I have this problem on more than one host, so the specific hardware is not an issue. Bare metal Ubuntu 12 is not creating a login process on the only serial port, in the default configuration. The serial port works correctly with the firmware. It works correctly with Grub2. I have even connected the serial line to 2 different external client boxes, so the problem is neither the hardware nor the remote client. When finally booted, the system fails to create the login process. root@xenpro3:~# ps ax | grep tty 1229 tty4 Ss+ 0:00 /sbin/getty -8 38400 tty4 1233 tty5 Ss+ 0:00 /sbin/getty -8 38400 tty5 1239 tty2 Ss+ 0:00 /sbin/getty -8 38400 tty2 1241 tty3 Ss+ 0:00 /sbin/getty -8 38400 tty3 1245 tty6 Ss+ 0:00 /sbin/getty -8 38400 tty6 1403 tty1 Ss+ 0:00 /sbin/getty -8 38400 tty1 1996 pts/0 S+ 0:00 grep --color=auto tty root@xenpro3:~# dmesg | grep tty [ 0.000000] Command line: BOOT_IMAGE=/vmlinuz-3.2.0-30-generic root=/dev/mapper/xenpro3-root ro console=ttyS0,115200n8 [ 0.000000] Kernel command line: BOOT_IMAGE=/vmlinuz-3.2.0-30-generic root=/dev/mapper/xenpro3-root ro console=ttyS0,115200n8 [ 0.000000] console [ttyS0] enabled [ 2.160986] serial8250: ttyS0 at I/O 0x3f8 (irq = 4) is a 16550A [ 2.203396] serial8250: ttyS1 at I/O 0x2f8 (irq = 3) is a 16550A [ 2.263296] 00:08: ttyS0 at I/O 0x3f8 (irq = 4) is a 16550A [ 2.323102] 00:09: ttyS1 at I/O 0x2f8 (irq = 3) is a 16550A root@xenpro3:~# uname -a Linux xenpro3 3.2.0-30-generic #48-Ubuntu SMP Fri Aug 24 16:52:48 UTC 2012 x86_64 x86_64 x86_64 GNU/Linux root@xenpro3:~# I have tried putting a ttyS0.conf file in /etc/initab, which solves the problem bare metal but I still cannot get the serial port to work when booting Ubuntu on top of Xen, as domain 0. My serial line output looks like this, when booting Xen /dev/ttyS0 at 0x03f8 (irq = 4) is a 16550A * Exporting directories for NFS kernel daemon... [ OK ] * Starting NFS kernel daemon [ OK ] SSL tunnels disabled, see /etc/default/stunnel4 [ 18.654627] XENBUS: Unable to read cpu state [ 18.659631] XENBUS: Unable to read cpu state [ 18.664398] XENBUS: Unable to read cpu state [ 18.669248] XENBUS: Unable to read cpu state * Starting Xen daemons [ OK ] mountall: Disconnected from Plymouth At this point, the serial line is no longer connected to a process. Xen itself is running just fine. Dmesg gives me a long list of [ 120.236841] init: ttyS0 main process ended, respawning [ 120.239717] ttyS0: LSR safety check engaged! [ 130.240265] init: ttyS0 main process (1631) terminated with status 1 [ 130.240294] init: ttyS0 main process ended, respawning [ 130.242970] ttyS0: LSR safety check engaged! which is no surprise because I see root@xenpro3:~# ls -l /dev/ttyS? crw-rw---- 1 root tty 4, 64 Nov 7 14:04 /dev/ttyS0 crw-rw---- 1 root dialout 4, 65 Nov 7 14:04 /dev/ttyS1 crw-rw---- 1 root dialout 4, 66 Nov 7 14:04 /dev/ttyS2 crw-rw---- 1 root dialout 4, 67 Nov 7 14:04 /dev/ttyS3 crw-rw---- 1 root dialout 4, 68 Nov 7 14:04 /dev/ttyS4 crw-rw---- 1 root dialout 4, 69 Nov 7 14:04 /dev/ttyS5 crw-rw---- 1 root dialout 4, 70 Nov 7 14:04 /dev/ttyS6 crw-rw---- 1 root dialout 4, 71 Nov 7 14:04 /dev/ttyS7 crw-rw---- 1 root dialout 4, 72 Nov 7 14:04 /dev/ttyS8 crw-rw---- 1 root dialout 4, 73 Nov 7 14:04 /dev/ttyS9 If I manually change the group of /dev/ttyS0 to dialout, it gets changed back. I have made no changes to the default udev rules, so I cannot see where this problem is coming from. Sincerely, John