linux-kernel - Re: [patch 0/6] Guest page hinting version 6.

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Message-Id: <1205530358.14987.32.camel@bodhitayantram.eng.vmware.com>
Date:	Fri, 14 Mar 2008 14:32:38 -0700
From:	Zachary Amsden <zach@...are.com>
To:	Jeremy Fitzhardinge <jeremy@...p.org>
Cc:	Hugh Dickins <hugh@...itas.com>,
	Martin Schwidefsky <schwidefsky@...ibm.com>,
	linux-kernel@...r.kernel.org, linux-s390@...r.kernel.org,
	virtualization@...ts.osdl.org, akpm@...ux-foundation.org,
	nickpiggin@...oo.com.au, frankeh@...son.ibm.com,
	rusty@...tcorp.com.au, andrea@...ranet.com, clameter@....com,
	a.p.zijlstra@...llo.nl, Keir Fraser <keir@...source.com>,
	Ian Pratt <ian.pratt@...source.com>
Subject: Re: [patch 0/6] Guest page hinting version 6.

On Fri, 2008-03-14 at 11:30 -0700, Jeremy Fitzhardinge wrote:
> Zachary Amsden wrote:
> > How about a fake hypervisor, which is really just a random page evictor,
> > following the rules of CMM?
> >   
> 
> Probably simpler to just have variants of the page_set_* functions which 
> simulate the worst-possible host action immediately (ie, stealing pages, 
> logically swapping them, etc).  That wouldn't give you full coverage, 
> but it would go some way.  An async variant which schedules a change in 
> a few milliseconds would help too.
> 
> I guess that's equivalent to having a special-purpose hypervisor built 
> into the kernel (hm, sounds familiar...).

It needn't be that hard on s390, I believe you don't need to worry about
PTEs becoming asynchronous when stealing a page, since if I understand
the hypervisor architecture, there is a per-page mapping level
available, allowing you to generate discard faults on access.  It might
be possible to use this mapping layer without implementing a full blown
hypervisor.  Martin?

For x86, at discard time, you would have to manually walk and invalidate
any PTEs potentially mapping the discarded page, but there is already
this great thing called Xen paravirt-ops which actually does that for
completely different reasons (PT page protection).

I think a random exponential distribution for discard would be needed to
catch all the racey failure modes.

Zach

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