port-xen/59976: xen pv channels appear to be unusable

["campbell+netbsd%mumble.net@localhost via gnats" <gnats-admin%NetBSD.org@localhost>]

>Number:         59976
>Category:       port-xen
>Synopsis:       xen pv channels appear to be unusable
>Confidential:   no
>Severity:       serious
>Priority:       medium
>Responsible:    port-xen-maintainer
>State:          open
>Class:          sw-bug
>Submitter-Id:   net
>Arrival-Date:   Sun Feb 08 01:10:00 +0000 2026
>Originator:     Taylor R Campbell
>Release:        current, 11, 10, 9, ...
>Organization:
>Environment:
>Description:
Xen provides a mechanism called pv channels for the dom0 to transmit data directly to and from the domU: https://xenbits.xen.org/docs/unstable/man/xen-pv-channel.7.html

This is configured by, e.g., the following xl.cfg(5) fragment:

channel = [ 'connection=socket, name=com.example.mumblefrotz, path=/var/run/mumblefrotz.sock ']

The dom0 can then connect to the local socket /var/run/mumblefrotz.sock, and the domU can use the same tty interface as is used for the console.

...Except apparently a NetBSD/xen guest just ignores all such devices advertised by the host, not just /devices/console/0 which is configured specially as the console:

528		/*
529		 * console is configured through xen_start_info when
530		 * xencons is attaching to hypervisor, so avoid console
531		 * probing when configuring xenbus devices
532		 */
533		if (strcmp(dir[i], "console") == 0)
534			continue;

https://nxr.netbsd.org/xref/src/sys/arch/xen/xenbus/xenbus_probe.c?r=1.62#527
>How-To-Repeat:
try to use xen pv channels with a NetBSD guest
>Fix:
Yes, please!