This option seems fairly odd, especially because it doesn't only disable access
to the serial port in use by Xen (and even that only optionally), but to all 8250
style ones. I would want to reach a state where all ports are available to guests except for the at most one port Xen may use for its console (plus perhaps any others sharing the IRQ with it, although I can't see how Xen would be able to properly show their unavailability). But due to the lack of experience with the xen console code (and also more generally the Linux one) I can't really judge whether turning off $subject
config option will have any ill effect here, while I'm pretty convinced that it would
help the situation described above. In particular I wonder whether xc_mode shouldn't default to XC_OFF in dom0 when that config option is off.

Thanks for any suggestions or hints, Jan