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

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Date:	Wed, 07 May 2008 09:00:14 +0200
From:	Martin Schwidefsky <schwidefsky@...ibm.com>
To:	Zachary Amsden <zach@...are.com>
Cc:	Rik van Riel <riel@...hat.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 20:49 -0700, Zachary Amsden wrote:
> > 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.

That is why we invented the millicoded ESSA instruction on s390. We had
an emulation of the instruction to test things. It worked but was
awfully slow.

-- 
blue skies,
  Martin.

"Reality continues to ruin my life." - Calvin.


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