irqchip/gic-v3-its: Introduce virtual ITS

GICv3 allows supporting LPI without an ITS, and in order to support
such a platform, a virtual ITS is introduced. The virtual ITS has the
same software part as a real ITS: having an irq domain, maintaining
->collections and maintaining the list of devices. The only difference
is the virtual ITS doesn't have a backed ITS therefore it cannot issue
ITS commands nor set up device tables. The virtual ITS only manages LPIs
and the LPIs are configured via DirectLPI interfaces.

And currently the virtual ITS is initialized only if there is no ITS in
the system and yet DirectLPI is support.

The virtual ITS approach provides the support for LPI without an ITS and
reuses as much exisiting code as possible, and is the preparation for
virtual PCI support on ARM64 Hyper-V guests.

Co-developed-by: Sunil Muthuswamy <sunilmut@microsoft.com>
Signed-off-by: Boqun Feng <boqun.feng@gmail.com>

drivers/irqchip/irq-gic-v3-its.c

@@ -117,9 +117,16 @@ struct its_node {
 	int			vlpi_redist_offset;
 };
 
+/*
+ * LPI can be supported without ITS, in which case, a virtual its_node is
+ * initialized to allow configuring LPI with the DirectLPI approach.
+ */
+static struct its_node *virtual_its_node;
+
 #define is_v4(its)		(!!((its)->typer & GITS_TYPER_VLPIS))
 #define is_v4_1(its)		(!!((its)->typer & GITS_TYPER_VMAPP))
 #define device_ids(its)		(FIELD_GET(GITS_TYPER_DEVBITS, (its)->typer) + 1)
+#define is_virtual(its)		((its) == virtual_its_node)
 
 #define ITS_ITT_ALIGN		SZ_256
 
@@ -1096,6 +1103,10 @@ void name(struct its_node *its,						\
 	unsigned long flags;						\
 	u64 rd_idx;							\
 									\
+	/* Virtual ITS doesn't support ITS commands */			\
+	if (is_virtual(its))						\
+		return;							\
+									\
 	raw_spin_lock_irqsave(&its->lock, flags);			\
 									\
 	cmd = its_allocate_entry(its);					\
@@ -1464,7 +1475,8 @@ static void lpi_update_config(struct irq_data *d, u8 clr, u8 set)
 
 	lpi_write_config(d, clr, set);
 	if (gic_rdists->has_direct_lpi &&
-	    (is_v4_1(its_dev->its) || !irqd_is_forwarded_to_vcpu(d)))
+	    (is_v4_1(its_dev->its) || !irqd_is_forwarded_to_vcpu(d) ||
+	     is_virtual(its_dev->its)))
 		direct_lpi_inv(d);
 	else if (!irqd_is_forwarded_to_vcpu(d))
 		its_send_inv(its_dev, its_get_event_id(d));
@@ -1690,6 +1702,10 @@ static void its_irq_compose_msi_msg(struct irq_data *d, struct msi_msg *msg)
 	u64 addr;
 
 	its = its_dev->its;
+
+	/* Virtual ITS doesn't have ->get_msi_base function, skip */
+	if (!its->get_msi_base)
+		return;
 	addr = its->get_msi_base(its_dev);
 
 	msg->address_lo		= lower_32_bits(addr);
@@ -3217,7 +3233,17 @@ static void its_cpu_init_collections(void)
 	raw_spin_lock(&its_lock);
 
 	list_for_each_entry(its, &its_nodes, entry)
-		its_cpu_init_collection(its);
+		/*
+		 * Only initializes the software part of collections for virtual
+		 * ITS.
+		 */
+		if (is_virtual(its)) {
+			int cpu = smp_processor_id();
+
+			its->collections[cpu].col_id = cpu;
+		} else {
+			its_cpu_init_collection(its);
+		}
 
 	raw_spin_unlock(&its_lock);
 }
@@ -3364,7 +3390,8 @@ static struct its_device *its_create_device(struct its_node *its, u32 dev_id,
 	int nr_ites;
 	int sz;
 
-	if (!its_alloc_device_table(its, dev_id))
+	/* No need to allocate device table for virtual ITS */
+	if (!is_virtual(its) && !its_alloc_device_table(its, dev_id))
 		return NULL;
 
 	if (WARN_ON(!is_power_of_2(nvecs)))
@@ -3551,9 +3578,12 @@ static int its_irq_domain_alloc(struct irq_domain *domain, unsigned int virq,
 	if (err)
 		return err;
 
-	err = iommu_dma_prepare_msi(info->desc, its->get_msi_base(its_dev));
-	if (err)
-		return err;
+	/* Virtual ITS doesn't have ->get_msi_base function, skip */
+	if (its->get_msi_base) {
+		err = iommu_dma_prepare_msi(info->desc, its->get_msi_base(its_dev));
+		if (err)
+			return err;
+	}
 
 	for (i = 0; i < nr_irqs; i++) {
 		err = its_irq_gic_domain_alloc(domain, virq + i, hwirq + i);
@@ -5121,6 +5151,63 @@ static int __init its_probe_one(struct resource *res,
 	return err;
 }
 
+static int __init virtual_its_init(void)
+{
+	struct its_node *its;
+	struct fwnode_handle *fwnode;
+	int err;
+
+	fwnode = irq_domain_alloc_named_fwnode("Virtual ITS");
+	if (!fwnode)
+		return -ENOMEM;
+
+	/*
+	 * Use 0 as the ID for virtual ITS, since we only init virtual ITS if
+	 * there is no real ITS in the system, so it's fine.
+	 */
+	err = iort_register_domain_token(0, 0, fwnode);
+	if (err)
+		goto out_free_fwnode;
+
+	its = kzalloc(sizeof(*its), GFP_KERNEL);
+	if (!its) {
+		err = -ENOMEM;
+		goto out_unregister_fwnode;
+	}
+
+	raw_spin_lock_init(&its->lock);
+	mutex_init(&its->dev_alloc_lock);
+	INIT_LIST_HEAD(&its->entry);
+	INIT_LIST_HEAD(&its->its_device_list);
+	its->numa_node = NUMA_NO_NODE;
+	its->msi_domain_flags = IRQ_DOMAIN_FLAG_MSI_REMAP;
+	err = its_alloc_collections(its);
+	if (err)
+		goto out_free_its;
+
+	err = its_init_domain(fwnode, its);
+	if (err)
+		goto out_free_collections;
+
+	raw_spin_lock(&its_lock);
+	virtual_its_node = its;
+	list_add(&its->entry, &its_nodes);
+	raw_spin_unlock(&its_lock);
+
+	return 0;
+
+out_free_collections:
+	its_free_collections(its);
+out_free_its:
+	kvfree(its);
+out_unregister_fwnode:
+	iort_deregister_domain_token(0);
+out_free_fwnode:
+	irq_domain_free_fwnode(fwnode);
+	return err;
+
+}
+
 static bool gic_rdists_supports_plpis(void)
 {
 	return !!(gic_read_typer(gic_data_rdist_rd_base() + GICR_TYPER) & GICR_TYPER_PLPIS);
@@ -5414,8 +5501,19 @@ int __init its_init(struct fwnode_handle *handle, struct rdists *rdists,
 		its_acpi_probe();
 
 	if (list_empty(&its_nodes)) {
-		pr_warn("ITS: No ITS available, not enabling LPIs\n");
-		return -ENXIO;
+		/* Initialize virtual ITS only if DirectLPI is set. */
+		if (gic_rdists->has_direct_lpi) {
+			pr_info("ITS: No ITS available, using virtual ITS\n");
+			err = virtual_its_init();
+			if (err) {
+				pr_info("ITS: Virtual ITS creation fails\n");
+				return err;
+			}
+			pr_info("ITS: Virtual ITS created\n");
+		} else {
+			pr_warn("ITS: No ITS available, not enabling LPIs\n");
+			return -ENXIO;
+		}
 	}
 
 	err = allocate_lpi_tables();