[Bug 292494] powerpc64: booting qemu + mac99 results in crash (kernel DSI write trap) after panic: witness_save: witness_cold

https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=292494

--- Comment #4 from Adrian Chadd <adrian@freebsd.org> ---
ok, it's actually an access to virtual address 0, ie in the trap handler

```
(kgdb) print *frame
$9 = {fixreg = {-4611686018412418252, 4294967152, 536870912, 29420080, 
    4294967152, 0, -9223372036854775758, -4611686018401836104, 
    -4611686018412426768, -4611686018403041280, -4611686018403500032, 
    -4611686018403106816, -4611686018412427536, -4611686018399260192, 
    -4611686018402451456, 0, 0, -4611686018399240192, 0, -4611686018402979716, 
    -4609434218613651016, -4611686018403496689, -4611686018403072439, 
    -4611686018404260381, -4611686018403255967, -4611686018398996752, 
    -4611686018402311368, -4611686018401817192, -4611686018397992040, 
    -9223372036854775758, -4611686018403397158, -4609434218613651872}, 
  lr = 14988372, cr = 603980322, xer = 536870912, ctr = 4293928400, 
  srr0 = 4293928572, srr1 = 8240, exc = 768, dar = 0, cpu = {aim = {
      dsisr = 1107296256}, booke = {esr = 1107296256, 
      dbcr0 = -3518497339244552}}}
(kgdb)
```

it's a data storage interrupt, dar=0, it's /just/ inside the openfirmware code
as far as I can tell.

```
(kgdb) printf "0x%x 0x%x\n", framep->srr0, framep->cpu.aim.dsisr
0xfff0267c 0x42000000
(kgdb)
```

I'm not yet sure what the right thing to do is here; however getting a page
fault before the VM system is up likely should cause more drama and not an
infinitely looping set of faults.

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