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Re: [Xen-devel] Re: Xen's use of PAT and PV guests

To: "Jeremy Fitzhardinge" <jeremy@xxxxxxxx>, "Konrad Rzeszutek Wilk" <konrad.wilk@xxxxxxxxxx>
Subject: Re: [Xen-devel] Re: Xen's use of PAT and PV guests
From: "Jan Beulich" <JBeulich@xxxxxxxxxx>
Date: Wed, 31 Mar 2010 09:26:19 +0100
Cc: Xen-devel <xen-devel@xxxxxxxxxxxxxxxxxxx>, Keir Fraser <keir.fraser@xxxxxxxxxxxxx>
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>>> Jeremy Fitzhardinge <jeremy@xxxxxxxx> 30.03.10 20:43 >>>
>On 03/30/2010 09:57 AM, Konrad Rzeszutek Wilk wrote:
>> How does this work on pages shared across domains? Say Guest A makes the
>> page WC,Dom0 makes it WB and Xen puts it in WC, and Dom0 reads does a
>> Write/Read/Write, but in actuallity it is a Read/Write/Write. Or is
>> there no danger there since the grant table pages have UC set on them?
>Not sure.  That would invoke undefined behaviour, I'd assume.  Does Xen 
>keep track of memory type aliases?  Grant pages don't have to be UC do 
>they?  Pages between front and backends don't need to be (and shouldn't 
>be) UC.

Granted pages are RAM pages, and hence should always be WB

As to Xen's memory type handling - iirc the most recent memory type
used in a page table entry determines what Xen uses in its 1:1 mapping,
but I don't think global consistency is being enforced.


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