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

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Date:	Thu, 13 Mar 2008 17:55:38 +0100
From:	Martin Schwidefsky <schwidefsky@...ibm.com>
To:	Jeremy Fitzhardinge <jeremy@...p.org>
Cc:	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,
	Anthony Liguori <anthony@...emonkey.ws>, hugh@...itas.com
Subject: Re: [patch 6/6] Guest page hinting: s390 support.

On Thu, 2008-03-13 at 09:17 -0700, Jeremy Fitzhardinge wrote:
> Jeremy Fitzhardinge wrote:
> > Martin Schwidefsky wrote:
> >> Vz->Vr cannot happen. This would be a bug in the host.
> >>   
> >
> > Does that mean that Vz is effectively identical to Uz? 
> 
> Hm, on further thought:
> 
> If guests writes to Vz pages are disallowed, then the only way out of Vz 
> is if the guest sets it to something else (Uz,Sz).  If so, what's the 
> point of using that state?  Why not make:
>
>     Vr -> Uz      host discard
>     Pr -> Uz      host discard clean
>     Sp -> Uz      set volatile
>     Uz -> Uz      set volatile

Vz is the page discarded state. The difference to Uz is slim, both
states will cause a program check on access. Vz generates a discard
fault, Uz generates an addressing exception which is nice for debugging.
But I don't see a reason why an implementation that uses Uz instead of
Vz shouldn't work.

> But given how you've described V-state pages, I really would expect 
> writes to a Vz to work, or alternatively, all writes to V-state pages to 
> be disallowed.  Are there any real uses for a writable Vr page?

You mean in the section that speaks about the guests states S/U/V/P ?
Always keep in mind that you can access a V/P page only until it gets
discarded. Then the useful content of the page frame is lost and any
read of write to the not Vz page will be answered with a discard fault.

A Vr page is read-only. If a page gets mapped for writing it needs to
get into the Pr state. This is the hint for the host to look at the
dirty bit before it discards a page.
So yes, there is no use for a writable Vr page.

-- 
blue skies,
  Martin.

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

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