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Re: [Xen-devel] Xen 4.0.0x allows for data corruption in Dom0

To: Joanna Rutkowska <joanna@xxxxxxxxxxxxxxxxxxxxxx>
Subject: Re: [Xen-devel] Xen 4.0.0x allows for data corruption in Dom0
From: Jeremy Fitzhardinge <jeremy@xxxxxxxx>
Date: Mon, 08 Mar 2010 15:41:08 -0800
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On 03/08/2010 03:23 PM, Joanna Rutkowska wrote:
But the corruptions always happen in 32-bytes chunks, which might
suggest it's not a page-related problem (e.g. wrongly re-used page), as
in that case we would be observing (at least sometimes) much bigger
chunks of corrupted data, I think.

Given that the domU doesn't have any devices or much going on, it could easily be corrupting memory in only small amounts.

The reason why I still believe it's a hypervisor related thing, it that
I'm currently using the very *same* Dom0 kernel (very recent
xen/stable-2.6.31) with Xen 3.4.2 and the system is damn stable. And I
really mean extensive use with 5-7 VMs running all the time doing
various things from Web browsing to kernel building.

OK, it's always good to get some positive feedback.

If I was to make an educated guess I would say it's something related to
some interrupt handling, i.e. Xen mishandling it, e.g. the handler is
writing out-of-buffer somewhere and it just happens to land in the Dom0
fs buffer used by e.g. dd operation.

It would be interesting to see what happens if you write the file with the test domain paused (xm pause ...). If the corruption continues, then it is almost certainly Xen. If it stops, then it either means the corruption was caused by pages inappropriately shared between dom0 and domU, or something like vcpu context switch is corrupting memory (which would be very sad).


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