I maintain several xen machines. Most servers that i maintain use serial consoles (i should probably write another post in more detail about why serial consoles are the One True Way to manage a server).

Trouble is, the way that xen works with the serial console has changed between etch and lenny. So what's changed? From what i can tell:

• The way that the dom0 Linux kernel interacts with the hypervisor's console has changed between Linux 2.6.18 (etch) and 2.6.26 (lenny), and
• the Xen hypervisor's method of specifying the console itself has changed between Xen 3.0.3 (etch) and 3.2.1 (lenny)

In etch systems, i had a standard GRUB specification like this (as noted earlier):

title           Xen 3.0.3-1-i386-pae / Debian GNU/Linux, kernel 2.6.18-4-xen-686
root            (hd0,0)
kernel          /xen-3.0.3-1-i386-pae.gz dom0_mem=131072 com1=115200,8n1
module          /vmlinuz-2.6.18-4-xen-686 root=/dev/mapper/vg_monkey0-dom0 ro console=ttyS0,115200n8 
module          /initrd.img-2.6.18-4-xen-686
savedefault

but in lenny, i find it necessary to do this:

title           Xen 3.2-1-i386 / Debian GNU/Linux, kernel 2.6.26-2-xen-686
root            (hd0,0)
kernel          /xen-3.2-1-i386.gz dom0_mem=131072 com1=115200,8n1 console=com1
module          /vmlinuz-2.6.26-2-xen-686 root=/dev/mapper/vg_monkey0-dom0 ro console=hvc0
module          /initrd.img-2.6.26-2-xen-686

In particular, the hypervisor itself needs an additional console=com1 argument to make sure that it uses the serial console (i suppose the com1=... argument simply specifies how to use the serial console should you need it, instead of a requirement to use the console).

And the Linux kernel itself for the dom0 needs to adopt hvc0 as its console, which i believe is an acronym for something like the "hypervisor virtual console" -- this way, the dom0 kernel will come out multiplexed over the hypervisor's console, no matter where that hypervisor's console is directed (you might not even need to specify this explicitly -- it might be set up by default).

When i tried to use the etch settings for the console with the lenny hypervisor and kernel, i saw the hypervisor try to come out on the video card, and a dom0 crash!