KVM: add missing memory barrier in kvm_{make,check}_request

kvm_make_request and kvm_check_request imply a producer-consumer
relationship; add implicit memory barriers to them. There was indeed
already a place that was adding an explicit smp_mb() to order between
kvm_check_request and the processing of the request. That memory
barrier can be removed (as an added benefit, kvm_check_request can use
smp_mb__after_atomic which is free on x86).

Signed-off-by: Paolo Bonzini <[e-mail]>

arch/x86/kvm/irq_comm.c

···
 	u32 i, nr_ioapic_pins;
 	int idx;
 
-	/* kvm->irq_routing must be read after clearing
-	 * KVM_SCAN_IOAPIC. */
-	smp_mb();
 	idx = srcu_read_lock(&kvm->irq_srcu);
 	table = srcu_dereference(kvm->irq_routing, &kvm->irq_srcu);
 	nr_ioapic_pins = min_t(u32, table->nr_rt_entries,

include/linux/kvm_host.h

···
 
 static inline void kvm_make_request(int req, struct kvm_vcpu *vcpu)
 {
+	/*
+	 * Ensure the rest of the request is published to kvm_check_request's
+	 * caller.  Paired with the smp_mb__after_atomic in kvm_check_request.
+	 */
+	smp_wmb();
 	set_bit(req, &vcpu->requests);
 }
 
···
 {
 	if (test_bit(req, &vcpu->requests)) {
 		clear_bit(req, &vcpu->requests);
+
+		/*
+		 * Ensure the rest of the request is visible to kvm_check_request's
+		 * caller.  Paired with the smp_wmb in kvm_make_request.
+		 */
+		smp_mb__after_atomic();
 		return true;
 	} else {
 		return false;