Re: Detecting deadlocks with hypervisor..

[Anthony Liguori <aliguori@us.ibm.com>]

T S wrote:
>> From: Anthony Liguori <aliguori@us.ibm.com>
>> To: T S <thileepan_@hotmail.com>
>> CC: xen-devel@lists.xensource.com
>> Subject: Re: [Xen-devel] Detecting deadlocks with hypervisor..
>> Date: Fri, 24 Mar 2006 13:24:46 -0600
>>
>> T S wrote:
>>> This may sound a silly question (pardon me because i am relatively 
>>> new to linux kernel) .. will it be possible to continue running 
>>> reboot.c (or for that matter any kernel thread) when the kernel is 
>>> deadlocked ? In Linux, is the kernel a single process or a bunch of 
>>> parallelly executing entities? If later, then during a kernel 
>>> deadlock (eg: by loading a faulty module that disables interrupts 
>>> and do something silly) there can still be some other 
>>> processes/threads run, right?
>>
>> Sorry for not making this more clear previously. You cannot restore a 
>> dead-locked domain if a normal xm save doesn't work. One thing that 
>> makes Xen unique is that guests actually are aware of what physical 
>> pages are assigned to them. When one does a save/restore, the guest 
>> has to canonicalize all of it's internal references to physical 
>> pages. When it's restored, it then remaps it's newly assigned 
>> physical pages to all the old places where it needed to know about 
>> them for some reason or another.
>
> We took a look at the xc_linux_save() function ... and what we see is 
> that
> the canonicalize action is actually done by the Dom-0 (and not by the 
> Dom-U);

Take a look at linux-2.6-sparse/drivers/core/reboot.c:__do_suspend(). 
Canonicalization is done both in Dom-0 and in the guest itself. Dom-0 
attempts to do as much of it as it can but as I've said before, it 
cannot do all of it.

> Also, given that Dom-0 can access the page tables and other structures 
> of the deadlocked guest,
> can one of you be able to tell me what changes I need to do to 
> xm_linux_save( ) (and other related functions) to save the state of 
> the deadlocked guest without doing any handshake with the guest OS ?

If you want to attempt to futz with the state of a guest while it's 
running without the guest cooperating, your best bet is to do as Keir 
suggested and pause the domain, make your changes, and then unpause.

Regards,

Anthony Liguori

>
> thanks!
> - T
>
>
>> If the guest isn't responsive when you do a save, then it will never 
>> canonicalize itself and there is no way to restore the domain.
>>
>> Regards,
>>
>> Anthony Liguori
>>
>>> thanks
>>> TS
>>>
>>>>
>>>> If a suspend completes correctly, Xend will see it (another watch 
>>>> will fire),
>>>> and xc_linux_save will be free to complete the save.
>>>>
>>>> > Also, does it seem viable to clone a copy of a deadlocked guest 
>>>> OS in the
>>>> > first place?
>>>>
>>>> If you have a byte-for-byte copy of a deadlocked guest, even if you 
>>>> could
>>>> suspend it, surely it will be deadlocked when it is resumed. How do 
>>>> you
>>>> intend to break the deadlock, and how is it easier to do that from 
>>>> outside
>>>> than it is to perform deadlock detection in the guest?
>>>>
>>>> Ewan.
>>>>
>>>>
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>>>> http://lists.xensource.com/xen-devel
>>>
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