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Re: [Xen-devel] Essay on an important Xen decision (long)

To: Anthony Liguori <aliguori@xxxxxxxxxx>
Subject: Re: [Xen-devel] Essay on an important Xen decision (long)
From: Keir Fraser <Keir.Fraser@xxxxxxxxxxxx>
Date: Wed, 11 Jan 2006 16:38:28 +0000
Cc: "Magenheimer, Dan \(HP Labs Fort Collins\)" <dan.magenheimer@xxxxxx>, xen-devel <xen-devel@xxxxxxxxxxxxxxxxxxx>
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On 11 Jan 2006, at 16:25, Anthony Liguori wrote:

On an architecture where VP is cheaper to implement than on x86, it may well make sense to do that in preference to P2M. As you say, it makes certain future extensions less of a pain to implement.

Is VP on x86 expensive in terms of performance or complexity?

I imagine that you would have to always have shadow paging enable but you could still do bulk updates ala writable page tables so the performance cost should be minimal I would think.

Trying to understand the memory system in more details so any additional info is much appreciate :-)

Shadow page tables do have a measurable overhead, although it's not *that* big for most workloads. We already support a shadow-translate mode (well, the xenlinux support for it may be broken right now, but it's worked in the past) for paravirt guests and various people researching new xen features want to make use of that. I can imagine that we will support both modes even in x86 at some point in the future, and users can make the features/performance tradeoff.

 -- Keir

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