Re: [Xen-devel] write page table in user mode

To:	"Tim Deegan" <Tim.Deegan@xxxxxxxxxx>
Subject:	Re: [Xen-devel] write page table in user mode
From:	"Michael A Fetterman" <Michael.Fetterman@xxxxxxxxxxxx>
Date:	Mon, 4 Feb 2008 12:02:09 +0000
Cc:	xen-devel@xxxxxxxxxxxxxxxxxxx, "Su, Disheng" <disheng.su@xxxxxxxxx>, "Tian, Kevin" <kevin.tian@xxxxxxxxx>
Delivery-date:	Mon, 04 Feb 2008 04:02:36 -0800
Dkim-signature:	v=1; a=rsa-sha256; c=relaxed/relaxed; d=gmail.com; s=gamma; h=domainkey-signature:received:received:message-id:date:from:sender:to:subject:cc:in-reply-to:mime-version:content-type:content-transfer-encoding:content-disposition:references:x-google-sender-auth; bh=8d1N+HUGBKTRZ3ZeYByRvLo4zxBMbXqDs7NMFNfMBPA=; b=sLZgqISfSuYlbsK8jQl4yC7KfW28vVJmAZL4oy38aojQVD7tDYYsYzaWJ38LtscoWDao+A3zyMOcraJETCkkAa9pZNt6aMESfIyAD+vFODpQNveGdTJg5n/Us4bhB7E+3V9tkJw/gX3GJ8JBPqdD7wnjozbD2UKSjijgtKLgjWs=
Domainkey-signature:	a=rsa-sha1; c=nofws; d=gmail.com; s=gamma; h=message-id:date:from:sender:to:subject:cc:in-reply-to:mime-version:content-type:content-transfer-encoding:content-disposition:references:x-google-sender-auth; b=paEf4RSDX1IuFZd2r0CrWqtLTDh2PIRedKhFdjj+TgIkrZ9w/6Et1x7jHUQl7QudXGJRJ9Pbl4puc7NaCvtFMV3bqAouDk1iaD8bGBT+h2TGlMERubGAoqo42PCAZf1CoeBMQYTYUwpPnvBPEw8wrGSMZUc891L72/Y5I2FlxFY=
Envelope-to:	www-data@xxxxxxxxxxxxxxxxxx
In-reply-to:	<20080204110034.GA15990@xxxxxxxxxxxxxxxxxxxxx>
List-help:	<mailto:xen-devel-request@lists.xensource.com?subject=help>
List-id:	Xen developer discussion <xen-devel.lists.xensource.com>
List-post:	<mailto:xen-devel@lists.xensource.com>
List-subscribe:	<http://lists.xensource.com/cgi-bin/mailman/listinfo/xen-devel>, <mailto:xen-devel-request@lists.xensource.com?subject=subscribe>
List-unsubscribe:	<http://lists.xensource.com/cgi-bin/mailman/listinfo/xen-devel>, <mailto:xen-devel-request@lists.xensource.com?subject=unsubscribe>
References:	<D470B4E54465E3469E2ABBC5AFAC390F024D8F35@xxxxxxxxxxxxxxxxxxxxxxxxxxxx> <C3CA4388.1332B%Keir.Fraser@xxxxxxxxxxxx> <20080204110034.GA15990@xxxxxxxxxxxxxxxxxxxxx>
Sender:	xen-devel-bounces@xxxxxxxxxxxxxxxxxxx

> > > It's said to be a forward progress issue, that instruction page of 
> > > faulting
> > > IP falls into mapped virtual range by same L1 as the target frame it tries
> > > to update. So the implication is that the unshadow unfortunately
> > > succeeds.
>
> Do you mean that the guest instruction stream is changing the VM mapping
> of the page under %eip, from user-space?  Wow.

When I first read Kevin's description, I took it to mean that the same L1 *page*
(not entry) was mapping the current EIP and the target of the write.  If this is
case, then the current code's prefetching of L1 entries on the same page is
probably interacting with the user-space write heuristic, causing the
shadow->unshadow->shadow livelock, making for a much easier fix.

_______________________________________________
Xen-devel mailing list
Xen-devel@xxxxxxxxxxxxxxxxxxx
http://lists.xensource.com/xen-devel

WARNING - OLD ARCHIVES

xen-devel

Re: [Xen-devel] write page table in user mode