linux-kernel - Re: [PATCH] lguest32 kallsyms backtrace of guest.

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Date:	Thu, 05 Apr 2007 12:54:12 +1000
From:	Rusty Russell <rusty@...tcorp.com.au>
To:	Steven Rostedt <rostedt@...dmis.org>
Cc:	virtualization@...ts.osdl.org,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Glauber de Oliveira Costa <glommer@...il.com>
Subject: Re: [PATCH] lguest32 kallsyms backtrace of guest.

On Wed, 2007-04-04 at 14:23 -0400, Steven Rostedt wrote:
> This is taken from the work I did on lguest64.
> 
> When killing a guest, we read the guest stack to do a nice back trace of
> the guest and send it via printk to the host.
> 
> So instead of just getting an error message from the lguest launcher of:
> 
> lguest: bad read address 537012178 len 1
> 
> I also get in my dmesg:
> 
> called from  [<c0405f30>] show_trace_log_lvl+0x1a/0x2f

Hi Steven,

	This is a cool idea, but there are two issues with this patch.  The
first is that it's 500 lines of code: that's around +10% on lguest's
total code size!  The second is that it conflicts with the medium-term
plan to allow any user to run up lguests: this is why lg.ko never
printk()s about problems with the guest.

While it is useful for cases where a guest dies mysteriously before it
brings up the console, three alternatives come to mind:

1) Modify early_printk so Guests can use it.
2) Have a separate tool(-set?) for this kind of post-mortem.  Then you
just have to implement guest suspend! 8)
3) Put this in a CONFIG_LGUEST_DEBUG.

Note that options 1 or 2 make you do more work, but are probably better
in the long term.  I'm happy for #3 to sit as a patch in the tree for
the duration, tho!

Cheers,
Rusty.


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