Message-ID: <47BDEB57.5040203@zytor.com>
Date: Thu, 21 Feb 2008 13:21:27 -0800
From: "H. Peter Anvin" <hpa@zytor.com>
User-Agent: Thunderbird 2.0.0.9 (X11/20071115)
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To: Jeremy Fitzhardinge <jeremy@goop.org>
CC: Ian Campbell <ijc@hellion.org.uk>, Joel Becker <Joel.Becker@oracle.com>,
       Jody Belka <lists-lkml@pimb.org>, linux-kernel@vger.kernel.org,
       Ingo Molnar <mingo@elte.hu>, Thomas Gleixner <tglx@linutronix.de>,
       "Eric W. Biederman" <ebiederm@xmission.com>,
       Andi Kleen <andi@firstfloor.org>,
       Mika Penttila <mika.penttila@kolumbus.fi>
Subject: Re: 2.6.25-rc1 xen pvops regression
References: <20080212235404.GY7980@pimb.org> <47B2DBA5.6030001@goop.org>	 <20080214022744.GA4160@mail.oracle.com> <47B3F2DC.8080707@goop.org>	 <20080215202336.GE26034@mail.oracle.com>	 <1203274161.27987.6.camel@localhost.localdomain>	 <20080218104025.GA15899@ca-server1.us.oracle.com>	 <1203458366.26910.15.camel@cthulhu.hellion.org.uk>	 <47BBDA20.8030105@zytor.com>	 <1203497511.26910.39.camel@cthulhu.hellion.org.uk>	 <47BCA275.7000504@goop.org> <1203546597.26910.74.camel@cthulhu.hellion.org.uk> <47BDEA11.6010302@goop.org>
In-Reply-To: <47BDEA11.6010302@goop.org>
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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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