I meet this problem when finishing the lab of my OS course. We are trying to implement a kernel with the function of system call (platform: QEMU/i386). When testing the kernel, problem occurred that after kernel load user program to memory and change the CPU state from kernel mode to user mode using 'iret' instruction, CPU works in a strange way as following.
Your guest has probably ended up executing a block of zeroed out memory. In i386, zeroed memory disassembles to a succession of "add BYTE PTR [rax],al" instructions, each of which is two bytes long (0x00 0x00), and if rax happens to point to memory which reads as zeroes, this will effectively be a 2-byte-insn no-op, which corresponds to what you are seeing. This might happen because you set up the iret incorrectly and it isn't returning to the address you expected, or because you've got the MMU setup wrong and the userspace program isn't in the memory where you expect it to be, for instance.
You could confirm this theory using QEMU's debug options (eg -d in_asm,cpu,exec,int,unimp,guest_errors -D qemu.log will log a lot of execution information to a file), which should (among a lot of other data) show you what instructions it is actually executing.