Taking ttyS0 away from xen

On xen machines, (linux-Centos5 Dom0) xen is in the habit of grabbing /dev/ttyS0 for its console, so if you need the serial port for other purposes (a modem, or UPS control) you are royally screwed.

Symptoms:

# modprobe 8250

FATAL: Error inserting 8250
(/lib/modules/2.6.13-15.8-xen/kernel/drivers/serial/8250.ko): Device or
resource busy

# setserial -a /dev/ttyS0
Cannot get serial info: Invalid argument

The trick is finding the right parameters combination in the boot configuration. This is tedious, as it requires a reboot per test, and servers have slow reboot sequences. However, this grub.conf stanza fixed it for me:

title CentOS (2.6.18-194.11.3.el5xen)
 kernel /xen.gz-3.2.1 noirqbalance hap dom0_mem=2G dom0_max_vcpus=8 xencons=tty console=vga
 module /vmlinuz-2.6.18-194.11.3.el5xen ro root=LABEL=dom0_root console=vga xencons=tty 
 module /initrd-2.6.18-194.11.3.el5xen.img

I suppose that this should also work:

title CentOS (2.6.18-194.11.3.el5xen)
 kernel /xen.gz-3.2.1 noirqbalance hap dom0_mem=2G dom0_max_vcpus=8 xencons=tty
 module /vmlinuz-2.6.18-194.11.3.el5xen ro root=LABEL=dom0_root console=vga
 module /initrd-2.6.18-194.11.3.el5xen.img

But I really have no inclination to try it now.