linux-kernel - Re: 2.6.33-rc2: Xen/Guest switching to user mode with no user page tables

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Message-ID: <1263554950.16526.27033.camel@zakaz.uk.xensource.com>
Date:	Fri, 15 Jan 2010 11:29:10 +0000
From:	Ian Campbell <ijc@...lion.org.uk>
To:	Christian Kujau <lists@...dbynature.de>
Cc:	Cyrill Gorcunov <gorcunov@...il.com>,
	Brian Gerst <brgerst@...il.com>,
	"H. Peter Anvin" <hpa@...or.com>,
	Jeremy Fitzhardinge <jeremy@...p.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: 2.6.33-rc2: Xen/Guest switching to user mode with no user page
 tables

On Fri, 2010-01-15 at 00:36 -0800, Christian Kujau wrote:
> On Sun, 10 Jan 2010 at 12:59, Ian Campbell wrote:
> > The correct fix is for the Xen backend to declare kernel RPL == 0 for 64
> > bit guests -- the hyervisor already takes care of all the necessary
> > squashing to ring 3 transparently (because making the guest worry about
> > it would break the very common assumption that you can distinguish user
> > from kernel CS by RPL).
> 
> Yes' it's a 64bit guest, I should have mentioned this from the beginning. 

That's OK, I already knew because only 64 bit guests have a separate
user page table.

> With the 2 patches from Ian and Cyrill applied, the DomU is now booting 
> fine again (currently running mainline -git).

Excellent. These patches are both now in -tip. They are in the urgent
branch so I assume they will be heading to mainline before too long.

> Cyrill: with your patch alone (for arch/x86/kernel/process.c), the DomU
> is still not booting, Dom0 "xm dmesg" reporting the same error. As it's
> working with both patches applied, should I try to test with only Ian's
> patch (for arch/x86/xen/enlighten.c) applied?

It's OK, both patches are definitely required to fix 64 bit guests so
there is no point in testing just one or the other.

Ian.

-- 
Ian Campbell
Current Noise: Exodus - Scar Spangled Banner

War is much too serious a matter to be entrusted to the military.
		-- Clemenceau

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