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

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Date:	Wed, 12 Mar 2008 14:36:43 -0700
From:	Jeremy Fitzhardinge <jeremy@...p.org>
To:	Anthony Liguori <anthony@...emonkey.ws>
CC:	Jeremy Fitzhardinge <jeremy@...p.org>, 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, schwidefsky@...ibm.com,
	hugh@...itas.com
Subject: Re: [patch 6/6] Guest page hinting: s390 support.

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.

>>> Do the host states even really need visibility to the guest at all?  
>>> It may be useful for the guest to be able to distinguish between Ur 
>>> and Uz but it doesn't seem necessary.
>>>       
>> Well, you implicitly see the hypervisor state.  If you touch a [UV]z 
>> page then you get a fault telling you that the page has been taken 
>> away from you (I think).  And it would definitely help with debugging 
>> (seems likely there's lots of scope for race conditions if you 
>> prematurely tell the hypervisor you don't need the page any more...).
>>     
>
> 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...

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;


      J
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