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Re: [Xen-users] Debugging DomU

On 26/05/2015 11:15, Ian Campbell wrote:
On Tue, 2015-05-26 at 11:08 +0200, Julien Grall wrote:
Hi Ian,

On 26/05/2015 11:03, Ian Campbell wrote:
On Tue, 2015-05-26 at 10:48 +0200, Julien Grall wrote:
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.

So long as it is configured to use the same underlying device as Xen's
console then it shouldn't result in crashes, I thought, due to the very
simplistic uart stuff in xen/arch/arm/vuart.c.

For dom0 yes. But we are talking about domU here.

Ah yes, I hadn't paged in all the context of this thread, sorry.

The UART emulated by vuart.c re-uses the hardware UART region. In domU,
this region may clash with other part of the memory layout. Therefore
it's not possible to use it.


/me wonders about a once-per-domain warning when an MMIO trap occurs to
what would be the vuart address if it were available... Would save a lot
of hair pulling (and if it happens to be ram we miss the trap, but
that's fine)

Well, the warning would be confusing if the UART is clashing with the GICv2 and/or come later in the boot.

I think the CONFIG_DEBUG_LL option is well-documented in Linux and the problem in the wiki [1] to avoid taking care of this problem in Xen.

This issue would have been the same if the user is using the wrong kernel on bare-metal. He has to read the doc to see which options has to be enabled before building the kernel.


[1] http://wiki.xen.org/wiki/Xen_ARM_with_Virtualization_Extensions#Common_DomU_Pitfalls

Julien Grall

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