Re: [Xen-devel] Debugging Dom0 kernel over serial

[Ian Campbell <Ian.Campbell@xxxxxxxxxx>]

On Wed, 2012-06-06 at 00:24 +0100, James Paton wrote:
> Hi,
> 
> Apologies if this is not the appropriate list, but I thought I would 
> have better luck here than on xen-users.
> 
> I'm a CS graduate student hacking on the xen-blkback driver and would 
> like to be able to debug it using gdb over serial. I have spent a great 
> deal of time on this with no success (more details can be found at 
> http://bit.ly/KMEF6o (Stack Overflow)). 
> 
> It eventually occurred to me that it might not even be possible to do 
> what I'm trying to do if Xen intercepts interrupts from the serial port 
> and tries to direct them to dom0 through some unexpected channel. Then, 
> after I've done `echo g > /proc/sysrq_trigger`, the kernel debugger 
> might not see the interrupt. Can anyone confirm or disconfirm this? 

It's not something I've ever tried but I expect that if you avoid
console=com1 (or com*) on your hypervisor command line then the com port
should be available for dom0 just like on native.

Alternatively you could perhaps separate the two by using com1 for Xen
and com2/ttyS1 for dom0/kdb etc.

I've no idea if kdb would be expected to work over hvc*. I suspect you'd
be treading fresh ground with that one...

I'd also start by taking virtual box out of the equation on the host
side. Start by using a MacOS terminal emulator and checking that you can
see the dom0 console messages and login via a getty running on ttyS0
before trying to hook up gdb.

> What is the usual procedure for debugging the Dom0 kernel?

Printk ;-)

Ian.


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