KVM: nVMX: clean up posted interrupt descriptor try_cmpxchg

Rely on try_cmpxchg64 for re-reading the PID on failure, using READ_ONCE
only right before the first iteration.
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

arch/x86/kvm/vmx/posted_intr.c

@@ -34,7 +34,7 @@ static inline struct pi_desc *vcpu_to_pi_desc(struct kvm_vcpu *vcpu)
 	return &(to_vmx(vcpu)->pi_desc);
 }
 
-static int pi_try_set_control(struct pi_desc *pi_desc, u64 old, u64 new)
+static int pi_try_set_control(struct pi_desc *pi_desc, u64 *pold, u64 new)
 {
 	/*
 	 * PID.ON can be set at any time by a different vCPU or by hardware,
@@ -42,7 +42,7 @@ static int pi_try_set_control(struct pi_desc *pi_desc, u64 old, u64 new)
 	 * update must be retried with a fresh snapshot an ON change causes
 	 * the cmpxchg to fail.
 	 */
-	if (!try_cmpxchg64(&pi_desc->control, &old, new))
+	if (!try_cmpxchg64(&pi_desc->control, pold, new))
 		return -EBUSY;
 
 	return 0;
@@ -96,8 +96,9 @@ void vmx_vcpu_pi_load(struct kvm_vcpu *vcpu, int cpu)
 	if (!x2apic_mode)
 		dest = (dest << 8) & 0xFF00;
 
+	old.control = READ_ONCE(pi_desc->control);
 	do {
-		old.control = new.control = READ_ONCE(pi_desc->control);
+		new.control = old.control;
 
 		/*
 		 * Clear SN (as above) and refresh the destination APIC ID to
@@ -111,7 +112,7 @@ void vmx_vcpu_pi_load(struct kvm_vcpu *vcpu, int cpu)
 		 * descriptor was modified on "put" to use the wakeup vector.
 		 */
 		new.nv = POSTED_INTR_VECTOR;
-	} while (pi_try_set_control(pi_desc, old.control, new.control));
+	} while (pi_try_set_control(pi_desc, &old.control, new.control));
 
 	local_irq_restore(flags);
 
@@ -156,12 +157,12 @@ static void pi_enable_wakeup_handler(struct kvm_vcpu *vcpu)
 
 	WARN(pi_desc->sn, "PI descriptor SN field set before blocking");
 
+	old.control = READ_ONCE(pi_desc->control);
 	do {
-		old.control = new.control = READ_ONCE(pi_desc->control);
-
 		/* set 'NV' to 'wakeup vector' */
+		new.control = old.control;
 		new.nv = POSTED_INTR_WAKEUP_VECTOR;
-	} while (pi_try_set_control(pi_desc, old.control, new.control));
+	} while (pi_try_set_control(pi_desc, &old.control, new.control));
 
 	/*
 	 * Send a wakeup IPI to this CPU if an interrupt may have been posted