kernel: mmu: install demand mappings for the on-demand linker sections

This sets initial unpaged mappings for __ondemand_func code and
__ondemand_rodata variables. To achieve this, we have to augment the
backing store API.

Signed-off-by: Nicolas Pitre <npitre@baylibre.com>

Kconfig.zephyr

@@ -262,6 +262,7 @@ config LINKER_USE_PINNED_SECTION
 
 config LINKER_USE_ONDEMAND_SECTION
 	bool "Use Evictable Linker Section"
+	depends on DEMAND_MAPPING
 	depends on !LINKER_USE_PINNED_SECTION
 	depends on !ARCH_MAPS_ALL_RAM
 	help

doc/kernel/memory_management/demand_paging.rst

@@ -179,6 +179,11 @@ which must be implemented:
   free a backing store location (the ``location`` token) which can
   then be used for subsequent page out operation.
 
+* :c:func:`k_mem_paging_backing_store_location_query()` is called to obtain
+  the ``location`` token corresponding to storage content to be virtually
+  mapped and paged-in on demand. Most useful with
+  :kconfig:option:`CONFIG_DEMAND_MAPPING`.
+
 * :c:func:`k_mem_paging_backing_store_page_in()` copies a data page
   from the backing store location associated with the provided
   ``location`` token to the page pointed by ``K_MEM_SCRATCH_PAGE``.

include/zephyr/kernel/mm/demand_paging.h

@@ -349,6 +349,22 @@ int k_mem_paging_backing_store_location_get(struct k_mem_page_frame *pf,
  */
 void k_mem_paging_backing_store_location_free(uintptr_t location);
 
+/**
+ * Obtain persistent location token for on-demand content
+ *
+ * Unlike k_mem_paging_backing_store_location_get() this does not allocate
+ * any backing store space. Instead, it returns a location token corresponding
+ * to some fixed storage content to be paged in on demand. This is expected
+ * to be used in conjonction with CONFIG_LINKER_USE_ONDEMAND_SECTION and the
+ * K_MEM_MAP_UNPAGED flag to create demand mappings at boot time. This may
+ * also be used e.g. to implement file-based mmap().
+ *
+ * @param addr Virtual address to obtain a location token for
+ * @param [out] location storage location token
+ * @return 0 for success or negative error code
+ */
+int k_mem_paging_backing_store_location_query(void *addr, uintptr_t *location);
+
 /**
  * Copy a data page from K_MEM_SCRATCH_PAGE to the specified location
  *

kernel/mmu.c

@@ -1050,6 +1050,34 @@ static void mark_linker_section_pinned(void *start_addr, void *end_addr,
 }
 #endif /* CONFIG_LINKER_USE_BOOT_SECTION) || CONFIG_LINKER_USE_PINNED_SECTION */
 
+#ifdef CONFIG_LINKER_USE_ONDEMAND_SECTION
+static void z_paging_ondemand_section_map(void)
+{
+	uint8_t *addr;
+	size_t size;
+	uintptr_t location;
+	uint32_t flags;
+
+	size = (uintptr_t)lnkr_ondemand_text_size;
+	flags = K_MEM_MAP_UNPAGED | K_MEM_PERM_EXEC | K_MEM_CACHE_WB;
+	VIRT_FOREACH(lnkr_ondemand_text_start, size, addr) {
+		k_mem_paging_backing_store_location_query(addr, &location);
+		arch_mem_map(addr, location, CONFIG_MMU_PAGE_SIZE, flags);
+		sys_bitarray_set_region(&virt_region_bitmap, 1,
+					virt_to_bitmap_offset(addr, CONFIG_MMU_PAGE_SIZE));
+	}
+
+	size = (uintptr_t)lnkr_ondemand_rodata_size;
+	flags = K_MEM_MAP_UNPAGED | K_MEM_CACHE_WB;
+	VIRT_FOREACH(lnkr_ondemand_rodata_start, size, addr) {
+		k_mem_paging_backing_store_location_query(addr, &location);
+		arch_mem_map(addr, location, CONFIG_MMU_PAGE_SIZE, flags);
+		sys_bitarray_set_region(&virt_region_bitmap, 1,
+					virt_to_bitmap_offset(addr, CONFIG_MMU_PAGE_SIZE));
+	}
+}
+#endif /* CONFIG_LINKER_USE_ONDEMAND_SECTION */
+
 void z_mem_manage_init(void)
 {
 	uintptr_t phys;
@@ -1126,6 +1154,11 @@ void z_mem_manage_init(void)
 		}
 	}
 #endif /* CONFIG_DEMAND_PAGING */
+
+#ifdef CONFIG_LINKER_USE_ONDEMAND_SECTION
+	z_paging_ondemand_section_map();
+#endif
+
 #if __ASSERT_ON
 	page_frames_initialized = true;
 #endif