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

[Weinan Liu]

From: Weinan Liu <wnliu@stu.xmu.edu.cn>

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 <wnliu@stu.xmu.edu.cn>
---
 accel/kvm/kvm-all.c | 9 +++++++++
 1 file changed, 9 insertions(+)

diff --git a/accel/kvm/kvm-all.c b/accel/kvm/kvm-all.c
index 7e6a6076b1..840da7630e 100644
--- a/accel/kvm/kvm-all.c
+++ b/accel/kvm/kvm-all.c
@@ -719,6 +719,15 @@ static uint64_t kvm_dirty_ring_reap_locked(KVMState *s, 
CPUState* cpu)
         total = kvm_dirty_ring_reap_one(s, cpu);
     } else {
         CPU_FOREACH(cpu) {
+            /*
+             * Must ensure kvm_init_vcpu is finished, so cpu->kvm_dirty_gfns is
+             * available.
+             */
+            while (cpu->created == false) {
+                qemu_mutex_unlock_iothread();
+                qemu_mutex_lock_iothread();
+            }
+
             total += kvm_dirty_ring_reap_one(s, cpu);
         }
     }
-- 
2.25.1