parisc: document the parisc gateway page

/* 

 * Linux/PA-RISC Project (http://www.parisc-linux.org/)

 * 

 * System call entry code Copyright (c) Matthew Wilcox 1999 <willy@bofh.ai>

 * System call entry code / Linux gateway page

 * Copyright (c) Matthew Wilcox 1999 <willy@bofh.ai>

 * Licensed under the GNU GPL.

 * thanks to Philipp Rumpf, Mike Shaver and various others

 * sorry about the wall, puffin..

 */

/*

How does the Linux gateway page on PA-RISC work?

------------------------------------------------

The Linux gateway page on PA-RISC is "special".

It actually has PAGE_GATEWAY bits set (this is linux terminology; in parisc

terminology it's Execute, promote to PL0) in the page map.  So anything

executing on this page executes with kernel level privilege (there's more to it

than that: to have this happen, you also have to use a branch with a ,gate

completer to activate the privilege promotion).  The upshot is that everything

that runs on the gateway page runs at kernel privilege but with the current

user process address space (although you have access to kernel space via %sr2).

For the 0x100 syscall entry, we redo the space registers to point to the kernel

address space (preserving the user address space in %sr3), move to wide mode if

required, save the user registers and branch into the kernel syscall entry

point.  For all the other functions, we execute at kernel privilege but don't

flip address spaces. The basic upshot of this is that these code snippets are

executed atomically (because the kernel can't be pre-empted) and they may

perform architecturally forbidden (to PL3) operations (like setting control

registers).

*/

#include <asm/asm-offsets.h>

#include <asm/unistd.h>

#include <asm/errno.h>