Re: [Xen-devel] Re: [PATCH] Xen Guest Kexec

[Keir Fraser <Keir.Fraser@xxxxxxxxxxxx>]

On 3 Mar 2006, at 08:39, Gerd Hoffmann wrote:

> > The critical thing that disables the trap is flags==0: that sets the
> > 'dpl' for the 'trap gate' to zero so it effectively is unusable.
> Well, xen still tries to forward the trap to that vector then, but I
> want to avoid exactly that.
>
> The problem I have is that my kernel faults somewhere, but I'm already
> that far in doing kexec that the usual linux kernel trap handling isn't
> going to work any more.  What I'd like to see is xen print out a
> register dump, with EIP being the faulting instruction, not the
> (non-working) fault handler entry point.
>
> Guess I have to hack xen a bit for that ...
Well, the trick I described is good for at least 'non-exceptions' 
(i.e., it'll disable 'int x' software interrupts). For exceptions, I 
guess you should simply avoid causing them? :-)
In fact, why not install a dummy handler for all vectors (0-255). That 
is, a valid cs:eip that vectors to a small kexec function to print the 
state you desire? Setting flags==0 will ensure that softints don't 
work, but you can print out a useful message if you take a fault. You 
could try and put in a safe-ish place so it continues to work even for 
the early stages of the new kernel boot.
 -- Keir


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