Re: [Qemu-devel] icount and tb chaining

[陳韋任]

> previously returned tb. This is usually 0/1. When running icount, you
> can also get a 2 value in these least significant digits, indicating
> that the translation block was restarted due to the
> icount_decr.u16.low field being exhausted but having instructions left
> to execute in icount_extra. This 2 value falls through to tb_add_jump,
> which then updates the tb's jmp_first field, as both tb and next_tb
> refer to the same translation block. My question is why is this
> necessary, why not do nothing, and leave the previous chaining intact?
> I hope this is clearer and thanks for the response. -Jim

  I have a question here. Look at gen_icount_start and gen_icount_end in 
gen-icount.h, I think the least significant bits are set to 2 by following
code.

  tcg_gen_exit_tb((tcg_target_long)tb + 2);

And you said,

  "indicating that the translation block was restarted due to the
   icount_decr.u16.low field being exhausted but having instructions left
   to execute in icount_extra."

>From the code snipt of gen_icount_start below, I can only figure out something
(icount_decr.u32?) is exhausted so it will jump to label set by gen_icount_end
and execute "tcg_gen_exit_tb((tcg_target_long)tb + 2)". I only see icount_extra
and icount_decr.u16.low are dealt in cpu_exec (cpu-exec.c). Do I miss something
or misunderstand what you said? Thanks. 

---
    icount_label = gen_new_label();
    count = tcg_temp_local_new_i32();
    tcg_gen_ld_i32(count, cpu_env, offsetof(CPUState, icount_decr.u32));
    /* This is a horrid hack to allow fixing up the value later.  */
    icount_arg = gen_opparam_ptr + 1;
    tcg_gen_subi_i32(count, count, 0xdeadbeef);

    tcg_gen_brcondi_i32(TCG_COND_LT, count, 0, icount_label);
---

Regards,
chenwj

-- 
Wei-Ren Chen (陳韋任)
Computer Systems Lab, Institute of Information Science,
Academia Sinica, Taiwan (R.O.C.)
Tel:886-2-2788-3799 #1667
Homepage: http://people.cs.nctu.edu.tw/~chenwj