Hi, all I am trying to figure out why QEMU put some constraints on block linking (chaining). Take x86 as an example, there are two places put constraints on block linking, gen_goto_tb and cpu_exec.
----------------- gen_goto_tb (target-i386/translate.c) --------------- /* NOTE: we handle the case where the TB spans two pages here */ if ((pc & TARGET_PAGE_MASK) == (tb->pc & TARGET_PAGE_MASK) || (pc & TARGET_PAGE_MASK) == ((s->pc - 1) & TARGET_PAGE_MASK)) { /* jump to same page: we can use a direct jump */ tcg_gen_goto_tb(tb_num); gen_jmp_im(eip); tcg_gen_exit_tb((tcg_target_long)tb + tb_num); } else { /* jump to another page: currently not optimized */ gen_jmp_im(eip); gen_eob(s); } ----------------------------------------------------------------------- ----------------------- cpu_exec (cpu-exec.c) ------------------------- /* see if we can patch the calling TB. When the TB spans two pages, we cannot safely do a direct jump. */ if (next_tb != 0 && tb->page_addr[1] == -1) { tb_add_jump((TranslationBlock *)(next_tb & ~3), next_tb & 3, tb); } ----------------------------------------------------------------------- Is it just because we cannot optimize block linking which crosses page boundary, or there are some correctness/safety issues should be considered? I did some experiments myself. First, I removed the if-else condition in gen_goto_tb (always go to if branch) and leave cpu_exec alone. In this case, user mode works fine, but system mode crashes while booting linux. Then, I removed the "tb->page_addr[1]" check and leave gen_goto_tb alone. This time, both user mode and system mode works fine. I use the disk image and user mode tests downloaded from the website as the test case. Could someone kindly explain why there are constraints on block linking? 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