cpus: don't credit executed instructions before they have run

Outside of the vCPU thread icount time will only be tracked against
timers_state.qemu_icount. We no longer credit cycles until they have
completed the run. Inside the vCPU thread we adjust for passage of
time by looking at how many have run so far. This is only valid inside
the vCPU thread while it is running.

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

1 files changed, 19 insertions, 6 deletions

diff --git a/cpus.c b/cpus.c
index 6034b104c3..0ecb0b87f0 100644
--- a/cpus.c
+++ b/cpus.c
@@ -223,6 +223,15 @@ void qemu_tcg_configure(QemuOpts *opts, Error **errp)
     }
 }
 
+/* The current number of executed instructions is based on what we
+ * originally budgeted minus the current state of the decrementing
+ * icount counters in extra/u16.low.
+ */
+static int64_t cpu_get_icount_executed(CPUState *cpu)
+{
+    return cpu->icount_budget - (cpu->icount_decr.u16.low + cpu->icount_extra);
+}
+
 int64_t cpu_get_icount_raw(void)
 {
     int64_t icount;
@@ -234,7 +243,8 @@ int64_t cpu_get_icount_raw(void)
             fprintf(stderr, "Bad icount read\n");
             exit(1);
         }
-        icount -= (cpu->icount_decr.u16.low + cpu->icount_extra);
+        /* Take into account what has run */
+        icount += cpu_get_icount_executed(cpu);
     }
     return icount;
 }
@@ -1195,7 +1205,10 @@ static void prepare_icount_for_run(CPUState *cpu)
 
         count = tcg_get_icount_limit();
 
-        timers_state.qemu_icount += count;
+        /* 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;
@@ -1206,14 +1219,14 @@ static void prepare_icount_for_run(CPUState *cpu)
 static void process_icount_data(CPUState *cpu)
 {
     if (use_icount) {
-        /* Fold pending instructions back into the
-           instruction counter, and clear the interrupt flag.  */
-        timers_state.qemu_icount -= (cpu->icount_decr.u16.low
-                        + cpu->icount_extra);
+        /* Account for executed instructions */
+        timers_state.qemu_icount += cpu_get_icount_executed(cpu);
 
         /* Reset the counters */
         cpu->icount_decr.u16.low = 0;
         cpu->icount_extra = 0;
+        cpu->icount_budget = 0;
+
         replay_account_executed_instructions();
     }
 }