failed virt_to_phys attempt

kernel_module/README.md

@@ -27,6 +27,7 @@
         1.  [dep](dep.c)
         1.  [dep2](dep2.c)
     1.  [character_device](character_device.c)
+    1.  [virt_to_phys](virt_to_phys.c)
     1.  Hardware device drivers
         1.  [pci_min](pci_min.c)
         1.  [pci](pci.c)

kernel_module/virt_to_phys.c

@@ -0,0 +1,67 @@
+/*
+TODO not working. Tested with both:
+
+	/virt_to_phys.sh
+
+and on QEMU monitor:
+
+	xp 0x<vaddr>
+
+- https://stackoverflow.com/questions/5748492/is-there-any-api-for-determining-the-physical-address-from-virtual-address-in-li
+- https://stackoverflow.com/questions/43325205/can-we-use-virt-to-phys-for-user-space-memory-in-kernel-module
+*/
+
+#include <asm/io.h> /* virt_to_phys */
+#include <linux/debugfs.h>
+#include <linux/delay.h> /* usleep_range */
+#include <linux/kernel.h>
+#include <linux/kthread.h>
+#include <linux/module.h>
+#include <linux/seq_file.h> /* single_open, single_release */
+
+MODULE_LICENSE("GPL");
+
+static volatile u32 i = 0x12345678;
+
+static struct dentry *debugfs_file;
+
+static int show(struct seq_file *m, void *v)
+{
+	seq_printf(m,
+		"i 0x%llx\n"
+		"&i %p\n"
+		"virt_to_phys 0x%llx\n",
+		(unsigned long long)i,
+		&i,
+		(unsigned long long)virt_to_phys(&i)
+	);
+	return 0;
+}
+
+static int open(struct inode *inode, struct  file *file)
+{
+	return single_open(file, show, NULL);
+}
+
+static const struct file_operations fops = {
+	.llseek = seq_lseek,
+	.open = open,
+	.owner = THIS_MODULE,
+	.read = seq_read,
+	.release = single_release,
+};
+
+static int myinit(void)
+{
+	debugfs_file = debugfs_create_file(
+		"lkmc_virt_to_phys", S_IRUSR, NULL, NULL, &fops);
+	return 0;
+}
+
+static void myexit(void)
+{
+	debugfs_remove(debugfs_file);
+}
+
+module_init(myinit)
+module_exit(myexit)

rootfs_overlay/virt_to_phys.sh

@@ -0,0 +1,8 @@
+#!/bin/sh
+set -ex
+insmod /virt_to_phys.ko
+cat /sys/kernel/debug/lkmc_virt_to_phys
+addr=$(grep virt_to_phys /sys/kernel/debug/lkmc_virt_to_phys | cut -d ' ' -f 2)
+devmem2 "$addr"
+devmem2 "$addr" w 0x87654321
+rmmod virt_to_phys