NetBSD Xen guest freezes system + vif MAC address confusion (NetBSD 9.99.97 / Xen 4.15.2)

[Matthias Petermann <mp%petermann-it.de@localhost>]

Hello all,

currently I am not able to instantiate a NetBSD Xen guest on NetBSD 9.99 
(side fact: I also have problems with a Windows guest, but it is not 
that important at the moment).

The problem occurs in the following environment:

 - Xen Kernel 4.15.2 and matching Xen Tools from pkgsrc 2022Q1 (built 
29.04.2022)
 - NetBSD/Xen 9.99.97 (build 25.05.2022)

The host is booted with this boot.cfg (if this matters):

```
    menu=Boot Xen:load /netbsd-XEN3_DOM0.gz console=pc;multiboot 
/xen.gz dom0_mem=512M vga=keep console=vga
```

The guest config looks like this:

```
    name = "net"
    type="pv"
    kernel = "/netbsd-INSTALL_XEN3_DOMU.gz"
    #kernel = "/netbsd-XEN3_DOMU.gz"
    memory = 2048
    vcpus = 2
    vif = [ 'mac=00:16:3E:01:00:01,bridge=bridge0' ]
    disk = [
       'file:/data/vhd/net.img,hda,rw',
       'file:/data/vhd/net-export.img,hdb,rw'
    ]
```

When I try to instantiate the guest, I get the following output on the 
controlling terminal:

```
ganymed$ doas xl create net
Parsing config from net
libxl: error: libxl_device.c:1109:device_backend_callback: Domain 
1:unable to add device with path /local/domain/0/backend/vif/1/0
libxl: error: libxl_create.c:1862:domcreate_attach_devices: Domain 
1:unable to add vif devices
```

At the same time the following message appears on the system console:

```
[   184.680057] xbd backend: attach device vnd0d (size 1048576000) for 
domain 1
[   184.910057] xbd backend: attach device vnd1d (size 33554432) for 
domain 1
[   195.260077] xvif1i0: Ethernet address 00:16:3e:02:00:01
[   195.320059] xbd backend: detach device vnd1d for domain 1
[   195.350051] xbd backend: detach device vnd0d for domain 1
[   195.450054] xvif1i0: disconnecting
```

After the messages appear on the system console, the system does not 
respond to any input either via SSH or on the local console. It seems to 
be frozen. I can still activate the kernel debugger with 
Control+Alt+Escape.

What surprises me: the 4th digit of the MAC address in the system log 
seems to be 1 higher than specified in the guest configuration. I have 
checked this again because I initially assumed a configuration error. Is 
this somehow explainable or might this already be a indication of the 
root cause?

Kind regards
Matthias