Found through experimentation. If one sets the following in a loader.conf in a net boot nfs environment: console="comconsole" comconsole_speed="115200" comconsole_pcidev="0:22:3" If the loader is unable to find the pcidev (like you're booting multiple devices from the same nfs boot environment), loader will panic and a BTX halted message will be displayed on the video console. The serial port will emit one error message of "Invalid port" on a COM0 if the system has it. This seems to indicate that comc_pcidev_handle() calling into comc_port_set() isn't gracefully handling the error.
John: Do you have any thoughts here? Or should I just close this issue out.
We probably shouldn't panic but should unwind if possible, but I haven't looked to see how hard/easy that is.
https://reviews.freebsd.org/D13776 Ignore misconfiguration instead of panic'ing.
A commit references this bug: Author: sbruno Date: Fri Jan 5 23:50:50 UTC 2018 New revision: 327606 URL: https://svnweb.freebsd.org/changeset/base/327606 Log: Handle misconfigured/nonexistent pcidev for comconsole instead of BTX panic. PR: 203319 Reviewed by: imp jhb MFC after: 2 weeks Sponsored by: Limelight Networks Differential Revision: https://reviews.freebsd.org/D13776 Changes: head/stand/i386/libi386/comconsole.c
A commit references this bug: Author: kevans Date: Fri Feb 9 17:25:35 UTC 2018 New revision: 329070 URL: https://svnweb.freebsd.org/changeset/base/329070 Log: MFC r327606: Handle misconfigured/nonexistent pcidev for comconsole instead of BTX panic. [This is effectively a direct commit to stable/11 due to path restructuring in head. Diffs have simply been applied to previous location] PR: 203319 Changes: _U stable/11/ stable/11/sys/boot/i386/libi386/comconsole.c
bug fixed in repo