Re: [Xen-users] Debugging DomU

[Julien Grall <julien.grall@xxxxxxxxxx>]

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 
> arch/arm/boot/compressed/misc.c.
> 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.
Regards,

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

--
Julien Grall

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