cpu-exec: update icount after each TB_EXIT

There is no particular reason we shouldn't update the global system
icount time as we exit each TranslationBlock run. This ensures the
main-loop doesn't have to wait until we exit to the outer loop for
executed instructions to be credited to timer_state.

The prepare_icount_for_run function is slightly tweaked to match the
logic we run in cpu_loop_exec_tb.

Based on Paolo's original suggestion.

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>

cpu-exec.c

@@ -600,13 +600,13 @@ static inline void cpu_loop_exec_tb(CPUState *cpu, TranslationBlock *tb,
     /* Instruction counter expired.  */
     assert(use_icount);
 #ifndef CONFIG_USER_ONLY
-    if (cpu->icount_extra) {
-        /* Refill decrementer and continue execution.  */
-        cpu->icount_extra += insns_left;
-        insns_left = MIN(0xffff, cpu->icount_extra);
-        cpu->icount_extra -= insns_left;
-        cpu->icount_decr.u16.low = insns_left;
-    } else {
+    /* Ensure global icount has gone forward */
+    cpu_update_icount(cpu);
+    /* Refill decrementer and continue execution.  */
+    insns_left = MIN(0xffff, cpu->icount_budget);
+    cpu->icount_decr.u16.low = insns_left;
+    cpu->icount_extra = cpu->icount_budget - insns_left;
+    if (!cpu->icount_extra) {
         /* Execute any remaining instructions, then let the main loop
          * handle the next event.
          */

cpus.c

@@ -1211,8 +1211,7 @@ static void handle_icount_deadline(void)
 static void prepare_icount_for_run(CPUState *cpu)
 {
     if (use_icount) {
-        int64_t count;
-        int decr;
+        int insns_left;
 
         /* These should always be cleared by process_icount_data after
          * each vCPU execution. However u16.high can be raised
@@ -1221,17 +1220,10 @@ static void prepare_icount_for_run(CPUState *cpu)
         g_assert(cpu->icount_decr.u16.low == 0);
         g_assert(cpu->icount_extra == 0);
 
-
-        count = tcg_get_icount_limit();
-
-        /* To calculate what we have executed so far we need to know
-         * what we originally budgeted to run this cycle */
-        cpu->icount_budget = count;
-
-        decr = (count > 0xffff) ? 0xffff : count;
-        count -= decr;
-        cpu->icount_decr.u16.low = decr;
-        cpu->icount_extra = count;
+        cpu->icount_budget = tcg_get_icount_limit();
+        insns_left = MIN(0xffff, cpu->icount_budget);
+        cpu->icount_decr.u16.low = insns_left;
+        cpu->icount_extra = cpu->icount_budget - insns_left;
     }
 }