases we care about. I really don't know how much we
>> rely on that heuristic these days.
> Nor do I (since as Kevin points out we don't count this separately) but
> with the increasing completeness of the emulator, we've already lost a
> lot of heuristics for unshadowing pages -- this is what the
> unshadow-after-three-writes heuristic is trying to fix, for example.  It
> seems odd to be making the unshadowing more aggressive with one hand and
> less aggressive with the other.

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. OTOH it's arguable whether we care about running
VMware Workstation inside an HVM guest. :-)

 -- Keir

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