On 24/03/16 16:42, Alex Bennée wrote: >> diff --git a/include/exec/exec-all.h b/include/exec/exec-all.h >> > index 05a151da4a54..cc3d2ca25917 100644 >> > --- a/include/exec/exec-all.h >> > +++ b/include/exec/exec-all.h >> > @@ -257,20 +257,32 @@ struct TranslationBlock { >> > struct TranslationBlock *page_next[2]; >> > tb_page_addr_t page_addr[2]; >> > >> > - /* the following data are used to directly call another TB from >> > - the code of this one. */ >> > - uint16_t tb_next_offset[2]; /* offset of original jump target */ >> > + /* The following data are used to directly call another TB from >> > + * the code of this one. This can be done either by emitting direct or >> > + * indirect native jump instructions. These jumps are reset so that >> > the TB >> > + * just continue its execution. The TB can be linked to another one by >> > + * setting one of the jump targets (or patching the jump >> > instruction). Only >> > + * two of such jumps are supported. >> > + */ >> > + uint16_t jmp_reset_offset[2]; /* offset of original jump target */ >> > +#define TB_JMP_RESET_OFFSET_INVALID 0xffff /* indicates no jump generated >> > */ >> > #ifdef USE_DIRECT_JUMP >> > - uint16_t tb_jmp_offset[2]; /* offset of jump instruction */ >> > + uint16_t jmp_insn_offset[2]; /* offset of native jump instruction */ >> > #else >> > - uintptr_t tb_next[2]; /* address of jump generated code */ >> > + uintptr_t jmp_target_addr[2]; /* target address for indirect jump */ >> > #endif >> > - /* list of TBs jumping to this one. This is a circular list using >> > - the two least significant bits of the pointers to tell what is >> > - the next pointer: 0 = jmp_next[0], 1 = jmp_next[1], 2 = >> > - jmp_first */ >> > - struct TranslationBlock *jmp_next[2]; >> > - struct TranslationBlock *jmp_first; >> > + /* Each TB has an assosiated circular list of TBs jumping to this one. >> > + * jmp_list_first points to the first TB jumping to this one. >> > + * jmp_list_next is used to point to the next TB in a list. >> > + * Since each TB can have two jumps, it can participate in two lists. >> > + * The two least significant bits of a pointer are used to choose >> > which >> > + * data field holds a pointer to the next TB: >> > + * 0 => jmp_list_next[0], 1 => jmp_list_next[1], 2 => jmp_list_first. >> > + * In other words, 0/1 tells which jump is used in the pointed TB, >> > + * and 2 means that this is a pointer back to the target TB of this >> > list. >> > + */ >> > + struct TranslationBlock *jmp_list_next[2]; >> > + struct TranslationBlock *jmp_list_first; > OK I found that tricky to follow. Where does the value of the pointer > come from that sets these bottom bits? The TB jumping to this TB sets it?
Yeah, that's not easy to describe. Initially, we set: tb->jmp_list_first = tb | 2 That makes an empty list: jmp_list_first just points to the this TB and the low bits are 2. After that we can add a TB to the list in tb_add_jump(): tb->jmp_list_next[n] = tb_next->jmp_list_first; tb_next->jmp_list_first = tb | n; where 'tb' is going to jump to 'tb_next', 'n' (can be 0 or 1) is an index of jump target of 'tb'. (I simplified the code here) Any ideas how to make it more clear in the comment? Kind regards, Sergey