[Qemu-devel] Question about adding IBTC (Indirect Branch Translation Cache) into QEMU

[陳韋任 <chenwj@iis.sinica.edu.tw>]

Hi, all

  I want to add a binary tranlation optimization, called IBTC
(Indirect Branch Translation Cache) to QEMU. IBTC is a data
structure used to cache the mapping between guest IB (indirect
branch) and its corresponding translated code cache address.
If IBTC get a hit, then there is no need to jump back to QEMU,
next translated code cache can be excuted directly. An example
below. 

=------------------- target-i386/translation.c -------------------=
static target_ulong disas_insn(CPUState *env, DisasContext *s, target_ulong pc_start)
{
    // Place IBTC lookup in the following cases
    case 2: /* call Ev */
    case 4: /* jmp Ev */
    case 0xc2: /* ret im */
        /*
         *  generate IBTC lookup TCG IR in the code cache while
         *  encounting indirect branch (ret im)
         */
        if (s->cc_op != CC_OP_DYNAMIC)
            gen_op_set_cc_op(s->cc_op);
        TCGv_ptr ibtc_host_eip = tcg_temp_new_ptr();
        gen_helper_lookup_ibtc(ibtc_host_eip, cpu_T[0]);
        *gen_opc_ptr++ = INDEX_op_jmp;
        *gen_opparam_ptr++ = GET_TCGV_PTR(ibtc_host_eip);
        tcg_temp_free_ptr(ibtc_host_eip);

        gen_eob(s);
        break;
    case 0xc3: /* ret */
}
=------------------- target-i386/translation.c -------------------=

  IBTC works fine in the process (user) mode, but not in the system
mode. In process mode, since the IBTC allocated is big enough, I
don't need to invalidate an IBTC entry or flush the entire IBTC. I
suspect this is why IBTC doesn't work in the system mode. I spot a
few place to might be needed to call flush_ibtc() to flush the entire
IBTC, but I am not sure if there are the right place. Or I might miss
some place. The places I insert flush_ibtc are below,

-----
1. cpu-exec.c

int cpu_exec(CPUState *env1)
{
  if (tb_invalidated_flag) {
    next_tb = 0;
    flush_ibtc(); // flush IBTC
    tb_invalidated_flag = 0;
  }
}

2. exec.c

TranslationBlock *tb_gen_code(CPUState *env,
                              target_ulong pc, target_ulong cs_base,
                              int flags, int cflags)
{
  if (!tb) {
      /* flush must be done */
      tb_flush(env);
      flush_ibtc(); // flush IBTC
      /* cannot fail at this point */
      tb = tb_alloc(pc);
      /* Don't forget to invalidate previous TB info.  */
      tb_invalidated_flag = 1;
  }
}

3. exec.c

void tb_phys_invalidate(TranslationBlock *tb, tb_page_addr_t page_addr)
{
  tb_invalidated_flag = 1;

  /* remove the TB from the hash list */
  h = tb_jmp_cache_hash_func(tb->pc);
  for(env = first_cpu; env != NULL; env = env->next_cpu) {
      if (env->tb_jmp_cache[h] == tb)
          env->tb_jmp_cache[h] = NULL;
  }

  flush_ibtc(); // flush IBTC
}
-----

  Have I already considered all kind of situations? Or something else
I need to do to make IBTC work in system mode?

  Thanks!

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