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

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Date:	Tue, 06 May 2008 20:49:47 -0700
From:	Zachary Amsden <zach@...are.com>
To:	Rik van Riel <riel@...hat.com>
Cc:	schwidefsky@...ibm.com, Hugh Dickins <hugh@...itas.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, jeremy@...p.org, andrea@...ranet.com,
	clameter@....com, a.p.zijlstra@...llo.nl
Subject: Re: [patch 0/6] Guest page hinting version 6.


On Tue, 2008-05-06 at 15:46 -0400, Rik van Riel wrote:
> On Tue, 06 May 2008 17:33:02 +0200
> Martin Schwidefsky <schwidefsky@...ibm.com> wrote:
> > On Thu, 2008-03-13 at 16:57 +0000, Hugh Dickins wrote:
> > > It's very encouraging to see Jeremy and Rusty weighing in.  I hope
> > > Zach will too, and I've added Andrea: their support would count a lot.
> > > You have Nick on the list, good, I've added Christoph and Peter
> > > (if you do resend, linux-mm might prove more useful than linux-kernel).
> > > 
> > > With support from rival virtualizers,
> > > I do think you've a good chance of getting in.
> > 
> > Traffic on the guest page hinting patches died down again. Until another
> > user shows up I guess that's it for the full version. 
> 
> I suspect one of the problems is that there are too many state transitions
> to have it implemented with a low overhead on anything but S390, and even
> there you need milicoded instructions to handle things.
> 
> If the number of transitions can be reduced, page hinting could be useful
> for KVM, too.

Spot on Rik, if every transition becomes a hypercall (and a synchronous
one at that), it isn't workable for us.  If, on the other hand, you
share the state bits between the guest and hypervisor, you need a giant
(standalone) bit array for per-page state, which is neither convenient
for Linux nor the hypervisor.  I believe s390 has an 'instruction' to
migrate the state bits into the hypervisor per-physical-page data
without requiring a hypercall.

Zach

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