Re: Question: Redirect guest kernel's message via serial port to a file on dom0

[Ian Campbell <ian.campbell@citrix.com>]

On Tue, 2015-08-25 at 09:04 -0400, Meng Xu wrote:
> 2015-08-25 3:42 GMT-04:00 Andrew Cooper <andrew.cooper3@citrix.com>:
> > On 25/08/2015 03:48, Meng Xu wrote:
> > > Hi Andrew,
> > > 
> > > Thank you so much for your suggestion! I tried but have some 
> > > questions.
> > > 
> > > 2015-08-24 4:10 GMT-04:00 Andrew Cooper <andrew.cooper3@citrix.com>:
> > > > On 24/08/2015 04:01, Meng Xu wrote:
> > > > 
> > > > Hi,
> > > > 
> > > > I'm trying to use a PV guest VM on Xen to help debug Linux.
> > > > I was using VirtualBox to help debug Linux kernel by redirecting 
> > > > the output of the serial port of the VM to a file in the host. I 
> > > > can do it in VirtualBox.
> > > > 
> > > > [Why do I want to achieve this?]
> > > > It is much faster to reboot a VM than rebooting the physical 
> > > > machine.
> > > > I don't need another machine to physically connect to the serial 
> > > > port of the development machine.
> > > > I want to use Xen for as many things as possible. ;-)
> > > > 
> > > > I tried to google a tutorial or manual about how to configure it, 
> > > > but didn't find any. :-(
> > > > 
> > > > In my understanding, I need to do the following things:
> > > > 1) I need to add a line (something like "serial=") in the guest's 
> > > > configuration file to specify the serial port device to the VM;
> > > > 2) I need some configuration to redirect the output of the serial 
> > > > device to a file in domU;
> > > > 3) After that, I can configure the kernel command line in the VM to 
> > > > dump the kernel message via the serial port of the VM. (I know how 
> > > > to do this step.)
> > > > 
> > > > Did anyone have tried this before and have some configuration I can 
> > > > refer to? or
> > > > Could anyone give me some references that describes how to 
> > > > configure the above three steps?
> > > > 
> > > > I really appreciate any help or suggestion or comment.
> > > > 
> > > > 
> > > > Configure xenconsoled to log guest consoles to file "--log=guest" 
> > > > at which point anything sent to hvc0 will be logged to files in 
> > > > /var/log/xen/guest/console (configurable with "--log-dir=")
> > > 
> > > I set "XENCONSOLED_TRACE=guest" under /etc/default/xencommons ,
> > > because in /etc/init.d/xencommons, it has: test -z
> > > "$XENCONSOLED_TRACE" || XENCONSOLED_ARGS=" --log=$XENCONSOLED_TRACE"
> > > 
> > > So I think this is what you mean by "--log=guest"; After I set this
> > > variable and restart the xencommons by "service xencommons restart" 
> > > on
> > > dom0 (Ubuntu 12.04LTS), I still couldn't find the file in
> > > /var/log/xen/guest/console when I reboot the VM.
> > > Actually, "find /var -name console" returns no result.
> > > 
> > > **My question is:**
> > > Is there anything else I need to configure to get the
> > > /var/log/xen/guest/console?
> > > 
> > > I don't see the folder under /var/log/xen. :-(
> > 
> > You probably need to mkdir it then, or extend the args list with
> > --log-dir=/var/log/xen
> 
> I tried both, but neither of them works. :-(
> 
> I'm wondering if anyone has some document about how to do this?
> If there is some document about how to debug this configuration issue,
> it could also be helpful...
> But I didn't find it after googling and trying for 2 hours...:-(
> 
> Below is the detail of what I tried:
> 
> I tried to mkdir /var/log/xen/guest

The correct (default) path is /var/log/xen/console I think.

>  and restart the xencommons by
> "service xencommons restart",

Are you using sysvinit or systemd? If the latter are you using the systemd
units of the sysvinit scripts via systemd's LSB compat mode?

If you are using the systemd units then I think they might be configured
elsewhere.

Ian.