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RE: [Xen-devel] Shadow Page Tables in Xen

To: Gianluca Guida <glguida@xxxxxxxxx>, priya sehgal <priyagps@xxxxxxxxxxx>
Subject: RE: [Xen-devel] Shadow Page Tables in Xen
From: Ian Pratt <Ian.Pratt@xxxxxxxxxxxxx>
Date: Tue, 21 Apr 2009 20:26:31 +0100
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Cc: Tim Deegan <Tim.Deegan@xxxxxxxxxxxxx>, Gianluca Guida <Gianluca.Guida@xxxxxxxxxxxxx>, "xen-devel@xxxxxxxxxxxxxxxxxxx" <xen-devel@xxxxxxxxxxxxxxxxxxx>, Ian Pratt <Ian.Pratt@xxxxxxxxxxxxx>
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Thread-topic: [Xen-devel] Shadow Page Tables in Xen
> Have you actually measured this? I think that the major cause of page
> faults and VM slowdown is -- rather than page faults on write access
> -- the fact that we blow the shadow pagetables away everytime we clean
> the dirty bitmap, and this requires a long operation to remove from
> top to bottom all reference counts and reconstructing later the shadow
> pagetables on the next memory accesses.

Yep, ideally we'd just walking all the shadow leaf PTE's to just remove write 
access. I'd guess that this would be a much more worthwhile optimization than 
trying to 'pre-fetch' write faults.

Ian  

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