vl: introduce vm_shutdown()

Commit 00d09fdbbae5f7864ce754913efc84c12fdf9f1a ("vl: pause vcpus before
stopping iothreads") and commit dce8921b2baaf95974af8176406881872067adfa
("iothread: Stop threads before main() quits") tried to work around the
fact that emulation was still active during termination by stopping
iothreads.  They suffer from race conditions:
1. virtio_scsi_handle_cmd_vq() racing with iothread_stop_all() hits the
   virtio_scsi_ctx_check() assertion failure because the BDS AioContext
   has been modified by iothread_stop_all().
2. Guest vq kick racing with main loop termination leaves a readable
   ioeventfd that is handled by the next aio_poll() when external
   clients are enabled again, resulting in unwanted emulation activity.

This patch obsoletes those commits by fully disabling emulation activity
when vcpus are stopped.

Use the new vm_shutdown() function instead of pause_all_vcpus() so that
vm change state handlers are invoked too.  Virtio devices will now stop
their ioeventfds, preventing further emulation activity after vm_stop().

Note that vm_stop(RUN_STATE_SHUTDOWN) cannot be used because it emits a
QMP STOP event that may affect existing clients.

It is no longer necessary to call replay_disable_events() directly since
vm_shutdown() does so already.

Drop iothread_stop_all() since it is no longer used.

Cc: Fam Zheng <famz@redhat.com>
Cc: Kevin Wolf <kwolf@redhat.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: Fam Zheng <famz@redhat.com>
Acked-by: Paolo Bonzini <pbonzini@redhat.com>
Message-id: 20180307144205.20619-5-stefanha@redhat.com
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>

1 files changed, 13 insertions, 3 deletions

diff --git a/cpus.c b/cpus.c
index 9bcff7d63c..d8fe90eafe 100644
--- a/cpus.c
+++ b/cpus.c
@@ -993,7 +993,7 @@ void cpu_synchronize_all_pre_loadvm(void)
     }
 }
 
-static int do_vm_stop(RunState state)
+static int do_vm_stop(RunState state, bool send_stop)
 {
     int ret = 0;
 
@@ -1002,7 +1002,9 @@ static int do_vm_stop(RunState state)
         pause_all_vcpus();
         runstate_set(state);
         vm_state_notify(0, state);
-        qapi_event_send_stop(&error_abort);
+        if (send_stop) {
+            qapi_event_send_stop(&error_abort);
+        }
     }
 
     bdrv_drain_all();
@@ -1012,6 +1014,14 @@ static int do_vm_stop(RunState state)
     return ret;
 }
 
+/* Special vm_stop() variant for terminating the process.  Historically clients
+ * did not expect a QMP STOP event and so we need to retain compatibility.
+ */
+int vm_shutdown(void)
+{
+    return do_vm_stop(RUN_STATE_SHUTDOWN, false);
+}
+
 static bool cpu_can_run(CPUState *cpu)
 {
     if (cpu->stop) {
@@ -1994,7 +2004,7 @@ int vm_stop(RunState state)
         return 0;
     }
 
-    return do_vm_stop(state);
+    return do_vm_stop(state, true);
 }
 
 /**