Re: Re: Xen's use of PAT and PV guests

From: "Jan Beulich" <JBeulich@novell.com>
To: Jeremy Fitzhardinge <jeremy@goop.org>,
	Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Cc: Xen-devel <xen-devel@lists.xensource.com>,
	Keir Fraser <keir.fraser@eu.citrix.com>
Subject: Re: Re: Xen's use of PAT and PV guests
Date: Wed, 31 Mar 2010 09:26:19 +0100	[thread overview]
Message-ID: <4BB3234B0200007800038153@vpn.id2.novell.com> (raw)
In-Reply-To: <4BB2463A.9090401@goop.org>

>>> Jeremy Fitzhardinge <jeremy@goop.org> 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
everywhere.

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.

Jan