x86_64, vsyscall: Rewrite comment and clean up headers in vsyscall code

/*

 * Copyright (c) 2012-2014 Andy Lutomirski <luto@amacapital.net>

 *

 * Based on the original implementation which is:

 *  Copyright (C) 2001 Andrea Arcangeli <andrea@suse.de> SuSE

 *  Copyright 2003 Andi Kleen, SuSE Labs.

 *

 *  [ NOTE: this mechanism is now deprecated in favor of the vDSO. ]

 *  Parts of the original code have been moved to arch/x86/vdso/vma.c

 *

 * This file implements vsyscall emulation.  vsyscalls are a legacy ABI:

 * Userspace can request certain kernel services by calling fixed

 * addresses.  This concept is problematic:

 *

 *  Thanks to hpa@transmeta.com for some useful hint.

 *  Special thanks to Ingo Molnar for his early experience with

 *  a different vsyscall implementation for Linux/IA32 and for the name.

 * - It interferes with ASLR.

 * - It's awkward to write code that lives in kernel addresses but is

 *   callable by userspace at fixed addresses.

 * - The whole concept is impossible for 32-bit compat userspace.

 * - UML cannot easily virtualize a vsyscall.

 *

 *  vsyscall 1 is located at -10Mbyte, vsyscall 2 is located

 *  at virtual address -10Mbyte+1024bytes etc... There are at max 4

 *  vsyscalls. One vsyscall can reserve more than 1 slot to avoid

 *  jumping out of line if necessary. We cannot add more with this

 *  mechanism because older kernels won't return -ENOSYS.

 * As of mid-2014, I believe that there is no new userspace code that

 * will use a vsyscall if the vDSO is present.  I hope that there will

 * soon be no new userspace code that will ever use a vsyscall.

 *

 *  Note: the concept clashes with user mode linux.  UML users should

 *  use the vDSO.

 * The code in this file emulates vsyscalls when notified of a page

 * fault to a vsyscall address.

 */

#define pr_fmt(fmt) KBUILD_MODNAME ": " fmt

#include <linux/time.h>

#include <linux/init.h>

#include <linux/kernel.h>

#include <linux/timer.h>

#include <linux/seqlock.h>

#include <linux/jiffies.h>

#include <linux/sysctl.h>

#include <linux/topology.h>

#include <linux/timekeeper_internal.h>

#include <linux/getcpu.h>

#include <linux/cpu.h>

#include <linux/smp.h>

#include <linux/notifier.h>

#include <linux/syscalls.h>

#include <linux/ratelimit.h>

#include <asm/vsyscall.h>

#include <asm/pgtable.h>

#include <asm/compat.h>

#include <asm/page.h>

#include <asm/unistd.h>

#include <asm/fixmap.h>

#include <asm/errno.h>

#include <asm/io.h>

#include <asm/segment.h>

#include <asm/desc.h>

#include <asm/topology.h>

#include <asm/traps.h>

#define CREATE_TRACE_POINTS