xen/balloon: Mark unallocated host memory as UNUSABLE

Commit f5775e0b6116 ("x86/xen: discard RAM regions above the maximum
reservation") left host memory not assigned to dom0 as available for
memory hotplug.

Unfortunately this also meant that those regions could be used by
others. Specifically, commit fa564ad96366 ("x86/PCI: Enable a 64bit BAR
on AMD Family 15h (Models 00-1f, 30-3f, 60-7f)") may try to map those
addresses as MMIO.

To prevent this mark unallocated host memory as E820_TYPE_UNUSABLE (thus
effectively reverting f5775e0b6116) and keep track of that region as
a hostmem resource that can be used for the hotplug.

Signed-off-by: Boris Ostrovsky <[e-mail]>
Reviewed-by: Juergen Gross <[e-mail]>

arch/x86/xen/enlighten.c

···
+#ifdef CONFIG_XEN_BALLOON_MEMORY_HOTPLUG
+#include <linux/bootmem.h>
+#endif
 #include <linux/cpu.h>
 #include <linux/kexec.h>
 
 #include <xen/features.h>
 #include <xen/page.h>
+#include <xen/interface/memory.h>
 
 #include <asm/xen/hypercall.h>
 #include <asm/xen/hypervisor.h>
···
 }
 EXPORT_SYMBOL(xen_arch_unregister_cpu);
 #endif
+
+#ifdef CONFIG_XEN_BALLOON_MEMORY_HOTPLUG
+void __init arch_xen_balloon_init(struct resource *hostmem_resource)
+{
+	struct xen_memory_map memmap;
+	int rc;
+	unsigned int i, last_guest_ram;
+	phys_addr_t max_addr = PFN_PHYS(max_pfn);
+	struct e820_table *xen_e820_table;
+	const struct e820_entry *entry;
+	struct resource *res;
+
+	if (!xen_initial_domain())
+		return;
+
+	xen_e820_table = kmalloc(sizeof(*xen_e820_table), GFP_KERNEL);
+	if (!xen_e820_table)
+		return;
+
+	memmap.nr_entries = ARRAY_SIZE(xen_e820_table->entries);
+	set_xen_guest_handle(memmap.buffer, xen_e820_table->entries);
+	rc = HYPERVISOR_memory_op(XENMEM_machine_memory_map, &memmap);
+	if (rc) {
+		pr_warn("%s: Can't read host e820 (%d)\n", __func__, rc);
+		goto out;
+	}
+
+	last_guest_ram = 0;
+	for (i = 0; i < memmap.nr_entries; i++) {
+		if (xen_e820_table->entries[i].addr >= max_addr)
+			break;
+		if (xen_e820_table->entries[i].type == E820_TYPE_RAM)
+			last_guest_ram = i;
+	}
+
+	entry = &xen_e820_table->entries[last_guest_ram];
+	if (max_addr >= entry->addr + entry->size)
+		goto out; /* No unallocated host RAM. */
+
+	hostmem_resource->start = max_addr;
+	hostmem_resource->end = entry->addr + entry->size;
+
+	/*
+	 * Mark non-RAM regions between the end of dom0 RAM and end of host RAM
+	 * as unavailable. The rest of that region can be used for hotplug-based
+	 * ballooning.
+	 */
+	for (; i < memmap.nr_entries; i++) {
+		entry = &xen_e820_table->entries[i];
+
+		if (entry->type == E820_TYPE_RAM)
+			continue;
+
+		if (entry->addr >= hostmem_resource->end)
+			break;
+
+		res = kzalloc(sizeof(*res), GFP_KERNEL);
+		if (!res)
+			goto out;
+
+		res->name = "Unavailable host RAM";
+		res->start = entry->addr;
+		res->end = (entry->addr + entry->size < hostmem_resource->end) ?
+			    entry->addr + entry->size : hostmem_resource->end;
+		rc = insert_resource(hostmem_resource, res);
+		if (rc) {
+			pr_warn("%s: Can't insert [%llx - %llx) (%d)\n",
+				__func__, res->start, res->end, rc);
+			kfree(res);
+			goto  out;
+		}
+	}
+
+ out:
+	kfree(xen_e820_table);
+}
+#endif /* CONFIG_XEN_BALLOON_MEMORY_HOTPLUG */

arch/x86/xen/setup.c

···
 	addr = xen_e820_table.entries[0].addr;
 	size = xen_e820_table.entries[0].size;
 	while (i < xen_e820_table.nr_entries) {
-		bool discard = false;
 
 		chunk_size = size;
 		type = xen_e820_table.entries[i].type;
···
 				xen_add_extra_mem(pfn_s, n_pfns);
 				xen_max_p2m_pfn = pfn_s + n_pfns;
 			} else
-				discard = true;
+				type = E820_TYPE_UNUSABLE;
 		}
 
-		if (!discard)
-			xen_align_and_add_e820_region(addr, chunk_size, type);
+		xen_align_and_add_e820_region(addr, chunk_size, type);
 
 		addr += chunk_size;
 		size -= chunk_size;

drivers/xen/balloon.c

···
 	kfree(resource);
 }
 
+/*
+ * Host memory not allocated to dom0. We can use this range for hotplug-based
+ * ballooning.
+ *
+ * It's a type-less resource. Setting IORESOURCE_MEM will make resource
+ * management algorithms (arch_remove_reservations()) look into guest e820,
+ * which we don't want.
+ */
+static struct resource hostmem_resource = {
+	.name   = "Host RAM",
+};
+
+void __attribute__((weak)) __init arch_xen_balloon_init(struct resource *res)
+{}
+
 static struct resource *additional_memory_resource(phys_addr_t size)
 {
-	struct resource *res;
-	int ret;
+	struct resource *res, *res_hostmem;
+	int ret = -ENOMEM;
 
 	res = kzalloc(sizeof(*res), GFP_KERNEL);
 	if (!res)
···
 	res->name = "System RAM";
 	res->flags = IORESOURCE_SYSTEM_RAM | IORESOURCE_BUSY;
 
-	ret = allocate_resource(&iomem_resource, res,
-				size, 0, -1,
-				PAGES_PER_SECTION * PAGE_SIZE, NULL, NULL);
-	if (ret < 0) {
-		pr_err("Cannot allocate new System RAM resource\n");
-		kfree(res);
-		return NULL;
+	res_hostmem = kzalloc(sizeof(*res), GFP_KERNEL);
+	if (res_hostmem) {
+		/* Try to grab a range from hostmem */
+		res_hostmem->name = "Host memory";
+		ret = allocate_resource(&hostmem_resource, res_hostmem,
+					size, 0, -1,
+					PAGES_PER_SECTION * PAGE_SIZE, NULL, NULL);
+	}
+
+	if (!ret) {
+		/*
+		 * Insert this resource into iomem. Because hostmem_resource
+		 * tracks portion of guest e820 marked as UNUSABLE noone else
+		 * should try to use it.
+		 */
+		res->start = res_hostmem->start;
+		res->end = res_hostmem->end;
+		ret = insert_resource(&iomem_resource, res);
+		if (ret < 0) {
+			pr_err("Can't insert iomem_resource [%llx - %llx]\n",
+				res->start, res->end);
+			release_memory_resource(res_hostmem);
+			res_hostmem = NULL;
+			res->start = res->end = 0;
+		}
+	}
+
+	if (ret) {
+		ret = allocate_resource(&iomem_resource, res,
+					size, 0, -1,
+					PAGES_PER_SECTION * PAGE_SIZE, NULL, NULL);
+		if (ret < 0) {
+			pr_err("Cannot allocate new System RAM resource\n");
+			kfree(res);
+			return NULL;
+		}
 	}
 
 #ifdef CONFIG_SPARSEMEM
···
 			pr_err("New System RAM resource outside addressable RAM (%lu > %lu)\n",
 			       pfn, limit);
 			release_memory_resource(res);
+			release_memory_resource(res_hostmem);
 			return NULL;
 		}
 	}
···
 	set_online_page_callback(&xen_online_page);
 	register_memory_notifier(&xen_memory_nb);
 	register_sysctl_table(xen_root);
+
+	arch_xen_balloon_init(&hostmem_resource);
 #endif
 
 #ifdef CONFIG_XEN_PV

include/xen/balloon.h

···
 {
 }
 #endif
+
+#ifdef CONFIG_XEN_BALLOON_MEMORY_HOTPLUG
+struct resource;
+void arch_xen_balloon_init(struct resource *hostmem_resource);
+#endif