linux-kernel - Re: Interaction between Xen and XFS: stray RW mappings

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Date:	Mon, 22 Oct 2007 11:37:49 -0700
From:	Jeremy Fitzhardinge <jeremy@...p.org>
To:	Nick Piggin <nickpiggin@...oo.com.au>
CC:	dean gaudet <dean@...tic.org>, David Chinner <dgc@....com>,
	xfs@....sgi.com, Xen-devel <xen-devel@...ts.xensource.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Mark Williamson <mark.williamson@...cam.ac.uk>,
	Morten Bøgeskov 
	<xen-users@...ten.bogeskov.dk>, xfs-masters@....sgi.com
Subject: Re: Interaction between Xen and XFS: stray RW mappings

Nick Piggin wrote:
> You could call it a bug I think. I don't know much about Xen though,
> whether or not it expects to be able to run an arbitrary OS kernel.
>   

Xen's paravirtualized mode always requires a guest OS to be modified;
certainly some operating systems would be very hard to make work with
Xen.  But you can always fall back to using shadow pagetables or full
hvm (VT/SVM) mode.

> Presumably, the hypervisor _could_ write protect and trap writes to
> *all* page table page mappings.
>   

Xen manages this stuff with refcounts; it doesn't maintain an rmap for
these pages, so it would have to exhaustively search to do this.  But
aside from that, Xen never actively modifies pagetables, so this would
be a new behaviour in the interface.

    J
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