linux-kernel - Re: 2.6.25-rc1 xen pvops regression

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Message-Id: <1203497511.26910.39.camel@cthulhu.hellion.org.uk>
Date:	Wed, 20 Feb 2008 08:51:50 +0000
From:	Ian Campbell <ijc@...lion.org.uk>
To:	"H. Peter Anvin" <hpa@...or.com>
Cc:	Joel Becker <Joel.Becker@...cle.com>,
	Jeremy Fitzhardinge <jeremy@...p.org>,
	Jody Belka <lists-lkml@...b.org>, linux-kernel@...r.kernel.org,
	Ingo Molnar <mingo@...e.hu>,
	Thomas Gleixner <tglx@...utronix.de>,
	"Eric W. Biederman" <ebiederm@...ssion.com>,
	Andi Kleen <andi@...stfloor.org>,
	Mika Penttila <mika.penttila@...umbus.fi>
Subject: Re: 2.6.25-rc1 xen pvops regression


On Tue, 2008-02-19 at 23:43 -0800, H. Peter Anvin wrote:
> Ian Campbell wrote:
> > On Mon, 2008-02-18 at 02:40 -0800, Joel Becker wrote:
> >> On Sun, Feb 17, 2008 at 06:49:21PM +0000, Ian Campbell wrote:
> > 
> >>> x86/xen: Do not scan for DMI unless the DMI region is reserved by e820.
> > 
> >> 	This fixed it.  I'm now booting successfully.  Thank you!
> > 
> > Excellent. Jeremy, are you happy for this to go in?
> > 
> 
> NAK!
> 
> It's pretty standard for 0xf0000...0x100000 to be marked RESERVED in 
> E820 on real hardware (including the system I'm typing on right now.) 
> It is so marked to indicate that hardware cannot be mapped into that 
> space.  However, you can't rely on this fact -- heck, you can't rely on 
> E820 even existing on a real machine.  I have specimens of real-life 
> machines that go both ways.
> 
> This patch WILL break real hardware.
> 
> What's particularly damning is that it's titled "x86/xen: Do not scan 
> for DMI unless the DMI region is reserved by e820." whereas in fact it 
> changes (breaks) generic code.

Sorry, I was trying to indicate that it was a generic change which was
motivated by Xen support, but you're right it did look like I was saying
it was a Xen only change.

As far as the actual change goes I was assuming that any machine that
has DMI/SMBIOS would easily be new enough to have an E820 which could be
expected to reserve this region. Looks like I was mistaken about how
long E820 had been around and/or how reliably it is used to reserve the
tables.

Anyway, will have to think of another solution.

Ian.

-- 
Ian Campbell

Just because the message may never be received does not mean it is
not worth sending.

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