[PATCH v2] KVM: dirty ring: check if vcpu is created before dirty_ring_reap_one

[Weinan Liu]

Failed to assert '(dirty_gfns && ring_size)' in kvm_dirty_ring_reap_one if
the vcpu has not been finished to create yet. This bug occasionally occurs
when I open 200+ qemu instances on my 16G 6-cores x86 machine. And it must
be triggered if inserting a 'sleep(10)' into kvm_vcpu_thread_fn as below--

 static void *kvm_vcpu_thread_fn(void *arg)
 {
     CPUState *cpu = arg;
     int r;

     rcu_register_thread();

+    sleep(10);
     qemu_mutex_lock_iothread();
     qemu_thread_get_self(cpu->thread);
     cpu->thread_id = qemu_get_thread_id();
     cpu->can_do_io = 1;

where dirty ring reaper will wakeup but then a vcpu has not been finished
to create.

Signed-off-by: Weinan Liu <liu-weinan@qq.com>
---
 accel/kvm/kvm-all.c | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/accel/kvm/kvm-all.c b/accel/kvm/kvm-all.c
index 7e6a6076b1..0070ad72b8 100644
--- a/accel/kvm/kvm-all.c
+++ b/accel/kvm/kvm-all.c
@@ -1416,6 +1416,11 @@ static void *kvm_dirty_ring_reaper_thread(void *data)
          */
         sleep(1);
 
+        /* ensure kvm_init_vcpu is finished, so cpu->kvm_dirty_gfns is ok */
+        if (!phase_check(PHASE_MACHINE_READY)) {
+            continue;
+        }
+
         /* keep sleeping so that dirtylimit not be interfered by reaper */
         if (dirtylimit_in_service()) {
             continue;
-- 
2.25.1