[Qemu-devel] cpu_exit every time for a jump tb (containing only the "b" instruction)

[阮安邦]

Hi, list

       I am currently profiling qemu-system-arm. I wrote a simple ARM assembly only to add r0 infinitely:

    add_nums:
            add r0, r0, r1      
            b add_nums

       I forced a tb to contain only one instruction, and I got cpu_exit every time the tb for "b add_nums" was executed. It exited the cpu_exec loop every time, and then reentered it and executed it without cpu_exit.

       Following are the executing sequences. I dumped the "in_asm" for a tb every time it was executed. And wrapped the tcg_qemu_tb_exec() with qemu_log().

IN:
0x8001000c:  b    0x80010008

before tcg_qemu_tb_exec
cpu_exit
after tcg_qemu_tb_exec

IN:
0x8001000c:  b    0x80010008

before tcg_qemu_tb_exec
after tcg_qemu_tb_exec

IN:
0x80010008:  add    r0, r0, r1

before tcg_qemu_tb_exec
after tcg_qemu_tb_exec


IN:
0x8001000c:  b    0x80010008

before tcg_qemu_tb_exec
cpu_exit
after tcg_qemu_tb_exec

      It seems like the cpu_exit was called when  tcg_qemu_tb_exec was executing. However,   tcg_qemu_tb_exec is only to execute the translated native code (am i right?), how can it call the qemu func (cpu_exit)? Or it is called by other func executing simultaneously with   tcg_qemu_tb_exec?  And why cpu_exit is called everytime, it really comsumes lots of ticks.~

      Any suggestions? Great thanks!


Regards,
Ryan