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[PULL,1/1] kvm: dirty-ring: Fix race with vcpu creation

Message ID 20230404164839.455941-2-pbonzini@redhat.com
State New
Headers show
Series [PULL,1/1] kvm: dirty-ring: Fix race with vcpu creation | expand

Commit Message

Paolo Bonzini April 4, 2023, 4:48 p.m. UTC 
From: Peter Xu <peterx@redhat.com>

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 | 9 +++++++++
 1 file changed, 9 insertions(+)
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Patch

diff --git a/accel/kvm/kvm-all.c b/accel/kvm/kvm-all.c
index f2a6ea6a68a2..cf3a88d90e92 100644
--- a/accel/kvm/kvm-all.c
+++ b/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);