cpus: kick all vCPUs when running thread=single

qemu_cpu_kick is used for a number of reasons including to indicate
there is work to be done. However when thread=single the old
qemu_cpu_kick_rr_cpu only advanced the vCPU to the next executing one
which can lead to a hang in the case that:

  a) the kick is from outside the vCPUs (e.g. iothread)
  b) the timers are paused (i.e. iothread calling run_on_cpu)

To avoid this lets split qemu_cpu_kick_rr into two functions. One for
the timer which continues to advance to the next timeslice and another
for all other kicks.

Message-Id: <20191001160426.26644-1-alex.bennee@linaro.org>
Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>

1 files changed, 18 insertions, 6 deletions

diff --git a/cpus.c b/cpus.c
index d2c61ff155..bee7209134 100644
--- a/cpus.c
+++ b/cpus.c
@@ -949,8 +949,8 @@ static inline int64_t qemu_tcg_next_kick(void)
     return qemu_clock_get_ns(QEMU_CLOCK_VIRTUAL) + TCG_KICK_PERIOD;
 }
 
-/* Kick the currently round-robin scheduled vCPU */
-static void qemu_cpu_kick_rr_cpu(void)
+/* Kick the currently round-robin scheduled vCPU to next */
+static void qemu_cpu_kick_rr_next_cpu(void)
 {
     CPUState *cpu;
     do {
@@ -961,6 +961,16 @@ static void qemu_cpu_kick_rr_cpu(void)
     } while (cpu != atomic_mb_read(&tcg_current_rr_cpu));
 }
 
+/* Kick all RR vCPUs */
+static void qemu_cpu_kick_rr_cpus(void)
+{
+    CPUState *cpu;
+
+    CPU_FOREACH(cpu) {
+        cpu_exit(cpu);
+    };
+}
+
 static void do_nothing(CPUState *cpu, run_on_cpu_data unused)
 {
 }
@@ -993,7 +1003,7 @@ void qemu_timer_notify_cb(void *opaque, QEMUClockType type)
 static void kick_tcg_thread(void *opaque)
 {
     timer_mod(tcg_kick_vcpu_timer, qemu_tcg_next_kick());
-    qemu_cpu_kick_rr_cpu();
+    qemu_cpu_kick_rr_next_cpu();
 }
 
 static void start_tcg_kick_timer(void)
@@ -1828,9 +1838,11 @@ void qemu_cpu_kick(CPUState *cpu)
 {
     qemu_cond_broadcast(cpu->halt_cond);
     if (tcg_enabled()) {
-        cpu_exit(cpu);
-        /* NOP unless doing single-thread RR */
-        qemu_cpu_kick_rr_cpu();
+        if (qemu_tcg_mttcg_enabled()) {
+            cpu_exit(cpu);
+        } else {
+            qemu_cpu_kick_rr_cpus();
+        }
     } else {
         if (hax_enabled()) {
             /*