port-xen/49869: Xen DomU fails to crash dump

[mlelstv%serpens.de@localhost]

>Number:         49869
>Category:       port-xen
>Synopsis:       Xen DomU fails to crash dump
>Confidential:   no
>Severity:       serious
>Priority:       low
>Responsible:    port-xen-maintainer
>State:          open
>Class:          sw-bug
>Submitter-Id:   net
>Arrival-Date:   Sat May 02 07:50:00 +0000 2015
>Originator:     Michael van Elst
>Release:        NetBSD 7.99.13
>Organization:
--
                                Michael van Elst
Internet: mlelstv%serpens.de@localhost
                                "A potential Snark may lurk in every tree."
>Environment:
        
        
System: NetBSD test3.local 7.99.13 NetBSD 7.99.13 (XEN3PAE_DOMU.201505020340Z) #0: Sat May 2 06:01:27 UTC 2015 builds%b48.netbsd.org@localhost:/home/builds/ab/HEAD/i386/201505020340Z-obj/home/source/ab/HEAD/src/sys/arch/i386/compile/XEN3PAE_DOMU i386
Architecture: i386
Machine: i386
>Description:
Neither a i386 nor a amd64 DomU are able to dump core after a system
panic.

amd64 provides an empty memory map (dumpsize = 0) to the dump functions.
These will then just abort with the message "not possible".

i386 panics when you try to run the 'sync' command in ddb.

db{0}> sync

dumping to dev 142,1 offset 16431
dump fatal page fault in supervisor mode
trap type 6 code 2 eip c01558ed cs 9 eflags 10202 cr2 e7acbfff ilevel 8 esp c05c99c0
curlwp 0xc0b39d20 pid 0 lid 2 lowest kstack 0xc71b22c0
panic: trap
cpu0: Begin traceback...
vpanic(c04ecfa5,c71b4a60,c71b4ae4,c03d2d46,c04ecfa5,c71b4af0,c71b4af0,2,c71b22c0,10202) at netbsd:vpanic+0x11a
panic(c04ecfa5,c71b4af0,c71b4af0,2,c71b22c0,10202,e7acbfff,8,c05c99c0,100) at netbsd:panic+0x18
trap() at netbsd:trap+0xbe6
--- trap (number 6) ---
dodumpsys(c71b4cbc,8,100,c02aae3d,8,1104b14,0,c049e11c,c057c11c,c71b4ba4) at netbsd:dodumpsys+0x34d
dumpsys(100,0,c71b4c4c,c013ec40,0,0,0,c71b4bc8,8,0) at netbsd:dumpsys+0x14
db_sync_cmd(0,0,0,c71b4bc8,8,0,0,4857f550,a,d) at netbsd:db_sync_cmd+0x2e
...



>How-To-Repeat:
On a console of a i386 or amd64 DomU, break into ddb with +++++ and
issue a sync command.

>Fix:
        

>Unformatted: