-current has been failing to boot for me on XEN3 (i386) for certainly
a few weeks now, with 4.99.64 (but not necessarily from then on)
and -current from yesterday. The machine doesn't have a serial
port, so I hand-copied a few details from console output, hoping
they are useful to someone...

...
pmsprobe: reset error 5
xenbus0: using event channel 11

[ at this point it hangs; breaking into ddb yields: ]

fatal breakpoint trap in supervisor mode

[ 'bt' ]

breakpoint
cpu_Debugger
internal_command
wskbd_translate
wskbd_input
pckbd_input
pckbportintr
pckbcintr
pirq_interrupt(..., fatal page fault in supervisor mode
kernel: supervisor trap page fault, code=0

Of special interest seems to be the line before the last, where in
the argument list there appears (another?) page fault, so I copied
this, too. I don't have a real clue what the problem might be,
though. Also I'm not sure whether I actually changed anything before
it stopped booting, so maybe it's even hardware-related; at least
noone else seems to suffer from this problem. Anyway, any help
would be greatly appreciated.

TIA,

-- Dennis den Brok