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

[Anthony Liguori <aliguori@xxxxxxxxxx>]

T S wrote:
> > 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).
Yes, although this strictly isn't necessary.

> 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" ?
Not currently, although, as Keir pointed out, you still have to contend 
with the fact that a guest may have a cached PFN somewhere (for 
instance, because it's in the process of updating a page table).
Regards,

Anthony Liguori

> 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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