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