xen 3.2 serial

Getting access to the serial port in a Xen 3.2 dom0 is somewhat complicated. This is the magic incantation for your grub menu.lst file to get console at 115200 bps on the first physical serial port, as well as on the screen.

serial --unit=0 --speed=115200
terminal --timeout=5 serial console

title Xen 3
root    (hd0,0)
kernel    /boot/xen-3.gz com1=115200,8n1 console=com1,vga
module    /boot/vmlinuz-2.6.18.8-xen root=/dev/md0 ro xencons=ttyS0 console=tty0 console=ttyS0,115200n8
module    /boot/initrd.img-2.6.18.8-xen
boot

The ‘com1=115200,8n1 console=com1,vga’ arguments on the kernel line make sure that xen writes its output to the serial port (with the right speed, stop bit etc) as well as to the vga device. The ‘console=tty0 console=ttyS0,115200n8′ on the first module line tells the kernel to do the same. Xen controls the serial port hardware at this point (that’s the default in 3.2: XEN_DISABLE_CONSOLE is set in the dom0 kernel config!), and in order for the dom0 kernel and Xen to share that serial port, we have to tell the kernel to use the xencons virtual console driver – hence the ‘xencons=ttyS0′ parameter.

Debugging Xen with Serial Console has it almost right, but the xencons parameter is missing. Without xencons you’ll see serial output until the dom0 kernel taks over; that kernel won’t see any serial ports, which makes (m)getty very unhappy…