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

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Date:	Thu, 13 Mar 2008 09:17:47 -0700
From:	Jeremy Fitzhardinge <jeremy@...p.org>
To:	schwidefsky@...ibm.com
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.

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

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?

On the other hand, removing Vz->Vr does clean up the dot graph a lot...

    J

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