Re: [Xen-devel] Using debug-key 'o: Dump IOMMU p2m table, locks up machine

[Jeremy Fitzhardinge <jeremy@xxxxxxxx>]

On 09/20/2012 01:08 AM, Jan Beulich wrote:
> Ping?

I've been meaning to work up a reply, but I haven't had time to swap in
all the context again.

    J

>
>>>> On 04.09.12 at 11:26, Jan Beulich wrote:
>>>>> On 04.09.12 at 10:13, Sander Eikelenboom <linux@xxxxxxxxxxxxxx> wrote:
>>> Hmm don't know how to get the file/line, only thing i have found is:
>>>
>>> serveerstertje:/boot# gdb xen-syms-4.2.0-rc4-pre
>>> GNU gdb (GDB) 7.0.1-debian
>>> Copyright (C) 2009 Free Software Foundation, Inc.
>>> License GPLv3+: GNU GPL version 3 or later 
>>> <http://gnu.org/licenses/gpl.html>
>>> This is free software: you are free to change and redistribute it.
>>> There is NO WARRANTY, to the extent permitted by law.  Type "show copying"
>>> and "show warranty" for details.
>>> This GDB was configured as "x86_64-linux-gnu".
>>> For bug reporting instructions, please see:
>>> <http://www.gnu.org/software/gdb/bugs/>...
>>> Reading symbols from /boot/xen-syms-4.2.0-rc4-pre...done.
>>> (gdb) x/i 0xffff82c48015c9ee
>>> 0xffff82c48015c9ee <context_switch+916>:        mov    %edx,%gs
>>> (gdb)
>> I'm not really a gdb expert, so I don't know off the top of my
>> head either. I thought I said in a previous reply that people
>> generally appear to use the addr2line utility for that purpose.
>>
>> But the disassembly already tells us where precisely the
>> problem is: The selector value (0x0063) attempted to be put
>> into %gs is apparently wrong in the context of the current
>> GDT. Now, that's GDT_ENTRY_TLS_MIN on the Linux side,
>> and ought to be valid. I'm surprised the guest (and the current
>> process in it) survives this (as the failure here results in a failsafe
>> callback into the guest).
>>
>> Looking at the Linux side of things, this has been that way
>> forever, and I think has always been broken: On x86-64, it
>> should also clear %gs here (since 32-bit processes use it for
>> their TLS, and there's nothing wrong for a 64-bit process to put
>> something in there either), albeit not via loadsegment(), but
>> through xen_load_gs_index(). And I neither see why on 32-bit
>> it only clears %gs - %fs can as much hold a selector that might
>> get invalidated with the TLS descriptor updates. Eduardo,
>> Jeremy, Konrad?
>>
>> Jan
>>
>
>


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