linux-kernel - Re: [patch 6/6] Guest page hinting: s390 support.

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Date:	Thu, 13 Mar 2008 10:45:08 +0100
From:	Martin Schwidefsky <schwidefsky@...ibm.com>
To:	Jeremy Fitzhardinge <jeremy@...p.org>
Cc:	Anthony Liguori <anthony@...emonkey.ws>, akpm@...l.org,
	linux-s390@...r.kernel.org, frankeh@...son.ibm.com,
	nickpiggin@...oo.com.au, linux-kernel@...r.kernel.org,
	virtualization@...ts.osdl.org, hugh@...itas.com
Subject: Re: [patch 6/6] Guest page hinting: s390 support.

On Wed, 2008-03-12 at 14:36 -0700, Jeremy Fitzhardinge wrote:
> Anthony Liguori wrote:
> >> Vp should never happen, since you'd never preserve a V page.  And 
> >> surely it would be Pr -> Sr, since the hypervisor wouldn't push the 
> >> page to backing store when you change the client state.
> >>     
> >
> > You're right, I meant Vp/Pp but they are invalid states.  I think one of 
> > the things that keeps tripping me up is that the host can change both 
> > the host and guest page states.  My initial impression was that the host 
> > handled the host state and the guest handled the guest state.
> >   
> 
> Yes.  And it seems to me that you get unfortunate outcomes if you have a 
> Pr->Vz->Vr transition.

Vz->Vr cannot happen. This would be a bug in the host.

> > I was thinking that it may be useful to know a Ur verses a Uz when 
> > allocating memory.  In this case, you'd rather allocate Ur pages verses 
> > Uz to avoid the fault.  I don't read s390 arch code well, is the host 
> > state explicit to the guest?
> >   
> 
> Yes, reusing Ur pages might well be better, but who knows - they've 
> probably got an instruction which makes Uz cheap...

Yes, faulting in a Uz page is cheap on s390. Isn't it a lovely
architecture :-)

> Stuff like this suggets that both parts of the state are packed 
> together, and are guest-visible:
> 
> +	return (state & ESSA_USTATE_MASK) == ESSA_USTATE_VOLATILE &&
> +		(state & ESSA_CSTATE_MASK) == ESSA_CSTATE_ZERO;
> 

Yes, the return value of the ESSA instruction has both the guest state
and the host state.

-- 
blue skies,
  Martin.

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


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