NetBSD 8.10 does not boot on 7043-140

Hi,

as others have already observed, NetBSD is broken on the IBM 7043-140:

https://mail-index.netbsd.org/port-prep/2017/04/02/msg000109.html [1]
https://mail-index.netbsd.org/port-prep/2017/06/27/msg000119.html [2]
https://mail-index.netbsd.org/port-prep/2018/08/27/msg000131.html [3]

Artyom Tarasenkos guess was that the breakage exists since 3.99.22,
but his quick fix (from [1]) does not work anymore.

The bootup of my 7043 (on a serial console) looks similar to [2]:

>> NetBSD/prep BOOT, Revision 1.9 (Tue Jul 17 14:59:51 UTC 2018)

INTRF
phase mismatch without command
selection timeout without command
unhandled scsi interrupt, sist=0xffff sstat1=0xff DSA=0xffffffff DSP=0x7f7e8fff
XXXXX: fatal error, need reset the bus...
INTRF
phase mismatch without command
selection timeout without command
unhandled scsi interrupt, sist=0xffff sstat1=0xff DSA=0xffffffff DSP=0x7f7e8fff
XXXXX: fatal error, need reset the bus...
INTRF

... And so on until you'll switch off power. I am pretty sure that this
is not a hardware fault, as the same machine boots Linux just fine and
the other boot failures are looking very similar, ever the one under qemu.

Can somebody point me in the right direction where to start digging?
I mean, the INTRF and "phase mismatch without command" prints are most
probably from sys/arch/prep/stand/boot/siop.c, function siop_intr, but
as this is my first look into the NetBSD kernel tree, I'd be glad for
every help ;-)

TIA,
Uli
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