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

[Anthony Liguori <aliguori@xxxxxxxxxx>]

T S wrote:
> > 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);
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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