llext: Keep constant data in flash.

This patch adds a new Kconfig option that allows read-only data sections
without relocations to be mapped directly (remain in flash) instead of
being copied to RAM during extension loading. This helps extensions that
have large constant data, such as certificates, lookup tables etc.. to
save saving significant RAM, especially with MPU is enabled.

This feature requires the extension's linker script to place rodata
sections with and without relocations into separate, non-overlapping
section called .rodata.reloc.

Signed-off-by: Ibrahim Abdalkader <[email protected]>

Author: iabdalkader

include/zephyr/llext/llext.h

@@ -45,6 +45,9 @@ enum llext_mem {
 	LLEXT_MEM_TEXT,         /**< Executable code */
 	LLEXT_MEM_DATA,         /**< Initialized data */
 	LLEXT_MEM_RODATA,       /**< Read-only data */
+#ifdef CONFIG_LLEXT_RODATA_NO_RELOC
+	LLEXT_MEM_RODATA_NO_RELOC,  /**< Read-only data without relocations (kept in flash) */
+#endif
 	LLEXT_MEM_BSS,          /**< Uninitialized data */
 	LLEXT_MEM_EXPORT,       /**< Exported symbol table */
 	LLEXT_MEM_SYMTAB,       /**< Symbol table */
@@ -62,6 +65,11 @@ enum llext_mem {
 /* Number of memory partitions used by LLEXT */
 #define LLEXT_MEM_PARTITIONS (LLEXT_MEM_BSS+1)
 
+#ifdef CONFIG_LLEXT_RODATA_NO_RELOC
+/* Section name prefix for read-only data kept in flash */
+#define LLEXT_SECTION_RODATA_NO_RELOC ".rodata.noreloc"
+#endif
+
 struct llext_loader;
 /** @endcond */
 

subsys/llext/Kconfig

@@ -108,6 +108,21 @@ config LLEXT_STORAGE_WRITABLE
 	  Select if LLEXT storage is writable, i.e. if extensions are stored in
 	  RAM and can be modified in place
 
+config LLEXT_RODATA_NO_RELOC
+	bool "Keep read-only data without relocations in flash"
+	help
+	  When enabled, read-only data sections (.rodata) that do not have
+	  any relocations are kept in their original location (typically flash)
+	  instead of being copied to RAM.
+
+	  This is useful for large constant data like certificates, lookup
+	  tables, or string literals that don't contain any pointers.
+
+	  This option requires the extension's linker script to place read-only
+	  data with relocations and read-only data without relocations into
+	  separate, non-overlapping sections. If these sections overlap in the
+	  ELF file, loading will fail.
+
 config LLEXT_EXPORT_DEVICES
 	bool "Export all DT devices to llexts"
 	select LLEXT_EXPORT_SYMBOL_GROUP_DEVICE

subsys/llext/llext_load.c

@@ -262,6 +262,10 @@ static int llext_map_sections(struct llext_loader *ldr, struct llext *ext,
 				mem_idx = LLEXT_MEM_TEXT;
 			} else if (shdr->sh_flags & SHF_WRITE) {
 				mem_idx = LLEXT_MEM_DATA;
+#ifdef CONFIG_LLEXT_RODATA_NO_RELOC
+			} else if (strcmp(name, LLEXT_SECTION_RODATA_NO_RELOC) == 0) {
+				mem_idx = LLEXT_MEM_RODATA_NO_RELOC;
+#endif
 			} else {
 				mem_idx = LLEXT_MEM_RODATA;
 			}

subsys/llext/llext_mem.c

@@ -66,6 +66,9 @@ static void llext_init_mem_part(struct llext *ext, enum llext_mem mem_idx,
 			ext->mem_parts[mem_idx].attr = K_MEM_PARTITION_P_RW_U_RW;
 			break;
 		case LLEXT_MEM_RODATA:
+#ifdef CONFIG_LLEXT_RODATA_NO_RELOC
+		case LLEXT_MEM_RODATA_NO_RELOC:
+#endif
 			ext->mem_parts[mem_idx].attr = K_MEM_PARTITION_P_RO_U_RO;
 			break;
 		default:
@@ -132,10 +135,14 @@ static int llext_copy_region(struct llext_loader *ldr, struct llext *ext,
 			ext->mem[mem_idx] = llext_peek(ldr, region->sh_offset);
 			if (ext->mem[mem_idx]) {
 				if ((IS_ALIGNED(ext->mem[mem_idx], region_align) ||
+#ifdef CONFIG_LLEXT_RODATA_NO_RELOC
+				    (mem_idx == LLEXT_MEM_RODATA_NO_RELOC) ||
+#endif
 				     ldr_parm->pre_located) &&
 				    ((mem_idx != LLEXT_MEM_TEXT) ||
 				     INSTR_FETCHABLE(ext->mem[mem_idx], region_alloc))) {
 					/* Map this region directly to the ELF buffer */
+					LOG_DBG("Region %d kept at %p", mem_idx, ext->mem[mem_idx]);
 					llext_init_mem_part(ext, mem_idx,
 							    (uintptr_t)ext->mem[mem_idx],
 							    region_alloc);