Kernel-provided User Helpers
  ============================
  
  These are segment of kernel provided user code reachable from user space
  at a fixed address in kernel memory.  This is used to provide user space
  with some operations which require kernel help because of unimplemented
  native feature and/or instructions in many ARM CPUs. The idea is for this
  code to be executed directly in user mode for best efficiency but which is
  too intimate with the kernel counter part to be left to user libraries.
  In fact this code might even differ from one CPU to another depending on
  the available instruction set, or whether it is a SMP systems. In other
  words, the kernel reserves the right to change this code as needed without
  warning. Only the entry points and their results as documented here are
  guaranteed to be stable.
  
  This is different from (but doesn't preclude) a full blown VDSO
  implementation, however a VDSO would prevent some assembly tricks with
  constants that allows for efficient branching to those code segments. And
  since those code segments only use a few cycles before returning to user
  code, the overhead of a VDSO indirect far call would add a measurable
  overhead to such minimalistic operations.
  
  User space is expected to bypass those helpers and implement those things
  inline (either in the code emitted directly by the compiler, or part of
  the implementation of a library call) when optimizing for a recent enough
  processor that has the necessary native support, but only if resulting
  binaries are already to be incompatible with earlier ARM processors due to

  usage of similar native instructions for other things.  In other words

  don't make binaries unable to run on earlier processors just for the sake
  of not using these kernel helpers if your compiled code is not going to
  use new instructions for other purpose.
  
  New helpers may be added over time, so an older kernel may be missing some
  helpers present in a newer kernel.  For this reason, programs must check
  the value of __kuser_helper_version (see below) before assuming that it is
  safe to call any particular helper.  This check should ideally be
  performed only once at process startup time, and execution aborted early
  if the required helpers are not provided by the kernel version that
  process is running on.
  
  kuser_helper_version
  --------------------
  
  Location:	0xffff0ffc
  
  Reference declaration:
  
    extern int32_t __kuser_helper_version;
  
  Definition:
  
    This field contains the number of helpers being implemented by the
    running kernel.  User space may read this to determine the availability
    of a particular helper.
  
  Usage example:
  
  #define __kuser_helper_version (*(int32_t *)0xffff0ffc)
  
  void check_kuser_version(void)
  {
  	if (__kuser_helper_version < 2) {
  		fprintf(stderr, "can't do atomic operations, kernel too old
  ");
  		abort();
  	}
  }
  
  Notes:
  
    User space may assume that the value of this field never changes
    during the lifetime of any single process.  This means that this
    field can be read once during the initialisation of a library or
    startup phase of a program.
  
  kuser_get_tls
  -------------
  
  Location:	0xffff0fe0
  
  Reference prototype:
  
    void * __kuser_get_tls(void);
  
  Input:
  
    lr = return address
  
  Output:
  
    r0 = TLS value
  
  Clobbered registers:
  
    none
  
  Definition:
  
    Get the TLS value as previously set via the __ARM_NR_set_tls syscall.
  
  Usage example:
  
  typedef void * (__kuser_get_tls_t)(void);
  #define __kuser_get_tls (*(__kuser_get_tls_t *)0xffff0fe0)
  
  void foo()
  {
  	void *tls = __kuser_get_tls();
  	printf("TLS = %p
  ", tls);
  }
  
  Notes:
  
    - Valid only if __kuser_helper_version >= 1 (from kernel version 2.6.12).
  
  kuser_cmpxchg
  -------------
  
  Location:	0xffff0fc0
  
  Reference prototype:
  
    int __kuser_cmpxchg(int32_t oldval, int32_t newval, volatile int32_t *ptr);
  
  Input:
  
    r0 = oldval
    r1 = newval
    r2 = ptr
    lr = return address
  
  Output:
  
    r0 = success code (zero or non-zero)
    C flag = set if r0 == 0, clear if r0 != 0
  
  Clobbered registers:
  
    r3, ip, flags
  
  Definition:
  
    Atomically store newval in *ptr only if *ptr is equal to oldval.
    Return zero if *ptr was changed or non-zero if no exchange happened.
    The C flag is also set if *ptr was changed to allow for assembly
    optimization in the calling code.
  
  Usage example:
  
  typedef int (__kuser_cmpxchg_t)(int oldval, int newval, volatile int *ptr);
  #define __kuser_cmpxchg (*(__kuser_cmpxchg_t *)0xffff0fc0)
  
  int atomic_add(volatile int *ptr, int val)
  {
  	int old, new;
  
  	do {
  		old = *ptr;
  		new = old + val;
  	} while(__kuser_cmpxchg(old, new, ptr));
  
  	return new;
  }
  
  Notes:
  
    - This routine already includes memory barriers as needed.
  
    - Valid only if __kuser_helper_version >= 2 (from kernel version 2.6.12).
  
  kuser_memory_barrier
  --------------------
  
  Location:	0xffff0fa0
  
  Reference prototype:
  
    void __kuser_memory_barrier(void);
  
  Input:
  
    lr = return address
  
  Output:
  
    none
  
  Clobbered registers:
  
    none
  
  Definition:
  
    Apply any needed memory barrier to preserve consistency with data modified
    manually and __kuser_cmpxchg usage.
  
  Usage example:
  
  typedef void (__kuser_dmb_t)(void);
  #define __kuser_dmb (*(__kuser_dmb_t *)0xffff0fa0)
  
  Notes:
  
    - Valid only if __kuser_helper_version >= 3 (from kernel version 2.6.15).

  
  kuser_cmpxchg64
  ---------------
  
  Location:	0xffff0f60
  
  Reference prototype:
  
    int __kuser_cmpxchg64(const int64_t *oldval,
                          const int64_t *newval,
                          volatile int64_t *ptr);
  
  Input:
  
    r0 = pointer to oldval
    r1 = pointer to newval
    r2 = pointer to target value
    lr = return address
  
  Output:
  
    r0 = success code (zero or non-zero)
    C flag = set if r0 == 0, clear if r0 != 0
  
  Clobbered registers:
  
    r3, lr, flags
  
  Definition:
  
    Atomically store the 64-bit value pointed by *newval in *ptr only if *ptr
    is equal to the 64-bit value pointed by *oldval.  Return zero if *ptr was
    changed or non-zero if no exchange happened.
  
    The C flag is also set if *ptr was changed to allow for assembly
    optimization in the calling code.
  
  Usage example:
  
  typedef int (__kuser_cmpxchg64_t)(const int64_t *oldval,
                                    const int64_t *newval,
                                    volatile int64_t *ptr);
  #define __kuser_cmpxchg64 (*(__kuser_cmpxchg64_t *)0xffff0f60)
  
  int64_t atomic_add64(volatile int64_t *ptr, int64_t val)
  {
  	int64_t old, new;
  
  	do {
  		old = *ptr;
  		new = old + val;
  	} while(__kuser_cmpxchg64(&old, &new, ptr));
  
  	return new;
  }
  
  Notes:
  
    - This routine already includes memory barriers as needed.
  
    - Due to the length of this sequence, this spans 2 conventional kuser
      "slots", therefore 0xffff0f80 is not used as a valid entry point.
  
    - Valid only if __kuser_helper_version >= 5 (from kernel version 3.1).