$ gdb qemu-kvm
GNU gdb (GDB) 7.4.1
Copyright (C) 2012 Free Software Foundation, Inc.
License GPLv3+: GNU GPL version 3 or later <http://gnu.org/licenses/gpl.html>
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and "show warranty" for details.
This GDB was configured as "x86_64-unknown-linux-gnu".
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<http://www.gnu.org/software/gdb/bugs/>...
Reading symbols from /usr/bin/qemu-kvm...done.
(gdb) run -net nic,model=virtio -net user,hostfwd=tcp::7272-:22 arch.img
Starting program: /usr/bin/qemu-kvm -net nic,model=virtio -net user,hostfwd=tcp::7272-:22 arch.img
[Thread debugging using libthread_db enabled]
Using host libthread_db library "/lib/libthread_db.so.1".
[New Thread 0x7ffff041f700 (LWP 1182)]
[New Thread 0x7fffefc1e700 (LWP 1183)]

Program received signal SIGSEGV, Segmentation fault.
0x0000555555685960 in if_output (so=so@entry=0x0, ifm=ifm@entry=0x55555651e420) at slirp/if.c:80
80                      if (so == ifq->ifq_so) {
(gdb) bt
#0  0x0000555555685960 in if_output (so=so@entry=0x0, ifm=ifm@entry=0x55555651e420) at slirp/if.c:80
#1  0x0000555555686968 in ip_output (so=so@entry=0x0, m0=m0@entry=0x55555651e420) at slirp/ip_output.c:84
#2  0x000055555568e892 in udp_output2 (so=so@entry=0x0, m=m@entry=0x55555651e420, saddr=saddr@entry=
    0x7fffffff0f90, daddr=daddr@entry=0x7fffffff0fa0, iptos=iptos@entry=16) at slirp/udp.c:275
#3  0x000055555568533a in bootp_reply (bp=<optimized out>, slirp=<optimized out>) at slirp/bootp.c:309
#4  bootp_input (m=m@entry=0x555556514880) at slirp/bootp.c:317
#5  0x000055555568ecea in udp_input (m=0x555556514880, iphlen=<optimized out>) at slirp/udp.c:126
#6  0x00005555556892b0 in slirp_input (slirp=<optimized out>, pkt=0x7fffffff1110 "\377\377\377\377\377\377RT",
    pkt_len=pkt_len@entry=370) at slirp/slirp.c:688
#7  0x0000555555660870 in net_slirp_receive (nc=<optimized out>, buf=<optimized out>, size=370)
    at net/slirp.c:116
#8  0x000055555565df32 in vc_sendv_compat (vc=0x555556324300, iov=<optimized out>, iovcnt=<optimized out>)
    at net.c:543
#9  0x000055555565e175 in qemu_vlan_deliver_packet_iov (sender=0x5555563d5c10, flags=<optimized out>, iov=
    0x7fffffffa1e0, iovcnt=1, opaque=<optimized out>) at net.c:592
#10 0x000055555566061b in qemu_net_queue_deliver_iov (iovcnt=1, iov=0x7fffffffa1e0, flags=0, sender=
    0x5555563d5c10, queue=0x555556323e80) at net/queue.c:169
#11 qemu_net_queue_send_iov (queue=0x555556323e80, sender=0x5555563d5c10, flags=0, iov=0x7fffffffa1e0, iovcnt=
    1, sent_cb=<optimized out>) at net/queue.c:212
#12 0x00005555557885e1 in virtio_net_flush_tx (n=0x55555641d020, vq=0x5555564291a0)
    at /opt/arch/svn-packages/qemu-kvm/trunk/src/qemu-kvm-1.0.1/hw/virtio-net.c:750
#13 0x00005555557887e6 in virtio_net_tx_bh (opaque=0x55555641d020)
    at /opt/arch/svn-packages/qemu-kvm/trunk/src/qemu-kvm-1.0.1/hw/virtio-net.c:838
#14 0x00005555555c733f in qemu_bh_poll () at async.c:71
#15 0x000055555565b05f in main_loop_wait (nonblocking=<optimized out>) at main-loop.c:472
#16 0x00005555555c2049 in main_loop () at /opt/arch/svn-packages/qemu-kvm/trunk/src/qemu-kvm-1.0.1/vl.c:1482
#17 main (argc=<optimized out>, argv=<optimized out>, envp=<optimized out>)
    at /opt/arch/svn-packages/qemu-kvm/trunk/src/qemu-kvm-1.0.1/vl.c:3523