Now, of course, this hardware memory on the chip, the so-called I/O memory, is not RAM. The Linux kernel refuses the module or driver author direct access to such hardware I/O memory locations. We already know why: on a modern VM-based OS, all memory access has to be via the Memory Management Unit (MMU) and paging tables.
Let's quickly summarize the key aspect of what was seen in the companion guide Linux Kernel Programming in Chapter 7, Memory Management Internals – Essentials: by default, memory is virtualized, which means that all addresses are virtual and not physical (this includes the addresses within the kernel segment or VAS). Think of it this way: once a virtual address is accessed by a process (or the kernel) for reading or writing or execution, the system has to fetch the memory content at the corresponding physical address. This involves translating the virtual address to the physical address at runtime...
