virt_to_phys: fix %p vs %px

Move doc to README

README.adoc

@@ -4127,7 +4127,9 @@ The module also shows which handlers are registered for each IRQ, as we have obs
 
 When in text mode, we can also observe interrupt line 4 with handler `ttyS0` increase continuously as IO goes through the UART.
 
-=== Linux kernel utility functions
+=== Kernel utility functions
+
+https://github.com/torvalds/linux/blob/v4.17/Documentation/core-api/kernel-api.rst
 
 ==== kstrto
 
@@ -4151,6 +4153,54 @@ Sources:
 
 Bibliography: https://stackoverflow.com/questions/6139493/how-convert-char-to-int-in-linux-kernel/49811658#49811658
 
+==== virt_to_phys
+
+Convert a virtual address to physical:
+
+....
+insmod /virt_to_phys.ko
+cat /sys/kernel/debug/lkmc_virt_to_phys
+....
+
+Source: link:kernel_module/virt_to_phys.c[]
+
+Sample output:
+
+....
+*kmalloc_ptr = 0x12345678
+kmalloc_ptr = ffff88000e169ae8
+virt_to_phys(kmalloc_ptr) = 0xe169ae8
+static_var = 0x12345678
+&static_var = ffffffffc0002308
+virt_to_phys(&static_var) = 0x40002308
+....
+
+We can confirm that the `kmalloc_ptr` translation worked with:
+
+....
+./qemumonitor 'xp 0xe169ae8'
+....
+
+which reads four bytes from a given physical address, and gives the expected:
+
+....
+000000000e169ae8: 0x12345678
+....
+
+TODO it only works for kmalloc however, for the static variable:
+
+....
+./qemumonitor 'xp 0x40002308'
+....
+
+it gave a wrong value of `00000000`.
+
+Bibliography:
+
+* https://stackoverflow.com/questions/5748492/is-there-any-api-for-determining-the-physical-address-from-virtual-address-in-li/45128487#45128487
+* https://stackoverflow.com/questions/39134990/mmap-of-dev-mem-fails-with-invalid-argument-for-virt-to-phys-address-but-addre/45127582#45127582
+* https://stackoverflow.com/questions/43325205/can-we-use-virt-to-phys-for-user-space-memory-in-kernel-module
+
 === Linux kernel tracing
 
 Good overviews: