KVM: x86: Add lowest-priority support for vt-d posted-interrupts

Use vector-hashing to deliver lowest-priority interrupts for
VT-d posted-interrupts. This patch extends kvm_intr_is_single_vcpu()
to support lowest-priority handling.
Signed-off-by: Feng Wu <feng.wu@intel.com>
Reviewed-by: Radim Krčmář <rkrcmar@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

arch/x86/kvm/lapic.c

@@ -798,6 +798,20 @@ bool kvm_irq_delivery_to_apic_fast(struct kvm *kvm, struct kvm_lapic *src,
 	return ret;
 }
 
+/*
+ * This routine tries to handler interrupts in posted mode, here is how
+ * it deals with different cases:
+ * - For single-destination interrupts, handle it in posted mode
+ * - Else if vector hashing is enabled and it is a lowest-priority
+ *   interrupt, handle it in posted mode and use the following mechanism
+ *   to find the destinaiton vCPU.
+ *	1. For lowest-priority interrupts, store all the possible
+ *	   destination vCPUs in an array.
+ *	2. Use "guest vector % max number of destination vCPUs" to find
+ *	   the right destination vCPU in the array for the lowest-priority
+ *	   interrupt.
+ * - Otherwise, use remapped mode to inject the interrupt.
+ */
 bool kvm_intr_is_single_vcpu_fast(struct kvm *kvm, struct kvm_lapic_irq *irq,
 			struct kvm_vcpu **dest_vcpu)
 {
@@ -839,6 +853,33 @@ bool kvm_intr_is_single_vcpu_fast(struct kvm *kvm, struct kvm_lapic_irq *irq,
 		if (cid >= ARRAY_SIZE(map->logical_map))
 			goto out;
 
+		if (kvm_vector_hashing_enabled() &&
+				kvm_lowest_prio_delivery(irq)) {
+			int idx;
+			unsigned int dest_vcpus;
+
+			dest_vcpus = hweight16(bitmap);
+			if (dest_vcpus == 0)
+				goto out;
+
+			idx = kvm_vector_to_index(irq->vector, dest_vcpus,
+						  &bitmap, 16);
+
+			/*
+			 * We may find a hardware disabled LAPIC here, if that
+			 * is the case, print out a error message once for each
+			 * guest and return
+			 */
+			dst = map->logical_map[cid][idx];
+			if (!dst && !kvm->arch.disabled_lapic_found) {
+				kvm->arch.disabled_lapic_found = true;
+				printk(KERN_INFO
+					"Disabled LAPIC found during irq injection\n");
+				goto out;
+			}
+
+			*dest_vcpu = dst->vcpu;
+		} else {
 			for_each_set_bit(i, &bitmap, 16) {
 				dst = map->logical_map[cid][i];
 				if (++r == 2)
@@ -850,6 +891,7 @@ bool kvm_intr_is_single_vcpu_fast(struct kvm *kvm, struct kvm_lapic_irq *irq,
 			else
 				goto out;
 		}
+	}
 
 	ret = true;
 out: