demand_paging: add a flash-based backing store for qemu_x86_tiny

This adds a flash-based backing store for qemu_x86_tiny board for
testing demand paging. This allows us to test code execution where
.text section is not in physical memory at boot.

Signed-off-by: Daniel Leung <daniel.leung@intel.com>

subsys/demand_paging/backing_store/CMakeLists.txt

@@ -11,4 +11,9 @@ include_directories(
 if(NOT DEFINED CONFIG_BACKING_STORE_CUSTOM)
   zephyr_library()
   zephyr_library_sources_ifdef(CONFIG_BACKING_STORE_RAM   ram.c)
+
+  zephyr_library_sources_ifdef(
+    CONFIG_BACKING_STORE_QEMU_X86_TINY_FLASH
+    backing_store_qemu_x86_tiny.c
+    )
 endif()

subsys/demand_paging/backing_store/Kconfig

@@ -18,6 +18,16 @@ config BACKING_STORE_RAM
 	  This implements a backing store using physical RAM pages that the
 	  Zephyr kernel is otherwise unaware of. It is intended for
 	  demonstration and testing of the demand paging feature.
+
+config BACKING_STORE_QEMU_X86_TINY_FLASH
+	bool "Flash-based backing store on qemu_x86_tiny"
+	depends on BOARD_QEMU_X86_TINY
+	help
+	  This uses the "flash" memory area (in DTS) as the backing store
+	  for demand paging. The qemu_x86_tiny.ld linker script puts
+	  the symbols outside of boot and pinned sections into the flash
+	  area, allowing testing of the demand paging mechanism on
+	  code and data.
 endchoice
 
 if BACKING_STORE_RAM

subsys/demand_paging/backing_store/backing_store_qemu_x86_tiny.c

@@ -0,0 +1,78 @@
+/*
+ * Copyright (c) 2021 Intel Corporation
+ *
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+/**
+ * @file
+ * @brief Backing store on qemu_x86_tiny for testing
+ *
+ * This uses the "flash" memory area (in DTS) as the backing store
+ * for demand paging. The qemu_x86_tiny.ld linker script puts
+ * the symbols outside of boot and pinned sections into the flash
+ * area, allowing testing of the demand paging mechanism on
+ * code and data.
+ */
+
+#include <mmu.h>
+#include <string.h>
+#include <kernel_arch_interface.h>
+#include <linker/linker-defs.h>
+#include <sys/util.h>
+
+void *location_to_flash(uintptr_t location)
+{
+	uintptr_t ptr = location;
+
+	/* Offset from start of virtual address space */
+	ptr -= CONFIG_KERNEL_VM_BASE + CONFIG_KERNEL_VM_OFFSET;
+
+	/* Translate the offset into address to flash */
+	ptr += CONFIG_FLASH_BASE_ADDRESS;
+
+	__ASSERT_NO_MSG(ptr >= CONFIG_FLASH_BASE_ADDRESS);
+	__ASSERT_NO_MSG(ptr < (CONFIG_FLASH_BASE_ADDRESS
+			       + KB(CONFIG_FLASH_SIZE)
+			       - CONFIG_MMU_PAGE_SIZE));
+
+	return UINT_TO_POINTER(ptr);
+}
+
+int k_mem_paging_backing_store_location_get(struct z_page_frame *pf,
+					    uintptr_t *location,
+					    bool page_fault)
+{
+	/* Simply returns the virtual address */
+	*location = POINTER_TO_UINT(pf->addr);
+
+	return 0;
+}
+
+void k_mem_paging_backing_store_location_free(uintptr_t location)
+{
+	/* Nothing to do */
+}
+
+void k_mem_paging_backing_store_page_out(uintptr_t location)
+{
+	(void)memcpy(location_to_flash(location), Z_SCRATCH_PAGE,
+		     CONFIG_MMU_PAGE_SIZE);
+}
+
+void k_mem_paging_backing_store_page_in(uintptr_t location)
+{
+	(void)memcpy(Z_SCRATCH_PAGE, location_to_flash(location),
+		     CONFIG_MMU_PAGE_SIZE);
+}
+
+void k_mem_paging_backing_store_page_finalize(struct z_page_frame *pf,
+					      uintptr_t location)
+{
+	/* Nothing to do */
+}
+
+void k_mem_paging_backing_store_init(void)
+{
+	/* Nothing to do */
+}