Re: x86/AMD: Nested hvm crashes in 4.3

[Andrew Cooper <andrew.cooper3@citrix.com>]

On 27/06/13 11:24, Suravee Suthikulpanit wrote:
> On 6/27/2013 5:08 AM, Jan Beulich wrote:
>>>>> On 27.06.13 at 11:20, Suravee Suthikulpanit
>>>>> <suravee.suthikulpanit@amd.com> wrote:
>>> On 6/27/2013 3:22 AM, Jan Beulich wrote:
>>>>>>> On 27.06.13 at 02:24, Suravee Suthikulanit
>>>>>>> <suravee.suthikulpanit@amd.com> wrote:
>>>>> I have found an issue in where the system crash right when I start
>>>>> another HVM guest inside an HVM guest.  I have traced back to the
>>>>> patch
>>>>> which the issue started.
>>>>>
>>>>> commit f1bde87fc08ce8c818a1640a8fe4765d48923091
>>>>> Author: Jan Beulich <jbeulich@suse.com>
>>>>> Date:   Fri Feb 8 11:06:04 2013 +0100
>>>>>
>>>>>        x86: debugging code for testing 16Tb support on smaller
>>>>> memory systems
>>>>>
>>>>>        Signed-off-by: Jan Beulich <jbeulich@suse.com>
>>>>>        Acked-by: Keir Fraser <keir@xen.org>
>>>> We had issues exposed by this patch before, but any such issue
>>>> would just have been masked before that patch (and would
>>>> surface on a system with more than 5Tb of memory anyway).
>>> The system I am having the issue has 48GB of memory.
>> Which is why you're seeing the problem only with the debugging
>> code enabled.
> Is the "debugging" enabled by default?  I didn't specify any debug
> when building.
> How can I check and disable debugging?
>
>> (And of course I didn't really expect you to have
>> tried this on a huge memory system - they're just too rare still
>> for this to be likely.)
>>
>>>> So it is very unlikely for the patch itself to be at fault.
>>> I have traced the issue and found that the system crashing starts
>>> from this
>>> commit id and onward.
>>> (i.e. The system does not crash with commit id
>>> ed759d20249197cf87b338ff0ed328052ca3b8e7)
>>> So, I am still believe that this patch has somehow triggered the issue.
>> As said - I'm pretty certain this merely unmasked an already
>> lurking issue.
> I'm not quite sure what you meant here.  Are you saying that this
> "crashing" is a known issue?
>
>>   And that's what the purpose of that patch is.
> This patch is crashing the system. What do you mean by "And that's
> what the purpose of that patch is"?
>
> Suravee
>>
>> Jan
>>
>>
>
>

It means that this patch is exposing a latent bug where the nested hvm
code is already wrong.  It will be something in the nested hvm code
which is not using map_domain_page() when it really should be.

Without posting a stack trace, there is nothing we can do to help narrow
down the issue.

~Andrew


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