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

["James Harper" <james.harper@xxxxxxxxxxxxxxxx>]

> 
> 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
should
> > only be invoked if the cache is exhausted when trying to fix up a
PoD
> > entry.
> 
> But yes, there will be a performance impact because all accesses to
the
> 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
Dom0.

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...

James


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