KVM: X86: correct meaningless kvm_apicv_activated() check

After test_and_set_bit() for kvm->arch.apicv_inhibit_reasons, we will
always get false when calling kvm_apicv_activated() because it's sure
apicv_inhibit_reasons do not equal to 0.

What the code wants to do, is check whether APICv was *already* active
and if so skip the costly request; we can do this using cmpxchg.

Reported-by: Miaohe Lin <linmiaohe@huawei.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index a7cb85231330023a53dc518df8734b28cab27882..e54c6ad628a810a047e1754f8196f2ec5e11ec42 100644 (file)
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -8049,19 +8049,26 @@ EXPORT_SYMBOL_GPL(kvm_vcpu_update_apicv);
  */
 void kvm_request_apicv_update(struct kvm *kvm, bool activate, ulong bit)
 {
+       unsigned long old, new, expected;
+
        if (!kvm_x86_ops->check_apicv_inhibit_reasons ||
            !kvm_x86_ops->check_apicv_inhibit_reasons(bit))
                return;
 
-       if (activate) {
-               if (!test_and_clear_bit(bit, &kvm->arch.apicv_inhibit_reasons) ||
-                   !kvm_apicv_activated(kvm))
-                       return;
-       } else {
-               if (test_and_set_bit(bit, &kvm->arch.apicv_inhibit_reasons) ||
-                   kvm_apicv_activated(kvm))
-                       return;
-       }
+       old = READ_ONCE(kvm->arch.apicv_inhibit_reasons);
+       do {
+               expected = new = old;
+               if (activate)
+                       __clear_bit(bit, &new);
+               else
+                       __set_bit(bit, &new);
+               if (new == old)
+                       break;
+               old = cmpxchg(&kvm->arch.apicv_inhibit_reasons, expected, new);
+       } while (old != expected);
+
+       if (!!old == !!new)
+               return;
 
        trace_kvm_apicv_update_request(activate, bit);
        if (kvm_x86_ops->pre_update_apicv_exec_ctrl)