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Re: [Xen-users] Debugging DomU

On 26/05/2015 01:10, Chris (Christopher) Brand wrote:
Hi Julien,

Hi Chris,

Currently Xen use a dummy implementation of Debug registers which let the guest 
think it's not possible to use hardware debugging.

Although, it looks like we miss some registers when it was implemented.

AFAICT DBDGDTRTXint register should be handled WI/RAZ. Can you give a try to 
the patch below?

The patch you provided does make the message go away.

Thanks, I will send the patch on the ML.

I can actually step reliably through the startup code with the JTAG now. It looks like that 
instruction was from the "putstr("Decompressing kernel...");" in 
With that change, xenctx shows me a much more reasonable value for PC and 
SVC_LR. Digging around then led me to disable CONFIG_DEBUG_LL, because it seems 
that the problems I'm seeing relate to trying to print something. With that, I 
think it's getting further still. Xenctx now tells me that it ends up in 
__loop_delay(), with LR pointing to panic(). I'm having problems reproducing 
this with the JTAG, though (it seems to behave slightly differently when I'm 
using JTAG to debug it), and I'm not seeing any output from DomU.

I confirm the CONFIG_DEBUG_LL should be disabled in Xen guest because it's not portable and not supported.

For __loop_delay my guess is the timer frequency is not correct because the ARCH timer is not built in the kernel or CNTFRQ_EL0 is not correctly set by the firmware.

Do you have a property "clock-frequency" in the arch timer node in DOM0? If so, it's likely that CNTFRQ_EL0 is not correct.

You may be able to get some message before the console is effectively setup by applying the patch in [1]. It will print every guest message on the Xen console.


[1] http://lists.xen.org/archives/html/xen-devel/2014-10/msg03436.html

Julien Grall

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