how to debug the QEMU quit without any backtrace at the breakpoint.

how to debug the QEMU quit without any backtrace at the breakpoint.

breakpoints -- Making program stop at certain points

1.launch a KVM guest with gdb.
# gdb /usr/libexec/qemu-kvm

2.set the breakpoint at exit.
(gdb) b exit
Breakpoint 1 at 0xaaf30

3.start the program.
(gdb) r -M pc -cpu host -enable-kvm -m 2048 -smp 2,sockets=2,cores=1,threads=1 -no-kvm-pit-reinjection -usb -device usb-tablet,id=input0 -name sluo -uuid 990ea161-6b67-47b2-b803-19fb01d30d30 -rtc base=localtime,clock=host,driftfix=slew...-monitor stdio

(qemu) [Thread 0x7fffeabf7700 (LWP 31736) exited]
[New Thread 0x7fffeabf7700 (LWP 31741)]

Breakpoint 1, 0x00007ffff2c9a250 in exit () from /lib64/libc.so.6

4.program received signals and got the backtrace.
(gdb) bt
#0  0x00007ffff2c9a250 in exit () from /lib64/libc.so.6
#1  0x0000555555795fe2 in virtqueue_num_heads (vq=0x55555811db48, idx=0)
    at /usr/src/debug/qemu-1.5.3/hw/virtio/virtio.c:291
#2  0x0000555555796673 in virtqueue_pop (vq=vq@entry=0x55555811db48, elem=elem@entry=0x55555767ec50)
    at /usr/src/debug/qemu-1.5.3/hw/virtio/virtio.c:441
......
#27 main (argc=<optimized out>, argv=<optimized out>, envp=<optimized out>) at vl.c:4360
(gdb)