Re: [Qemu-devel] tcg: improve MAX_CODE_GEN_BUFFER_SIZE for arm

[Aurelien Jarno <aurelien@aurel32.net>]

On 2015-12-08 10:43, TeLeMan wrote:
> I know MAX_CODE_GEN_BUFFER_SIZE is limited by the host direct branch
> instructions.But the arm's MAX_CODE_GEN_BUFFER_SIZE is so small.I
> tried improving MAX_CODE_GEN_BUFFER_SIZE.I wrote some check codes for
> the overflow offset in tcg_out_b(), tcg_out_bl(),
> tcg_out_blx_imm(),reloc_pc24(). But I didn't catch any overflow case
> when tb_size and MAX_CODE_GEN_BUFFER_SIZE were larger than 32MB. After
> the generated code size was larger than 32MB, qemu crashed.

Instrumenting all the tcg_out_* branch related functions do not work
here as the address is actually not known at code generation:

    case INDEX_op_goto_tb:
        if (s->tb_jmp_offset) {
            /* Direct jump method */
            s->tb_jmp_offset[args[0]] = tcg_current_code_size(s);
            tcg_out_b_noaddr(s, COND_AL);

It is patched later during TB linking.

> Any suggest for this issue?

I already posted a patch a long time ago to remove the 16MB limit on ARM
hosts:

http://lists.gnu.org/archive/html/qemu-devel/2012-10/msg01684.html

However as you can see in the thread, it has been rejected as it doesn't
not bring improvement in all cases.

Aurelien

-- 
Aurelien Jarno                          GPG: 4096R/1DDD8C9B
aurelien@aurel32.net                 http://www.aurel32.net