KVM: Do not report TPR write to userspace if new value bigger or equal to a previous one.

Saves many exits to userspace in a case of IRQ chip in userspace.
Signed-off-by: Gleb Natapov <gleb@redhat.com>
Signed-off-by: Avi Kivity <avi@redhat.com>

arch/x86/kvm/svm.c

@@ -1862,9 +1862,13 @@ static int emulate_on_interception(struct vcpu_svm *svm,
 
 static int cr8_write_interception(struct vcpu_svm *svm, struct kvm_run *kvm_run)
 {
+	u8 cr8_prev = kvm_get_cr8(&svm->vcpu);
+	/* instruction emulation calls kvm_set_cr8() */
 	emulate_instruction(&svm->vcpu, NULL, 0, 0, 0);
 	if (irqchip_in_kernel(svm->vcpu.kvm))
 		return 1;
+	if (cr8_prev <= kvm_get_cr8(&svm->vcpu))
+		return 1;
 	kvm_run->exit_reason = KVM_EXIT_SET_TPR;
 	return 0;
 }

arch/x86/kvm/vmx.c

@@ -2724,13 +2724,18 @@ static int handle_cr(struct kvm_vcpu *vcpu, struct kvm_run *kvm_run)
 			kvm_set_cr4(vcpu, kvm_register_read(vcpu, reg));
 			skip_emulated_instruction(vcpu);
 			return 1;
-		case 8:
-			kvm_set_cr8(vcpu, kvm_register_read(vcpu, reg));
-			skip_emulated_instruction(vcpu);
-			if (irqchip_in_kernel(vcpu->kvm))
-				return 1;
-			kvm_run->exit_reason = KVM_EXIT_SET_TPR;
-			return 0;
+		case 8: {
+				u8 cr8_prev = kvm_get_cr8(vcpu);
+				u8 cr8 = kvm_register_read(vcpu, reg);
+				kvm_set_cr8(vcpu, cr8);
+				skip_emulated_instruction(vcpu);
+				if (irqchip_in_kernel(vcpu->kvm))
+					return 1;
+				if (cr8_prev <= cr8)
+					return 1;
+				kvm_run->exit_reason = KVM_EXIT_SET_TPR;
+				return 0;
+			}
 		};
 		break;
 	case 2: /* clts */