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

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Date:	Tue, 26 Feb 2008 17:06:32 +0000
From:	Mark McLoughlin <markmc@...hat.com>
To:	Ian Campbell <ijc@...lion.org.uk>
Cc:	"H. Peter Anvin" <hpa@...or.com>,
	Jeremy Fitzhardinge <jeremy@...p.org>,
	Joel Becker <Joel.Becker@...cle.com>,
	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 Fri, 2008-02-22 at 07:25 +0000, Ian Campbell wrote:
> On Thu, 2008-02-21 at 14:58 -0800, H. Peter Anvin wrote:
> > 
> > Which it is on real hardware, because although it's not *reserved*
> > (type 2), it is certainly not made available as *normal memory* (type
> > 1).  If Xen maps this as type 1 then I definitely see the problem.
> > 
> > We can exclude type 1 memory from DMI scan, certainly.
> 
> I'd been meaning to ask this. So the machines you have which don't
> describe 0xf0000 as reserved also don't describe it as RAM? (I guess
> it's either a hole in the table or one of the other e820 types).
> Ian.

...

> >From 13bdb4ee9d80b83a81c3dbefa52464e511d1b4df Mon Sep 17 00:00:00 2001
> From: Ian Campbell <ijc@...lion.org.uk>
> Date: Fri, 22 Feb 2008 07:17:14 +0000
> Subject: [PATCH] x86: Do not scan for DMI if the DMI region is marked as RAM by e820.
> 
> Under Xen the memory at 0xf0000 is regular RAM and so can potentially contain a
> page table and hence cannot be mapped. The e820 map given to guest reflects
> this.
> 
> Signed-off-by: Ian Campbell <ijc@...lion.org.uk>

...

> @@ -371,6 +372,9 @@ void __init dmi_scan_machine(void)
>  		}
>  	}
>  	else {
> +		if (e820_all_mapped(0xF0000, 0xF0000+0x10000, E820_RAM))
> +			goto out;

	One issue with using the e820 map for this is that a Xen Dom0 will also
have this region marked as RAM in the e820 map, but will set up a fixmap
for it, allowing dmi_scan_machine() to map the region.

Cheers,
Mark.

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