Linux (Ubuntu), C language: Virtual to Physical Address Translation

Question

As the title suggests, I have a problem of obtaining the physical address from a virtual one.

Let me explain: Given a variable declaration in process space, how can

Answer 1

As partially answered by the original poster, the Linux kernel exposes its mapping to userland through a set of files in the /proc. The documentation can be found here. Short summary:

1. /proc/$pid/maps provides a list of mappings with their virtual addresses and the corresponding file for mapped files.
2. /proc/$pid/pagemap provides more information about each mapped page, including the physical address if it exists.

As the original poster found later, this example provides a sample implementation of how to use these files.

Answer 2

In user code, you can't know the physical address corresponding to a virtual address. This is information is simply not exported outside the kernel. It could even change at any time, especially if the kernel decides to swap out part of your process's memory.

In /proc/$pid/maps, you have information about what the virtual addresses in your program's address space correspond to (mmapped files, heap, stack, etc.). That's all you'll get.

If you're working on kernel code (which you aren't, apparently), you can find out the physical address corresponding to a page of memory. But even then virt_to_phys isn't the whole story; I recommend reading Linux Device Drivers (especially chapters 8 and 15).

The header asm/io.h is a kernel header. It's not available when you're compiling user code because its contents just wouldn't make sense. The functions it declares aren't available in any library, only in the kernel.

Answer 3

Pass the virtual address to the kernel using systemcall/procfs and use vmalloc_to_pfn. Return the Physical address through procfs/registers.