[Qemu-devel] TB chaining in QEMU

[Xin Tong]

It seems to me that when QEMU emits a TB to TB transition, it does not look for whether the code has already been generated or not ( at least x86 on x86 emulation) . it just lay down a 4 byte address, waiting to be patched later. Am I right ?


    case INDEX_op_goto_tb:
        if (s->tb_jmp_offset) {
            /* direct jump method */
            /* need to make sure that the jmp offset does not cross 32 byte boundary on Intel chip
             * and 8 byte boundary on AMD chip. As qemu is not checking for processor type. Assume
             * 8 byte boundary to be safe */
            tcg_out8(s, OPC_JMP_long); /* jmp im */
            s->tb_jmp_offset[args[0]] = s->code_ptr - s->code_buf;
            tcg_out32(s, 0);
        } else {
            /* indirect jump method */
            tcg_out_modrm_offset(s, OPC_GRP5, EXT5_JMPN_Ev, -1,
                                 (tcg_target_long)(s->tb_next + args[0]));
        }
        s->tb_next_offset[args[0]] = s->code_ptr - s->code_buf;
        break;


Thanks 

Xin