Re: Detecting deadlocks with hypervisor..

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

Thileepan Subramaniam wrote:
> Hello,
>
> I am trying to see if the hypervisor can be used to detect deadlocks 
> in the guest VMs. My goal is to detect if a guest OS is deadlocked, 
> and if it is, then create a clone of the deadlocked OS without the 
> locking condition, and letting the clone run. While the clone runs I 
> am hoping to generate some hints that could tell me what caused the 
> deadlock.
>
> I simulated a deadlock/hang situation in a guest OS (by loading a 
> badly written module to the kernel) and when the guestOS kernel was 
> hanging, I ran "xm save" from Dom-0. But this command waits forever.
>
> I tried to follow the flow of the .py files (XendCheckpoint.py etc.). 
> These seem to be called when I run 'xm save'. But beyond a point I am 
> not sure what the python scripts do. I also see some libxc files such 
> as xc_linux_save.c, but I am not sure who is using it (Dom-0 or Xen or 
> the XenU). Can someone help me by explaining me what happens behind 
> the scene when "xm save" is called ? Is there any good documentation 
> explaining which actions are done by which layers (eg: python layer, C 
> layer etc).
>
> Also, does it seem viable to clone a copy of a deadlocked guest OS in 
> the first place?

As Ewan pointed out, xm save is guest-assisted so a hung guest will not 
be savable.

You may want to look at xc_domain_dumpcore().  You could do some 
post-analysis of the core dump to determine where it locked.  
Determining why it dead-locked is of course impossible for the general 
case but you may be able to develop some interesting heuristics with 
appropriate static analysis.

As for recovering the guest, a really clever approach would be to 
rewrite some of the locking code (maybe temporarily?) by mapping the 
guest's code page into dom0's memory after examining EIP in the core.

I reckon there's a rather interesting paper to be written on something 
like this :-)

Regards,

Anthony Liguori

> thanks!
> - ts
>
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