PCI: vmd: Switch to pci_bus_find_emul_domain_nr()

The new common domain number allocator can replace the custom allocator
in VMD.

Beyond some code reuse benefits it does close a potential race whereby
vmd_find_free_domain() collides with new PCI buses coming online with a
conflicting domain number. Such a race has not been observed in practice,
hence not tagging this change as a fix.

As VMD uses pci_create_root_bus() rather than pci_alloc_host_bridge() +
pci_scan_root_bus_bridge() it has no chance to set ->domain_nr in the
bridge so needs to manage freeing the domain number on its own.

Signed-off-by: Dan Williams <dan.j.williams@intel.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Cc: Szymon Durawa <szymon.durawa@linux.intel.com>
Cc: Nirmal Patel <nirmal.patel@linux.intel.com>
Link: https://patch.msgid.link/20251024224622.1470555-3-dan.j.williams@intel.com

Author: djbw

drivers/pci/controller/vmd.c

@@ -565,22 +565,6 @@ static void vmd_detach_resources(struct vmd_dev *vmd)
 	vmd->dev->resource[VMD_MEMBAR2].child = NULL;
 }
 
-/*
- * VMD domains start at 0x10000 to not clash with ACPI _SEG domains.
- * Per ACPI r6.0, sec 6.5.6,  _SEG returns an integer, of which the lower
- * 16 bits are the PCI Segment Group (domain) number.  Other bits are
- * currently reserved.
- */
-static int vmd_find_free_domain(void)
-{
-	int domain = 0xffff;
-	struct pci_bus *bus = NULL;
-
-	while ((bus = pci_find_next_bus(bus)) != NULL)
-		domain = max_t(int, domain, pci_domain_nr(bus));
-	return domain + 1;
-}
-
 static int vmd_get_phys_offsets(struct vmd_dev *vmd, bool native_hint,
 				resource_size_t *offset1,
 				resource_size_t *offset2)
@@ -865,13 +849,6 @@ static int vmd_enable_domain(struct vmd_dev *vmd, unsigned long features)
 		.parent = res,
 	};
 
-	sd->vmd_dev = vmd->dev;
-	sd->domain = vmd_find_free_domain();
-	if (sd->domain < 0)
-		return sd->domain;
-
-	sd->node = pcibus_to_node(vmd->dev->bus);
-
 	/*
 	 * Currently MSI remapping must be enabled in guest passthrough mode
 	 * due to some missing interrupt remapping plumbing. This is probably
@@ -897,9 +874,24 @@ static int vmd_enable_domain(struct vmd_dev *vmd, unsigned long features)
 	pci_add_resource_offset(&resources, &vmd->resources[1], offset[0]);
 	pci_add_resource_offset(&resources, &vmd->resources[2], offset[1]);
 
+	sd->vmd_dev = vmd->dev;
+
+	/*
+	 * Emulated domains start at 0x10000 to not clash with ACPI _SEG
+	 * domains.  Per ACPI r6.0, sec 6.5.6, _SEG returns an integer, of
+	 * which the lower 16 bits are the PCI Segment Group (domain) number.
+	 * Other bits are currently reserved.
+	 */
+	sd->domain = pci_bus_find_emul_domain_nr(0, 0x10000, INT_MAX);
+	if (sd->domain < 0)
+		return sd->domain;
+
+	sd->node = pcibus_to_node(vmd->dev->bus);
+
 	vmd->bus = pci_create_root_bus(&vmd->dev->dev, vmd->busn_start,
 				       &vmd_ops, sd, &resources);
 	if (!vmd->bus) {
+		pci_bus_release_emul_domain_nr(sd->domain);
 		pci_free_resource_list(&resources);
 		vmd_remove_irq_domain(vmd);
 		return -ENODEV;
@@ -992,6 +984,7 @@ static int vmd_probe(struct pci_dev *dev, const struct pci_device_id *id)
 		return -ENOMEM;
 
 	vmd->dev = dev;
+	vmd->sysdata.domain = PCI_DOMAIN_NR_NOT_SET;
 	vmd->instance = ida_alloc(&vmd_instance_ida, GFP_KERNEL);
 	if (vmd->instance < 0)
 		return vmd->instance;
@@ -1057,6 +1050,7 @@ static void vmd_remove(struct pci_dev *dev)
 	vmd_detach_resources(vmd);
 	vmd_remove_irq_domain(vmd);
 	ida_free(&vmd_instance_ida, vmd->instance);
+	pci_bus_release_emul_domain_nr(vmd->sysdata.domain);
 }
 
 static void vmd_shutdown(struct pci_dev *dev)