KVM: x86/xen: Support per-vCPU event channel upcall via local APIC

Windows uses a per-vCPU vector, and it's delivered via the local APIC
basically like an MSI (with associated EOI) unlike the traditional
guest-wide vector which is just magically asserted by Xen (and in the
KVM case by kvm_xen_has_interrupt() / kvm_cpu_get_extint()).

Now that the kernel is able to raise event channel events for itself,
being able to do so for Windows guests is also going to be useful.
Signed-off-by: David Woodhouse <dwmw@amazon.co.uk>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Message-Id: <20220303154127.202856-15-dwmw2@infradead.org>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

arch/x86/include/asm/kvm_host.h

@@ -606,6 +606,7 @@ struct kvm_vcpu_hv {
 struct kvm_vcpu_xen {
 	u64 hypercall_rip;
 	u32 current_runstate;
+	u8 upcall_vector;
 	struct gfn_to_pfn_cache vcpu_info_cache;
 	struct gfn_to_pfn_cache vcpu_time_info_cache;
 	struct gfn_to_pfn_cache runstate_cache;

arch/x86/kvm/xen.c

@@ -314,6 +314,22 @@ void kvm_xen_update_runstate_guest(struct kvm_vcpu *v, int state)
 	mark_page_dirty_in_slot(v->kvm, gpc->memslot, gpc->gpa >> PAGE_SHIFT);
 }
 
+static void kvm_xen_inject_vcpu_vector(struct kvm_vcpu *v)
+{
+	struct kvm_lapic_irq irq = { };
+	int r;
+
+	irq.dest_id = v->vcpu_id;
+	irq.vector = v->arch.xen.upcall_vector;
+	irq.dest_mode = APIC_DEST_PHYSICAL;
+	irq.shorthand = APIC_DEST_NOSHORT;
+	irq.delivery_mode = APIC_DM_FIXED;
+	irq.level = 1;
+
+	/* The fast version will always work for physical unicast */
+	WARN_ON_ONCE(!kvm_irq_delivery_to_apic_fast(v->kvm, NULL, &irq, &r, NULL));
+}
+
 /*
  * On event channel delivery, the vcpu_info may not have been accessible.
  * In that case, there are bits in vcpu->arch.xen.evtchn_pending_sel which
@@ -374,6 +390,10 @@ void kvm_xen_inject_pending_events(struct kvm_vcpu *v)
 	}
 	read_unlock_irqrestore(&gpc->lock, flags);
 
+	/* For the per-vCPU lapic vector, deliver it as MSI. */
+	if (v->arch.xen.upcall_vector)
+		kvm_xen_inject_vcpu_vector(v);
+
 	mark_page_dirty_in_slot(v->kvm, gpc->memslot, gpc->gpa >> PAGE_SHIFT);
 }
 
@@ -708,6 +728,15 @@ int kvm_xen_vcpu_set_attr(struct kvm_vcpu *vcpu, struct kvm_xen_vcpu_attr *data)
 		r = 0;
 		break;
 
+	case KVM_XEN_VCPU_ATTR_TYPE_UPCALL_VECTOR:
+		if (data->u.vector && data->u.vector < 0x10)
+			r = -EINVAL;
+		else {
+			vcpu->arch.xen.upcall_vector = data->u.vector;
+			r = 0;
+		}
+		break;
+
 	default:
 		break;
 	}
@@ -795,6 +824,11 @@ int kvm_xen_vcpu_get_attr(struct kvm_vcpu *vcpu, struct kvm_xen_vcpu_attr *data)
 		r = 0;
 		break;
 
+	case KVM_XEN_VCPU_ATTR_TYPE_UPCALL_VECTOR:
+		data->u.vector = vcpu->arch.xen.upcall_vector;
+		r = 0;
+		break;
+
 	default:
 		break;
 	}
@@ -1228,6 +1262,12 @@ int kvm_xen_set_evtchn_fast(struct kvm_xen_evtchn *xe, struct kvm *kvm)
 				kick_vcpu = true;
 			}
 		}
+
+		/* For the per-vCPU lapic vector, deliver it as MSI. */
+		if (kick_vcpu && vcpu->arch.xen.upcall_vector) {
+			kvm_xen_inject_vcpu_vector(vcpu);
+			kick_vcpu = false;
+		}
 	}
 
  out_rcu:

include/uapi/linux/kvm.h

@@ -1754,6 +1754,7 @@ struct kvm_xen_vcpu_attr {
 			__u32 priority;
 			__u64 expires_ns;
 		} timer;
+		__u8 vector;
 	} u;
 };
 
@@ -1767,6 +1768,7 @@ struct kvm_xen_vcpu_attr {
 /* Available with KVM_CAP_XEN_HVM / KVM_XEN_HVM_CONFIG_EVTCHN_SEND */
 #define KVM_XEN_VCPU_ATTR_TYPE_VCPU_ID		0x6
 #define KVM_XEN_VCPU_ATTR_TYPE_TIMER		0x7
+#define KVM_XEN_VCPU_ATTR_TYPE_UPCALL_VECTOR	0x8
 
 /* Secure Encrypted Virtualization command */
 enum sev_cmd_id {