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

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Date:	Thu, 21 Feb 2008 13:21:27 -0800
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Jeremy Fitzhardinge <jeremy@...p.org>
CC:	Ian Campbell <ijc@...lion.org.uk>,
	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

Jeremy Fitzhardinge wrote:
> Ian Campbell wrote:
>> I'll see if I can track down where the page is getting used and have a
>> go at getting in there first. It must be pretty early to be allocated
>> already when dmi_scan_machine gets called.
>>
>> It's possible that the domain builder might have already allocated a PT
>> at this address. I haven't checked but I think currently the domain
>> builder always puts PT pages after the kernel so hopefully it's only a
>> theoretical problem.
>>   
> 
> Yes, it does.  And presumably the early pagetable builder is guaranteed 
> to avoid special memory like the DMI space.  But the bug definitely 
> seems to be a result of the DMI code trying to make a RW mapping of a 
> pagetable page, so something is amiss there.
> 
> Ooh, sleazy hack idea: make DMI always map RO, so even if it does get a 
> pagetable it causes no complaint...  A bit awkward, since there doesn't 
> seem to be an RO form of early_ioremap.
> 
>> Another option I was thinking of was a command line option to disable
>> DMI, which (maybe) isn't terribly useful in itself but it introduces an
>> associated variable to frob with. That's similar to how the TSC was
>> handled in the past (well, the opposite since TSC was forced on).
>>   
> 
> Yep, that would work too.
> 
> Still curious about why a pagetable page is ending up in that range 
> though.  Seems like it shouldn't be possible, since we shouldn't be 
> allowed to allocate from those pages, at least until the DMI probe has 
> happened...  Unless the early allocator is only excluded from e820 
> reserved pages, which would cause a problem on systems which don't 
> reserve the DMI space...  HPA?
> 

I thought the problem was a Xen-provided pagetable from before Linux 
started?

	-hpa
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