demand_paging: add a semihosting based backing store for on-demand sections

This is especially handy for tests and validation using QEMU.

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

subsys/demand_paging/backing_store/CMakeLists.txt

@@ -16,4 +16,9 @@ if(NOT DEFINED CONFIG_BACKING_STORE_CUSTOM)
     CONFIG_BACKING_STORE_QEMU_X86_TINY_FLASH
     backing_store_qemu_x86_tiny.c
     )
+
+  zephyr_library_sources_ifdef(
+    CONFIG_BACKING_STORE_ONDEMAND_SEMIHOST
+    backing_store_ondemand_semihost.c
+    )
 endif()

subsys/demand_paging/backing_store/Kconfig

@@ -28,6 +28,16 @@ config BACKING_STORE_QEMU_X86_TINY_FLASH
 	  the symbols outside of boot and pinned sections into the flash
 	  area, allowing testing of the demand paging mechanism on
 	  code and data.
+
+config BACKING_STORE_ONDEMAND_SEMIHOST
+	bool "Backing store for on-demand linker section using semihosting"
+	depends on SEMIHOST && LINKER_USE_ONDEMAND_SECTION
+	help
+	  This is used to do on-demand paging of code and data marked with
+	  __ondemand_func and __ondemand_rodata tags respectively. The compiled
+	  zephyr.bin on the host is used to retrieve needed data with the
+	  semihosting I/O facility.
+
 endchoice
 
 if BACKING_STORE_RAM

subsys/demand_paging/backing_store/backing_store_ondemand_semihost.c

@@ -0,0 +1,75 @@
+/*
+ * Copyright (c) 2024 BayLibre SAS
+ *
+ * SPDX-License-Identifier: Apache-2.0
+ */
+#include <mmu.h>
+#include <kernel_arch_interface.h>
+#include <zephyr/kernel/mm/demand_paging.h>
+#include <zephyr/linker/linker-defs.h>
+#include <zephyr/arch/common/semihost.h>
+
+/*
+ * semihost.h declares prototypes with longs but (at least on QEMU)
+ * returned values are 32-bits only. Let's use an int.
+ */
+static int semih_fd = -1;
+
+int k_mem_paging_backing_store_location_get(struct k_mem_page_frame *pf,
+					    uintptr_t *location,
+					    bool page_fault)
+{
+	if (k_mem_page_frame_is_backed(pf)) {
+		return k_mem_paging_backing_store_location_query(
+				k_mem_page_frame_to_virt(pf), location);
+	} else {
+		/* this is a read-only backing store */
+		return -ENOMEM;
+	}
+}
+
+void k_mem_paging_backing_store_location_free(uintptr_t location)
+{
+}
+
+void k_mem_paging_backing_store_page_out(uintptr_t location)
+{
+	__ASSERT(true, "not ever supposed to be called");
+	k_panic();
+}
+
+void k_mem_paging_backing_store_page_in(uintptr_t location)
+{
+	long size = CONFIG_MMU_PAGE_SIZE;
+
+	if (semihost_seek(semih_fd, (long)location) != 0 ||
+	    semihost_read(semih_fd, K_MEM_SCRATCH_PAGE, size) != size) {
+		k_panic();
+	}
+}
+
+void k_mem_paging_backing_store_page_finalize(struct k_mem_page_frame *pf,
+					      uintptr_t location)
+{
+	k_mem_page_frame_set(pf, K_MEM_PAGE_FRAME_BACKED);
+}
+
+int k_mem_paging_backing_store_location_query(void *addr, uintptr_t *location)
+{
+	uintptr_t offset = (uintptr_t)addr - (uintptr_t)lnkr_ondemand_start;
+	uintptr_t file_offset = (uintptr_t)lnkr_ondemand_load_start
+				- (uintptr_t)__text_region_start + offset;
+
+	__ASSERT(file_offset % CONFIG_MMU_PAGE_SIZE == 0, "file_offset = %#lx", file_offset);
+	*location = file_offset;
+	return 0;
+}
+
+void k_mem_paging_backing_store_init(void)
+{
+	semih_fd = semihost_open("./zephyr/zephyr.bin", SEMIHOST_OPEN_RB);
+	__ASSERT(semih_fd >= 0, "semihost_open() returned %d", semih_fd);
+	if (semih_fd < 0) {
+		k_panic();
+	}
+}