In host Linux shell, run below command to start gdb first.

[zsun@ubuntu:~]↥ % gdb-multiarch ~/workspace/code/linux/vmlinux
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Reading symbols from /home/zsun/workspace/code/linux/vmlinux...done.
(gdb) set architecture arm
The target architecture is assumed to be arm
(gdb) target remote localhost:1234
Remote debugging using localhost:1234

Start Qemu. Please note that “-s -S” is used for debugging. -s Shorthand for -gdb tcp::1234, i.e. open a gdbserver on TCP port 1234. -S Do not start CPU at startup (you must type ‘c’ in the monitor).

qemu-system-arm -M vexpress-a9 -kernel zImage -append "console=tty1 root=/dev/nfs nfsroot= rw ip=dhcp init=/sbin/init" -s -S

Then you can debug your kernel.