kernel: Add Kconfig option to disable LTO for kernel sources

Some SoCs require kernel code to be placed in RAM, which makes
link-time optimization (LTO) unsuitable for these files.
Disabling LTO allows the affected code to be linked as separate
objects and placed in specific memory regions.

Running kernel code from RAM can improve execution performance,
especially for timing-critical routines or context switch paths.

Signed-off-by: Tim Lin <tim2.lin@ite.corp-partner.google.com>

Author: GTLin08

kernel/CMakeLists.txt

@@ -195,6 +195,11 @@ target_link_libraries(kernel zephyr_interface)
 
 endif()
 
+# Optionally build kernel sources without LTO
+if(CONFIG_KERNEL_NO_LTO)
+  include(${CMAKE_CURRENT_LIST_DIR}/kernel_no_lto.cmake)
+endif()
+
 add_dependencies(kernel zephyr_generated_headers)
 
 unset(libkernel)

kernel/Kconfig

@@ -1138,6 +1138,19 @@ endif # BOOTARGS
 
 endmenu
 
+config KERNEL_NO_LTO
+	bool "Build selected kernel core files without LTO"
+	depends on LTO
+	depends on XIP
+	help
+	  Some SoCs require kernel code to be placed in RAM, which makes link-time
+	  optimization (LTO) unsuitable for these files (-fno-lto). Disabling LTO
+	  allows the affected code to be linked as separate objects and placed in
+	  specific memory regions.
+
+	  Running kernel code from RAM can improve execution performance, especially
+	  for timing-critical routines or context switch paths.
+
 rsource "Kconfig.device"
 rsource "Kconfig.vm"
 rsource "Kconfig.init"

kernel/kernel_no_lto.cmake

@@ -0,0 +1,50 @@
+# Copyright (c) 2025 ITE Corporation. All Rights Reserved.
+# SPDX-License-Identifier: Apache-2.0
+
+# Optional script to disable LTO for kernel files
+
+message(STATUS "[no-LTO] Building kernel files without LTO")
+
+# Retrieve all source files from the kernel library target
+if(TARGET kernel)
+  get_property(KERNEL_SRCS TARGET kernel PROPERTY SOURCES)
+else()
+  message(WARNING "[no-LTO] kernel target not found, skipping")
+  return()
+endif()
+
+# Check if there is a permitted kernel file that can enable LTO
+if(COMMAND kernel_lto_allow_list)
+  kernel_lto_allow_list(ADDITIONAL_ALLOWLIST)
+endif()
+
+# Default LTO allowlist
+set(DEFAULT_LTO_ALLOWLIST init.c errno.c fatal.c)
+# Append LTO allowlist
+list(APPEND DEFAULT_LTO_ALLOWLIST ${ADDITIONAL_ALLOWLIST})
+# Setting LTO allowlist
+set(LTO_ALLOWLIST ${DEFAULT_LTO_ALLOWLIST})
+
+# Apply -fno-lto to all C source files, except for some initialization files
+# (e.g. init.c, errno.c, fatal.c) and those that are less critical to be
+# executed in RAM. These files can be excluded from -fno-lto by using
+# ADDITIONAL_ALLOWLIST.
+foreach(src ${KERNEL_SRCS})
+  if(src MATCHES "\\.c$")
+
+    # Skip if filename matches any in allowlist
+    set(skip FALSE)
+    foreach(allow ${LTO_ALLOWLIST})
+      get_filename_component(basename ${src} NAME)
+      if("${basename}" STREQUAL "${allow}")
+        set(skip TRUE)
+        break()
+      endif()
+    endforeach()
+
+    if(NOT skip)
+      set_source_files_properties(${src} PROPERTIES COMPILE_FLAGS "-fno-lto -g")
+    endif()
+
+  endif()
+endforeach()