Red Hat Bugzilla – Bug 438573
Dom0 stops writing to serial console
Last modified: 2009-12-14 16:14:18 EST
Description of problem:
When booting a machine enabled for serial console use, xen does not write to the
console during the boot.
Version-Release number of selected component (if applicable):
0303 build of the -83 kernel
configure the serial port in BIOS, making sure to DISABLE console redirection
after boot (so 2.6 can take over). Mine was wired for 38400, RTS/DTS, direct,
using minicom (or some such) and a null modem cable, hook up the serial port, so
you can observe the output.
Steps to Reproduce:
1. configure /etc/inittab with a line like:
s0:2345:respawn:/sbin/agetty -L -f /etc/issueserial 38400 ttyS0 vt100
add "console=ttyS0, 38400 to both the xen and non-xen kernel entries
2. Boot the machine with the xen kernel
3. Boot the machine with the regular kernel
When booting with the xen kernel, nothing gets written to ttyS0 after the
'initrd' message. However, it works fine with the bare iron kernel.
Hm, what do you mean by "nothing gets written to ttyS0 after the 'initrd'
message"? Which initrd message are you talking about?
This is almost certainly a configuration issue, since the Xen serial console
behaves in different ways from the bare metal kernel. Basically what happens is
that the hypervisor is in control of the serial hardware, and emulates one so
that the dom0 kernel can also read/write to it. So what you need for
configuration in /boot/grub/grub.conf is:
title Red Hat Enterprise Linux Server (2.6.18-84.el5xen)
kernel /xen.gz-2.6.18-84.el5 com1=115200,8n1
module /vmlinuz-2.6.18-84.el5xen ro root=/dev/HostGroup/RHEL5_x86_64
Notice that the hypervisor line uses the "com1" syntax. Also notice that INIT
messages all go out to the *last* console you specify, namely console=ttyS0,
etc. Finally, if you had secondary serial port on this machine, you would
change the HV configuration to something like "com2=115200,8n1 console=com2L",
but you would *not* change the kernel line (it would still be ttyS0). This is
because the serial port is emulated to the dom0.
Make sure all of the above makes sense to you, and make sure you have it
configured correctly; it is one of the hard things to get right when using Xen
will give it a shot and get back to you
the above changes worked.
bug can be closed or withdrawn.