tests: demand_paging: add a test for on-demand sections

Exercises linker placement, the ondemand backing store, demand paging.

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

tests/kernel/mem_protect/demand_paging/ondemand_section/CMakeLists.txt

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+# SPDX-License-Identifier: Apache-2.0
+
+cmake_minimum_required(VERSION 3.20.0)
+find_package(Zephyr REQUIRED HINTS $ENV{ZEPHYR_BASE})
+project(ondemand_section)
+
+target_include_directories(app PRIVATE
+  ${ZEPHYR_BASE}/kernel/include
+  ${ZEPHYR_BASE}/arch/${ARCH}/include
+  )
+
+FILE(GLOB app_sources src/*.c)
+target_sources(app PRIVATE ${app_sources})

tests/kernel/mem_protect/demand_paging/ondemand_section/prj.conf

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+CONFIG_ZTEST=y
+CONFIG_DEMAND_PAGING=y
+CONFIG_DEMAND_PAGING_ALLOW_IRQ=y
+CONFIG_DEMAND_PAGING_STATS=y
+CONFIG_DEMAND_MAPPING=y
+CONFIG_LINKER_USE_ONDEMAND_SECTION=y

tests/kernel/mem_protect/demand_paging/ondemand_section/src/main.c

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+/*
+ * Copyright (c) 2024 BayLibre SAS
+ *
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+#include <zephyr/ztest.h>
+#include <zephyr/kernel/mm.h>
+#include <zephyr/kernel/mm/demand_paging.h>
+#include <zephyr/linker/sections.h>
+#include <mmu.h>
+
+static const char __ondemand_rodata message[] = "was evicted";
+
+static void __ondemand_func evictable_function(void)
+{
+	static int count;
+
+	printk("This %s code, count=%d\n", message, ++count);
+}
+
+ZTEST(ondemand_section, test_ondemand_basic)
+{
+	unsigned long faults_before, faults_after;
+	void *addr = (void *)ROUND_DOWN(&evictable_function, CONFIG_MMU_PAGE_SIZE);
+
+	printk("About to call unpaged code\n");
+	faults_before = k_mem_num_pagefaults_get();
+	evictable_function();
+	faults_after = k_mem_num_pagefaults_get();
+	zassert_not_equal(faults_before, faults_after, "should have faulted");
+
+	printk("Code should be resident on second call\n");
+	faults_before = k_mem_num_pagefaults_get();
+	evictable_function();
+	faults_after = k_mem_num_pagefaults_get();
+	zassert_equal(faults_before, faults_after, "should not have faulted");
+
+	printk("Forcefully evicting it from memory\n");
+	zassert_ok(k_mem_page_out(addr, CONFIG_MMU_PAGE_SIZE), "");
+
+	printk("Calling it again\n");
+	faults_before = k_mem_num_pagefaults_get();
+	evictable_function();
+	faults_after = k_mem_num_pagefaults_get();
+	zassert_not_equal(faults_before, faults_after, "should have faulted");
+
+	printk("Forcefully evicting it from memory again\n");
+	zassert_ok(k_mem_page_out(addr, CONFIG_MMU_PAGE_SIZE), "");
+
+	printk("Preemptively fetching it back in\n");
+	/* strangely, k_mem_page_in() returns void */
+	k_mem_page_in(addr, CONFIG_MMU_PAGE_SIZE);
+
+	printk("Code should be resident\n");
+	faults_before = k_mem_num_pagefaults_get();
+	evictable_function();
+	faults_after = k_mem_num_pagefaults_get();
+	zassert_equal(faults_before, faults_after, "should not have faulted");
+}
+
+ZTEST_SUITE(ondemand_section, NULL, NULL, NULL, NULL, NULL);

tests/kernel/mem_protect/demand_paging/ondemand_section/testcase.yaml

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+common:
+  tags:
+    - kernel
+    - mmu
+    - demand_paging
+tests:
+  kernel.demand_paging.ondemand_section.arm64_semihost:
+    platform_allow:
+      - qemu_cortex_a53
+    extra_configs:
+      - CONFIG_SEMIHOST=y
+      - CONFIG_BACKING_STORE_ONDEMAND_SEMIHOST=y