[Qemu-devel] [RFC v4 14/28] tcg: add kick timer for single-threaded vCPU emulation

["Alex Bennée" <alex.bennee@linaro.org>]

Currently we rely on the side effect of the main loop grabbing the
iothread_mutex to give any long running basic block chains a kick to
ensure the next vCPU is scheduled. As this code is being re-factored and
rationalised we now do it explicitly here.

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

---
v2
  - re-base fixes
  - get_ticks_per_sec() -> NANOSECONDS_PER_SEC
v3
  - add define for TCG_KICK_FREQ
  - fix checkpatch warning
v4
  - wrap next calc in inline qemu_tcg_next_kick() instead of macro
---
 cpus.c | 32 ++++++++++++++++++++++++++++++++
 1 file changed, 32 insertions(+)

diff --git a/cpus.c b/cpus.c
index b5b45b8..8c49d6c 100644
--- a/cpus.c
+++ b/cpus.c
@@ -1185,9 +1185,34 @@ static void deal_with_unplugged_cpus(void)
     }
 }
 
+/* Single-threaded TCG
+ *
+ * In the single-threaded case each vCPU is simulated in turn. If
+ * there is more than a single vCPU we create a simple timer to kick
+ * the vCPU and ensure we don't get stuck in a tight loop in one vCPU.
+ * This is done explicitly rather than relying on side-effects
+ * elsewhere.
+ */
+static void qemu_cpu_kick_no_halt(void);
+
+#define TCG_KICK_PERIOD (NANOSECONDS_PER_SECOND / 10)
+
+static inline int64_t qemu_tcg_next_kick(void)
+{
+    return qemu_clock_get_ns(QEMU_CLOCK_VIRTUAL) + TCG_KICK_PERIOD;
+}
+
+static void kick_tcg_thread(void *opaque)
+{
+    QEMUTimer *self = *(QEMUTimer **) opaque;
+    timer_mod(self, qemu_tcg_next_kick());
+    qemu_cpu_kick_no_halt();
+}
+
 static void *qemu_tcg_cpu_thread_fn(void *arg)
 {
     CPUState *cpu = arg;
+    QEMUTimer *kick_timer;
 
     rcu_register_thread();
 
@@ -1211,6 +1236,13 @@ static void *qemu_tcg_cpu_thread_fn(void *arg)
         }
     }
 
+    /* Set to kick if we have to do more than one vCPU */
+    if (CPU_NEXT(first_cpu)) {
+        kick_timer = timer_new_ns(QEMU_CLOCK_VIRTUAL,  kick_tcg_thread,
+                                  &kick_timer);
+        timer_mod(kick_timer, qemu_tcg_next_kick());
+    }
+
     /* process any pending work */
     atomic_mb_set(&exit_request, 1);
 
-- 
2.7.4