Re: [Xen-users] DomU Console Flow of Control

[Pasi Kärkkäinen <pasik@xxxxxx>]

On Sun, Feb 07, 2010 at 10:01:09AM -0600, John Riedl wrote:
> Hello all.  I've been running a Hardy Dom0/DomU for about 2.5 years.
> Recently I upgraded my test DomU to Karmic because I needed some
> upgraded tools on my desktop, and wanted to be sure I had an upgrade
> path that would work.  I ran into the /proc/<pid>/mountinfo problem,
> and had to upgrade the Dom0 to a newer Debian kernel.  (If you haven't
> been down this path: Karmic requires at least a 2.6.26 kernel because
> of some /proc updates.)  Then I had to upgrade my Dom0 to jaunty to
> get the Xen 3.3 so Xen could decompress the newer kernels from Karmic.
>  (If you haven't been down this path, newer linux kernels are
> sometimes compressed with lzma or bzip2, which Xen 3.2 doesn't
> understand.)  All seems to be well now -- except that I *cannot* get
> the console working with my karmic DomU.  Today I even used the
> xen-tools to create a brand spanking new karmic DomU.  Everything
> worked out of the box -- but no console.  (There is a practical part
> of me that says "quit worrying about having a console: embrace your
> inner ssh".  However, I know that eventually my ssh is going to break
> before a key deadline ...)
> 
> I've tried a million variants of hdc0/xdc0/tty0,

The correct parameters are hvc0 or xvc0.

> and I figure it's
> time to actually understand how this thing is supposed to work.  So:
> can someone please explain what needs to be true for a xen DomU
> console to work?  Tons of people are struggling with this problem in
> all different variants of Xen, but I haven't yet found a clear,
> concise outline of the basics of how it should work.  The key
> questions are below.  If some xen hero provides answers, I volunteer
> to write them up for the wiki to restore xen to the world :).  Here
> are the key questions:
> 
> 1) What is the relationship between xenconsole= and console= in the
> "extra = " config parameter?
>

Older xenlinux kernels use the xencons= parameter, while the newer upstream
pv_ops kernels use only the console=.

> 2) Is there a relationship between the Dom0 console and a DomU
> console?  Are there settings that are required on the Dom0 for the
> DomU console to work?
>

No. They're not related. Only domU configuration affects the domU console.

> 3) Is the foo in console=foo magic, interpreted by Xen in some
> mysterious way, or is it just a choice that connects to a normal tty
> in the domU?

No, that information is only handled by the domU kernel.

> 4) Does the DomU need to have a /dev/foo for console=foo to work?  If
> so, what should the properties of /dev/foo be?  (I created a /dev/xvc0
> on one of my karmic installs, and it got auto-removed on reboot.
> What's that all about?!)

Depends on the domU kernel.

> 5) Does the DomU need to run getty on the /dev/foo?  Conversely, is it
> important that the DomU not run getty?  Karmic has changed its way of
> running getty, which has added to the confusion, but once you find
> /etc/init it's pretty easy to follow.

Yes, it needs to do that.

> 6) Are there key modules that a DomU needs to have in its initrd to
> have the console work?
> 

Not sure. Console has always worked for me with correct settings :)

> Has anyone been down this path with karmic before?  I'd like to
> understand the general situation better, but specific advice would be
> very welcome.
> 

Take a look at this wiki page:
http://wiki.xensource.com/xenwiki/XenCommonProblems

-- Pasi


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