[Xen-users] Losing serial console output on boot

[Andy Smith <andy@xxxxxxxxxxxxxx>]

Hi,

I'm playing around with Xen 4 for the first time on a machine with a
serial console and I'm seeing some odd behaviour with a slow (9600
baud) serial console and the hypervisor.

Basically Xen is producing a vast amount of console output during
boot, and at some point after the dom0 kernel has been loaded it
will just stop printing it to the serial console. The point it does
this at isn't deterministic. Nothing more gets printed. It's often
part way through printing even the prefix, like:

[    0.0000

or similar.

If I tap a key at this point then I get a login prompt from getty on
hvc0 as I would expect. Everything appears to be working in fact;
it's almost like as soon as init in dom0 decided to put a getty on
hvc0, Xen gives up on its backlog of things to print on console.

I added the sync_console boot parameter, and with this everything is
printed from start of boot to login prompt on hvc0. I also get this
warning however:

(XEN) **********************************************
(XEN) ******* WARNING: CONSOLE OUTPUT IS SYNCHRONOUS
(XEN) ******* This option is intended to aid debugging of Xen by ensuring
(XEN) ******* that all output is synchronously delivered on the serial line.
(XEN) ******* However it can introduce SIGNIFICANT latencies and affect
(XEN) ******* timekeeping. It is NOT recommended for production use!
(XEN) **********************************************

which makes me think that maybe it isn't a good idea to keep that
set.

There are none of these problems using the same kernel on bare
metal. I'm unable to easily attach a VGA monitor so I'm not sure
what it looks like on there.

In terms of grub settings I am using those recommended in
http://wiki.debian.org/Xen#Dom0_.28host.29 under "Serial console
access", but I've tried what seems like every possible combination
and nothing makes any difference to this phenomenon except
sync_console.

Any ideas?

Cheers,
Andy

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