alpha and em mtu

[Sten Spans]

I upgraded my ds10 with an intel gigabit card,
and encountered the following issue (RELENG_5):

deepthought# ifconfig em0 mtu 1501
deepthought#
fatal kernel trap:

     trap entry     = 0x4 (unaligned access fault)
     faulting va    = 0xfffffc001847a02e
     opcode         = 0x28
     register       = 0x1
     pc             = 0xfffffc00004cd0f8
     ra             = 0xfffffc00004cce24
     sp             = 0xfffffe00187a9af0
     curthread      = 0xfffffc001f3a70e0
         pid = 14, comm = swi1: net

[thread 100006]
Stopped at      tcp_input+0x368:        srl     t0,#0x2,t0      <t0=0x45>
db> trace
tcp_input() at tcp_input+0x368
ip_input() at ip_input+0x9fc
netisr_processqueue() at netisr_processqueue+0xac
swi_net() at swi_net+0xf0
ithread_loop() at ithread_loop+0x1d4
fork_exit() at fork_exit+0x100
exception_return() at exception_return
--- root of call graph ---


deepthought# ifconfig em0 mtu 9014
deepthought#
fatal kernel trap:

     trap entry     = 0x4 (unaligned access fault)
     faulting va    = 0xfffffc0018c5202e
     opcode         = 0x28
     register       = 0x1
     pc             = 0xfffffc00004cd0f8
     ra             = 0xfffffc00004cce24
     sp             = 0xfffffe00187a9af0
     curthread      = 0xfffffc001f3a70e0
         pid = 14, comm = swi1: net

[thread 100006]
Stopped at      tcp_input+0x368:        srl     t0,#0x2,t0      <t0=0x45>
db> trace
tcp_input() at tcp_input+0x368
ip_input() at ip_input+0x9fc
netisr_processqueue() at netisr_processqueue+0xac
swi_net() at swi_net+0x148
ithread_loop() at ithread_loop+0x1d4
fork_exit() at fork_exit+0x100
exception_return() at exception_return
--- root of call graph ---

The box crashes on the first incoming tcp packet,
icmp is fine. mpsafenet=0 and net.inet.tcp.sack.enable=0
did not seem to help.
I have an intel box (fbsd5) with the same batch em0
which supports large mtu's just fine.

tcpdump:


22:20:29.219776 IP 192.168.1.4.35993 > 192.168.1.3.23: S 
2509718255:2509718255(0) win 17948 <mss 8974,sackOK,timestamp 160282313 
0,nop,wscale 2>
         0x0000:  0003 477a 3ef6 000d 5616 e035 0800 4510  ..Gz>...V..5..E.
         0x0010:  003c f426 4000 4006 c32d c0a8 0104 c0a8  .<.&@. at ..-......
         0x0020:  0103 8c99 0017 9597 42ef 0000 0000 a002  ........B.......
         0x0030:  461c 3ba9 0000 0204 230e 0402 080a 098d  F.;.....#.......
         0x0040:  b6c9 0000 0000 0103 0302                 ..........
22:20:29.220148 IP 192.168.1.3.23 > 192.168.1.4.35993: R 0:0(0) ack 
2509718256 win 0
         0x0000:  000d 5616 e035 0003 477a 3ef6 0800 4510  ..V..5..Gz>...E.
         0x0010:  0028 00d8 4000 4006 b690 c0a8 0103 c0a8  .(.. at .@.........
         0x0020:  0104 0017 8c99 0000 0000 9597 42f0 5014  ............B.P.
         0x0030:  0000 c740 0000 0000 0000 0000            ... at ........
22:20:44.098530 IP 192.168.1.4.35994 > 192.168.1.3.23: S 
2521517724:2521517724(0) win 17948 <mss 8974,sackOK,timestamp 160297194 
0,nop,wscale 2>
         0x0000:  0003 477a 3ef6 000d 5616 e035 0800 4510  ..Gz>...V..5..E.
         0x0010:  003c be76 4000 4006 f8dd c0a8 0104 c0a8  .<.v at .@.........
         0x0020:  0103 8c9a 0017 964b 4e9c 0000 0000 a002  .......KN.......
         0x0030:  461c f525 0000 0204 230e 0402 080a 098d  F..%....#.......
         0x0040:  f0ea 0000 0000 0103 0302                 ..........
22:20:47.097687 IP 192.168.1.4.35994 > 192.168.1.3.23: S 
2521517724:2521517724(0) win 17948 <mss 8974,sackOK,timestamp 160300194 
0,nop,wscale 2>
         0x0000:  0003 477a 3ef6 000d 5616 e035 0800 4510  ..Gz>...V..5..E.
         0x0010:  003c be78 4000 4006 f8db c0a8 0104 c0a8  .<.x at .@.........
         0x0020:  0103 8c9a 0017 964b 4e9c 0000 0000 a002  .......KN.......
         0x0030:  461c e96d 0000 0204 230e 0402 080a 098d  F..m....#.......
         0x0040:  fca2 0000 0000 0103 0302                 ..........
22:20:49.097336 arp who-has 192.168.1.3 tell 192.168.1.4
         0x0000:  0003 477a 3ef6 000d 5616 e035 0806 0001  ..Gz>...V..5....
         0x0010:  0800 0604 0001 000d 5616 e035 c0a8 0104  ........V..5....
         0x0020:  0000 0000 0000 c0a8 0103                 ..........


Any clues ?
-- 
Sten Spans

"There is a crack in everything, that's how the light gets in."
Leonard Cohen - Anthem