Re: [Xen-users] Debugging DomU

[Julien Grall <julien.grall@xxxxxxxxxx>]

Hi Christopher,

On 13/05/15 22:37, Chris (Christopher) Brand wrote:
> I did end up writing some code to do what you suggested (trap to the 
> hypervisor on an early exception), but never got it to succeed.
> 
> I have now got a JTAG on the system, which allows me to look at the page 
> tables when it's in the locked up state, and I see that address 0 isn't 
> mapped to any RAM at all, which would seem to be a problem because the 
> vmlinux includes exception table code that should live there.

This is normal that address 0 is not mapped to any RAM. The memory
layout of the guest (see xen/include/public/arch-arm.h) as the RAM guest
defined in higher memory.

Although, vmlinux is using virtual address, so 0 is likely a virtual
address. Do you know if the page table has been setup to the guest?

The error you describe in your mail is usually happening in the early setup.

If you are using arm64, you can give a look to arch/arm64/kernel/head.S
For arm32, give a look to arch/arm/boot/compressed/head.S and
arch/arm/kernel/head.S

> Can you point me towards the code that populates the page table for a new 
> DomU ?

I don't think this is the bug, as said above we don't map anything to
address 0 and this is normal because it's not part of the layout.

It would be useful if you provide the version of Xen and Linux you use.

Regards,

-- 
Julien Grall

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