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RE: [Xen-devel] map memory holes with same page

To: "Tim Deegan" <Tim.Deegan@xxxxxxxxxx>, "Paul Durrant" <Paul.Durrant@xxxxxxxxxx>
Subject: RE: [Xen-devel] map memory holes with same page
From: "James Harper" <james.harper@xxxxxxxxxxxxxxxx>
Date: Mon, 23 May 2011 20:29:05 +1000
Cc: Keir Fraser <keir.xen@xxxxxxxxx>, xen-devel@xxxxxxxxxxxxxxxxxxx
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Thread-topic: [Xen-devel] map memory holes with same page
> At 09:42 +0100 on 23 May (1306143771), Paul Durrant wrote:
> > > -----Original Message-----
> > [snip]
> > >
> > > Should there be a performance impact if Windows tries to touch a
> > > page that I have previously given pack with decrease_reservation?
> > > Will that invoke the PoD sweep?
> > >
> >
> > No. The p2m entry will be 'invalid', not 'PoD'. IIRC the sweep
> > only be invoked if the cache is exhausted when trying to fix up a
> > entry.
> But yes, there will be a performance impact because all accesses to
> missing page will be emulated by sending an ioreq to qemu, so it will
> run _very_ slowly.

Thanks. That matches what I'm seeing. For hibernate, it appears that
Windows compresses pages into a buffer and the buffer is what I pass to

I guess the only way to solve this is to stop every access hitting an
emulation. Either I can map a common read-only page into every hole
(which doesn't sound like a workable solution based on feedback so far),
or Xen could keep a common read-only page and map it into a hole every
time it is accessed (and then move the page when another hole is
accessed), which would reduce the problem to emulation on the first time
a different hole is accessed...


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