From: Gaurav Dhiman [mailto:gaurav4lkg@xxxxxxxxx] > > Linux kernel provides you with functions like virt_to_phys and friends to > convert a kernel mode virtual address to physical address and vice versa. >The functions you are talking are specifically for converting the >physical addresses to kernel virtual address not to user virtual >address space Infact, that was what I also meant. >As I looked into the functions "virt_to_phys" and "phys_to_virt" in >page.h file, i found the following macros: >#define __PAGE_OFFSET (0xC0000000UL) >#define PAGE_OFFSET ((unsigned long)__PAGE_OFFSET) >#define __pa(x) ((unsigned long)(x)-PAGE_OFFSET) >#define __va(x) ((void *)((unsigned long)(x)+PAGE_OFFSET)) >As per my understanding PAGE_OFFSET is the starting kernel virtual >address, that means kernel virtual address will start from >PAGE_OFFSET. >My question is, why we are subtracting PAGE_OFFSET from a given >address in __pa() macro, is it because physically kernel code is >placed in lower memory. If this is not the reason, can someone please >clarify this ? In the case of a 3:1 mapping, PAGE_OFFSET is 3 GB (0xc0000000) and the page tables are set such that , virtual address 0xc0000000 points to physical address 0x0 and so on. I have also seen systems with 2:2 mapping and there PAGE_OFFSET is 2GB and hence the pysical address = virt addr - 2GB. -- Kernelnewbies: Help each other learn about the Linux kernel. Archive: http://mail.nl.linux.org/kernelnewbies/ FAQ: http://kernelnewbies.org/faq/
