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

[Tim Deegan <Tim.Deegan@xxxxxxxxxx>]

At 12:03 +0100 on 23 May (1306152214), Olaf Hering wrote:
> On Mon, May 23, Tim Deegan wrote:
> 
> > 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. 
> 
> Isnt that what I just "fixed" for kdump, the new get_mem_type hvmop?
> Could that be reused for this Windows issue?

Unfortunately, hibernate tries to compress the pages before handing them
over to the driver, so there isn't a point where the driver can avoid
the access.  If there were such a point, the balloon driver could work
around it from its own records, without even needing the hypercall.

Cheers,

Tim.

-- 
Tim Deegan <Tim.Deegan@xxxxxxxxxx>
Principal Software Engineer, Xen Platform Team
Citrix Systems UK Ltd.  (Company #02937203, SL9 0BG)

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