arm64: Add memory hotplug support

Wire up the basic support for hot-adding memory. Since memory hotplug
is fairly tightly coupled to sparsemem, we tweak pfn_valid() to also
cross-check the presence of a section in the manner of the generic
implementation, before falling back to memblock to check for no-map
regions within a present section as before. By having arch_add_memory(()
create the linear mapping first, this then makes everything work in the
way that __add_section() expects.

We expect hotplug to be ACPI-driven, so the swapper_pg_dir updates
should be safe from races by virtue of the global device hotplug lock.
Signed-off-by: Robin Murphy <robin.murphy@arm.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>

arch/arm64/Kconfig

@@ -261,6 +261,9 @@ config ZONE_DMA32
 config HAVE_GENERIC_GUP
 	def_bool y
 
+config ARCH_ENABLE_MEMORY_HOTPLUG
+	def_bool y
+
 config SMP
 	def_bool y
 

arch/arm64/mm/init.c

@@ -291,6 +291,14 @@ int pfn_valid(unsigned long pfn)
 
 	if ((addr >> PAGE_SHIFT) != pfn)
 		return 0;
+
+#ifdef CONFIG_SPARSEMEM
+	if (pfn_to_section_nr(pfn) >= NR_MEM_SECTIONS)
+		return 0;
+
+	if (!valid_section(__nr_to_section(pfn_to_section_nr(pfn))))
+		return 0;
+#endif
 	return memblock_is_map_memory(addr);
 }
 EXPORT_SYMBOL(pfn_valid);

arch/arm64/mm/mmu.c

@@ -1046,3 +1046,20 @@ int pud_free_pmd_page(pud_t *pudp, unsigned long addr)
 	pmd_free(NULL, table);
 	return 1;
 }
+
+#ifdef CONFIG_MEMORY_HOTPLUG
+int arch_add_memory(int nid, u64 start, u64 size, struct vmem_altmap *altmap,
+		    bool want_memblock)
+{
+	int flags = 0;
+
+	if (rodata_full || debug_pagealloc_enabled())
+		flags = NO_BLOCK_MAPPINGS | NO_CONT_MAPPINGS;
+
+	__create_pgd_mapping(swapper_pg_dir, start, __phys_to_virt(start),
+			     size, PAGE_KERNEL, pgd_pgtable_alloc, flags);
+
+	return __add_pages(nid, start >> PAGE_SHIFT, size >> PAGE_SHIFT,
+			   altmap, want_memblock);
+}
+#endif

arch/arm64/mm/numa.c

@@ -466,3 +466,13 @@ void __init arm64_numa_init(void)
 
 	numa_init(dummy_numa_init);
 }
+
+/*
+ * We hope that we will be hotplugging memory on nodes we already know about,
+ * such that acpi_get_node() succeeds and we never fall back to this...
+ */
+int memory_add_physaddr_to_nid(u64 addr)
+{
+	pr_warn("Unknown node for memory at 0x%llx, assuming node 0\n", addr);
+	return 0;
+}