kvm: dirty-ring: Fix race with vcpu creation

It's possible that we want to reap a dirty ring on a vcpu that is during
creation, because the vcpu is put onto list (CPU_FOREACH visible) before
initialization of the structures.  In this case:

qemu_init_vcpu
    x86_cpu_realizefn
        cpu_exec_realizefn
            cpu_list_add      <---- can be probed by CPU_FOREACH
        qemu_init_vcpu
            cpus_accel->create_vcpu_thread(cpu);
                kvm_init_vcpu
                    map kvm_dirty_gfns  <--- kvm_dirty_gfns valid

Don't try to reap dirty ring on vcpus during creation or it'll crash.

Bugzilla: https://bugzilla.redhat.com/show_bug.cgi?id=2124756
Reported-by: Xiaohui Li <xiaohli@redhat.com>
Signed-off-by: Peter Xu <peterx@redhat.com>
Message-Id: <1d14deb6684bcb7de1c9633c5bd21113988cc698.1676563222.git.huangy81@chinatelecom.cn>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

accel/kvm/kvm-all.c

@@ -685,6 +685,15 @@ static uint32_t kvm_dirty_ring_reap_one(KVMState *s, CPUState *cpu)
     uint32_t ring_size = s->kvm_dirty_ring_size;
     uint32_t count = 0, fetch = cpu->kvm_fetch_index;
 
+    /*
+     * It's possible that we race with vcpu creation code where the vcpu is
+     * put onto the vcpus list but not yet initialized the dirty ring
+     * structures.  If so, skip it.
+     */
+    if (!cpu->created) {
+        return 0;
+    }
+
     assert(dirty_gfns && ring_size);
     trace_kvm_dirty_ring_reap_vcpu(cpu->cpu_index);