include/exec/exec-all: document common exit conditions

As a precursor to later patches attempt to come up with a more
concrete wording for what each of the common exit cases would be.

CC: Emilio G. Cota <cota@braap.org>
CC: Richard Henderson <rth@twiddle.net>
CC: Lluís Vilanova <vilanova@ac.upc.edu>
Signed-off-by: NAlex Bennée <alex.bennee@linaro.org>
Reviewed-by: NRichard Henderson <rth@twiddle.net>
Message-id: 20170713141928.25419-2-alex.bennee@linaro.org
Signed-off-by: NPeter Maydell <peter.maydell@linaro.org>

include/exec/exec-all.h

@@ -35,11 +35,34 @@ typedef abi_ulong tb_page_addr_t;
 typedef ram_addr_t tb_page_addr_t;
 #endif
 
-/* is_jmp field values */
+/* DisasContext is_jmp field values
+ *
+ * is_jmp starts as DISAS_NEXT. The translator will keep processing
+ * instructions until an exit condition is reached. If we reach the
+ * exit condition and is_jmp is still DISAS_NEXT (because of some
+ * other condition) we simply "jump" to the next address.
+ * The remaining exit cases are:
+ *
+ *   DISAS_JUMP    - Only the PC was modified dynamically (e.g computed)
+ *   DISAS_TB_JUMP - Only the PC was modified statically (e.g. branch)
+ *
+ * In these cases as long as the PC is updated we can chain to the
+ * next TB either by exiting the loop or looking up the next TB via
+ * the loookup helper.
+ *
+ *   DISAS_UPDATE  - CPU State was modified dynamically
+ *
+ * This covers any other CPU state which necessities us exiting the
+ * TCG code to the main run-loop. Typically this includes anything
+ * that might change the interrupt state.
+ *
+ * Individual translators may define additional exit cases to deal
+ * with per-target special conditions.
+ */
 #define DISAS_NEXT    0 /* next instruction can be analyzed */
 #define DISAS_JUMP    1 /* only pc was modified dynamically */
-#define DISAS_UPDATE  2 /* cpu state was modified dynamically */
-#define DISAS_TB_JUMP 3 /* only pc was modified statically */
+#define DISAS_TB_JUMP 2 /* only pc was modified statically */
+#define DISAS_UPDATE  3 /* cpu state was modified dynamically */
 
 #include "qemu/log.h"