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xen-devel
Re: [Xen-devel] Detecting deadlocks with hypervisor..
From: Anthony Liguori <aliguori@xxxxxxxxxx>
To: T S <thileepan_@xxxxxxxxxxx>
CC: ewan@xxxxxxxxxxxxx, edwin.zhai@xxxxxxxxx, rthelen@xxxxxxxxxx,
Xen-devel@xxxxxxxxxxxxxxxxxxx, Keir Fraser <Keir.Fraser@xxxxxxxxxxxx>
Subject: Re: [Xen-devel] Detecting deadlocks with hypervisor..
Date: Fri, 07 Apr 2006 12:41:20 -0500
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.
Anthony,
Thank you for your reply.
In linux-2.6-sparse/drivers/core/reboot.c:__do_suspend(), we see store_mfn
and console_mfn being canonicalized before the guest-OS goes to sleep (as
done in "xm save"). But before this canonicalization took place the python
layer writes the store_mfn and console_mfn into the save-file (in the file's
header area).
Does this mean the store_mfn and console_mfn values present in the header of
the file are re-written at a later part of the file ?
Other than the store & console mfn's are there any other parameters
canoicalized BY the guest OS during "xm save" ?
thanks.
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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