Hi Vaibhav, On Wed, Apr 11, 2012 at 2:22 PM, Vaibhav Jain <vjoss197@gmail.com> wrote:

I am not clear about the use of identity mapped paging while paging is being enabled by the operating system. Also I don't understand at what point are the identity mappings no longer useful.According to this article http://geezer.osdevbrasil.net/osd/mem/index.htm#identity - "The page table entries used to identity-map kernel memory can be deleted once paging and virtual addresses are enabled." Can somebody please explain?

The identity mapping are used when the MMU is first enabled. I'm most familiar with the ARM, so the rest of my comments are ARM specific. When the kernel first gets control, the MMU is off, so virtual addresses == physical addresses. The kernel sets up a small identity mapping (virtual == physical) as well as a the kernel direct mapping. This happens in assembler code before any C code in the kernel runs. http://lxr.linux.no/linux+v3.4.3/arch/arm/kernel/head.S#L203 The identity mapping is needed for the code that turns on the MMU. The identity mapping is only needed for a few instructions. If you look at http://lxr.linux.no/linux+v3.4.3/arch/arm/kernel/head.S#L451 The identity mapping is really only needed while lines 454 thru 459 execute. The mov pc, r13 instruction changes the PC to be in the 0xCxxxxxxx range (assuming a normal 3G user/ 1G kernel split is being used). Later on, in paging_init (which is written in C), that identity mapping is removed, and the MMU is setup properly for the rest of the kernel. -- Dave Hylands Shuswap, BC, Canada http://www.davehylands.com