Re: [Xen-devel] Detecting deadlocks with hypervisor..

["T S" <thileepan_@xxxxxxxxxxx>]

> From: Anthony Liguori <aliguori@xxxxxxxxxx>
> To: T S <thileepan_@xxxxxxxxxxx>
> CC: xen-devel@xxxxxxxxxxxxxxxxxxx
> 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);
Dom-0 is able to do this because it is able to access the page tables of 
Dom-U
as well as the pfn2mfn list of the Dom-U. Based on this, we think the Dom-0 
can
actually save the 'context' of the deadlocked Dom-U. Please correct me if 
this
claim is wrong.

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 ?
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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