port-i386/44540: ddb trace is broken

[bouyer%antioche.eu.org@localhost]

>Number:         44540
>Category:       port-i386
>Synopsis:       ddb trace is broken
>Confidential:   no
>Severity:       critical
>Priority:       high
>Responsible:    port-i386-maintainer
>State:          open
>Class:          sw-bug
>Submitter-Id:   net
>Arrival-Date:   Wed Feb 09 20:55:00 +0000 2011
>Originator:     Manuel Bouyer
>Release:        NetBSD 5.99.45
>Organization:
>Environment:
System: NetBSD  5.99.45 NetBSD 5.99.45 (XEN3PAE_DOMU) #8: Wed Feb  9 21:04:04 
CET 2011  
bouyer@hop:/dsk/l1/misc/bouyer/tmp/i386/obj/dsk/l1/misc/bouyer/quota2/src/sys/arch/i386/compile/XEN3PAE_DOMU
 i386
Architecture: i386
Machine: i386
>Description:

DDB on i386 (at last) is unable to provide a stack trace. I noticed
it with different kernel configuration, both on real harware and Xen,
and in different context (call to panic(), kernel trap from kernel thread
or interrupt handler, ...)
Here is an example:

unmounting 0xcb08730c / (/dev/xbd0a)...uvm_fault(0xca31ad40, 0, 1) -> 0xe
fatal page fault in supervisor mode
trap type 6 code 0 eip c023a8f4 cs 9 eflags 10287 cr2 0 ilevel 8
kernel: supervisor trap page fault, code=0
Stopped in pid 15.1 (halt) at   netbsd:turnstile_block+0x1b4:   movl    
0x10(%eax),%eax
db> tr
turnstile_block(0,1,ca31ea14,c04a262c,ca30b6dc,cb0b2d20,0,3,ca31ea14,0) at 
netbsd:turnstile_block+0x1b4
uvm_fault(0xca31ad40, 0, 1) -> 0xe
fatal page fault in supervisor mode
trap type 6 code 0 eip c0138211 cs 9 eflags 10246 cr2 1e9 ilevel 8
kernel: supervisor trap page fault, code=0
Faulted in DDB; continuing...

Entering ddb via the console cnmagic seems to paritialy work:
# ++++fatal breakpoint trap in supervisor mode
trap type 1 code 0 eip c012bb74 cs 9 eflags 202 cr2 bb501000 ilevel 6
Stopped in pid 0.2 (system) at  netbsd:breakpoint+0x4:  popl    %ebp
db> tr
breakpoint(3cd4d,1,0,1a,1,1b,ca60efa8,c0409fcf,ca5e6bf0,c066201a) at 
netbsd:breakpoint+0x4
xencons_tty_input(ca5e6bf0,c066201a,1,c0119efd,3b9aca00,0,6,0,4,2) at 
netbsd:xencons_tty_input+0xca
xencons_handler(ca5e6bf0,ca476ca0,c04ff450,6,ca30612c,4,0,40,c0184b95,2) at 
netbsd:xencons_handler+0x5f
evtchn_do_event(2,ca476ca0,ca476c58,c2c2c2c2,c2c2c2c2,c2c2c2c2,c2c2c2c2,c2c2c2c2,c2c2c2c2,c2c2c2c2)
 at netbsd:evtchn_do_event+0xd2
fatal page fault in supervisor mode
trap type 6 code 0 eip c0138211 cs 9 eflags 10246 cr2 1e9 ilevel 8
kernel: supervisor trap page fault, code=0
Faulted in DDB; continuing...

>How-To-Repeat:
        have a i386 kernel panic; try to see where it did come from
>Fix:
        please