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Re: [Xen-devel] garbage registers when domain killed by xen

It's probably repeatedly reentering your p-f handler at address 0. This will not cause the iret in Xen to fault (the fault will appear to occur in ring 1, address 0), and so the failsafe handler will not be triggered.

Yes, we should just domain_crash() if we see a callback to address 0. Even more helpful would be some extra crash context with an explanation (some way of stating it was a virtual 'double fault' of some kind), but I don;t know how you would represent that in a standard core dump file.

 -- Keir

On 7 May 2005, at 15:23, Kip Macy wrote:

There is an odd relationship between hitting the send button and epiphanies.

Show_guest_stack says the eip is 0 and the stack has eflags, eip, and
CS over and over and over again. That would indicate that I'm running
off my stack by trapping over and over again. However, I don't have
traps or callback handlers installed. What may be happening is xen
setting up a trapframe and then jumping to failsafe callback - over
and over again because jumping to failsafe callback itself causes a
page fault. In this case the eip is legitimately 0 - but not because
of me but because xen isn't checking that I've actually set my
failsafe_callback. I'll go look at FLT14 again to see if I'm on the
right track.


On 5/7/05, Kip Macy <kip.macy@xxxxxxxxx> wrote:
I'm not sure about ecx/edx but I know eip is bad. There are legitimate
cases of trying to call a null function pointer, but I know from the
context that that isn't the case. It appears to be a page fault - but
I don't have trap handlers installed yet.


On 5/7/05, Keir Fraser <Keir.Fraser@xxxxxxxxxxxx> wrote:

On 7 May 2005, at 02:26, Kip Macy wrote:

Is it really necessary for xen to overwrite part of a domain's context
on a fatal trap?

Which ones are overwritten -- ecx/edx? I'm not sure how that could
happen but it's clearly a bug rather than intentional.

  -- Keir

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