Date:	Wed, 09 Jul 2008 11:32:40 -0600
From:	Bob Montgomery <bob.montgomery@...com>
To:	Vivek Goyal <vgoyal@...hat.com>
Cc:	"kexec@...ts.infradead.org" <kexec@...ts.infradead.org>,
	Andi Kleen <ak@...e.de>, Dave Jones <rhkernel-list@...hat.com>,
	"airlied@...ux.ie" <airlied@...ux.ie>,
	linux kernel mailing list <linux-kernel@...r.kernel.org>
Subject: Re: AMD Family 10H machine check on vmcore read

On Tue, 2008-07-08 at 13:28 +0000, Vivek Goyal wrote:
> On Mon, Jul 07, 2008 at 05:08:06PM -0600, Bob Montgomery wrote:
> > We maintain a 2.6.18 derived kernel.
> > When testing kdump on a new AMD Family 10h (16) processor, once in the
> > kdump kernel, a read from either /proc/vmcore or /dev/oldmem that
> > corresponds to the area of memory identified in the original (crashing)
> > kernel by these boot messages:

> >
> > On a Family 15 AMD64 processor running this kernel and kdump kernel, I
> > can read the areas identified as being in the aperture from the kdump
> > kernel and get values, but on the new processor, reads from the kdump
> > kernel that are within that address range result in the machine check:
> >
> > HARDWARE ERROR
> > CPU 0: Machine Check Exception:                4 Bank 4: be0000010005001b
> > TSC 141bd974323de ADDR 1c000000 MISC e00c0ffe01000000
> >

> Hi Bob,
> 
> I am not sure what's happening here. Because in /proc/iomem, GART reserved
> area is reported as System RAM, kdump kernel will try to read this area
> and save it. Now I am not sure, what is so special about this area that
> mapping it and reading it in second kernel would cause a MCE.
> 
> CCing it to LKML, hoping people knowing GART will be able to provide some
> input.
> 
> > But I don't see this fix upstream in the kernel.  So I'm wondering if
> > some other patch protects other kdump kernels from this problem.  In
> > particular, a recent patch that informed the e820 map about the gart
> > aperture to prevent a normal kernel and a kexec kernel from putting it
> > at different addresses.  It didn't mention machine checks from kdump
> > kernels, but I wonder if it would have prevented access to that memory
> > area by having it be excluded from the /proc/vmcore list of areas??
> 
> Can you provide a link to the patch above? If /proc/iomem, does not report
> GART area as system ram then it will be excluded from the dump. (IIUC,
> IOMMU tables are in GART area and ideally one should be capturing it to
> find out how IOMMU tables looked like at the time of crash).

The patch that I thought might be related is:
 x86: disable the GART early, 64-bit

author     Yinghai Lu <Yinghai.Lu@....COM>
           Wed, 30 Jan 2008 12:33:09 +0000 (13:33 +0100)
committer  Ingo Molnar <mingo@...e.hu>
           Wed, 30 Jan 2008 12:33:09 +0000 (13:33 +0100)
commit     aaf230424204864e2833dcc1da23e2cb0b9f39cd
tree       a42042f5135aa63a780964bd053ae174211ab62f

I thought it might be relevant because of this included comment:

> hm, i'm wondering, instead of modifying the GART, why dont we simply
> _detect_ whatever GART settings we have inherited, and propagate that
> into our e820 maps? I.e. if there's inconsistency, then punch that out
> from the memory maps and just dont use that memory.

But this patch doesn't mention machine checks as the symptom that
initiated the patch.

And my reason for looking was because I didn't think I could be the
first person to try reading /proc/vmcore on a Family 10h processor.  So
I wondered why it hadn't been seen by some other tester, and thought
some other patch might have "fixed" it a different way on newer kernels
than mine.

Thanks,
Bob Montgomery





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