ARM: kuser: move interface documentation out of the source code

Digging into some assembly file in order to get information about the
kuser helpers is not that convivial. Let's move that information to
a better formatted file in Documentation/arm/ and improve on it a bit.

Thanks to Dave Martin <[e-mail]> for the initial cleanup and
clarifications.

Signed-off-by: Nicolas Pitre <[e-mail]>
Acked-by: Dave Martin <[e-mail]>

Documentation/arm/kernel_user_helpers.txt

···
+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
+useage 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\n");
+		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\n", 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).

arch/arm/kernel/entry-armv.S

···
 /*
  * 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 and restrictions like on
- * 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 are guaranteed to be stable.
- *
  * Each segment is 32-byte aligned and will be moved to the top of the high
  * vector page.  New segments (if ever needed) must be added in front of
  * existing ones.  This mechanism should be used only for things that are
  * really small and justified, and not be abused freely.
  *
- * User space is expected to implement those things inline when optimizing
- * for a processor that has the necessary native support, but only if such
- * resulting binaries are already to be incompatible with earlier ARM
- * processors due to the use of unsupported instructions other than what
- * is provided here.  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 the new instructions for other
- * purpose.
+ * See Documentation/arm/kernel_user_helpers.txt for formal definitions.
  */
  THUMB(	.arm	)
 
···
 	.globl	__kuser_helper_start
 __kuser_helper_start:
 
-/*
- * Reference prototype:
- *
- *	void __kernel_memory_barrier(void)
- *
- * Input:
- *
- *	lr = return address
- *
- * Output:
- *
- *	none
- *
- * Clobbered:
- *
- *	none
- *
- * Definition and user space usage example:
- *
- *	typedef void (__kernel_dmb_t)(void);
- *	#define __kernel_dmb (*(__kernel_dmb_t *)0xffff0fa0)
- *
- * Apply any needed memory barrier to preserve consistency with data modified
- * manually and __kuser_cmpxchg usage.
- *
- * This could be used as follows:
- *
- * #define __kernel_dmb() \
- *         asm volatile ( "mov r0, #0xffff0fff; mov lr, pc; sub pc, r0, #95" \
- *	        : : : "r0", "lr","cc" )
- */
-
 __kuser_memory_barrier:				@ 0xffff0fa0
 	smp_dmb	arm
 	usr_ret	lr
 
 	.align	5
-
-/*
- * Reference prototype:
- *
- *	int __kernel_cmpxchg(int oldval, int newval, int *ptr)
- *
- * Input:
- *
- *	r0 = oldval
- *	r1 = newval
- *	r2 = ptr
- *	lr = return address
- *
- * Output:
- *
- *	r0 = returned value (zero or non-zero)
- *	C flag = set if r0 == 0, clear if r0 != 0
- *
- * Clobbered:
- *
- *	r3, ip, flags
- *
- * Definition and user space usage example:
- *
- *	typedef int (__kernel_cmpxchg_t)(int oldval, int newval, int *ptr);
- *	#define __kernel_cmpxchg (*(__kernel_cmpxchg_t *)0xffff0fc0)
- *
- * Atomically store newval in *ptr if *ptr is equal to oldval for user space.
- * 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.
- *
- * Notes:
- *
- *    - This routine already includes memory barriers as needed.
- *
- * For example, a user space atomic_add implementation could look like this:
- *
- * #define atomic_add(ptr, val) \
- *	({ register unsigned int *__ptr asm("r2") = (ptr); \
- *	   register unsigned int __result asm("r1"); \
- *	   asm volatile ( \
- *	       "1: @ atomic_add\n\t" \
- *	       "ldr	r0, [r2]\n\t" \
- *	       "mov	r3, #0xffff0fff\n\t" \
- *	       "add	lr, pc, #4\n\t" \
- *	       "add	r1, r0, %2\n\t" \
- *	       "add	pc, r3, #(0xffff0fc0 - 0xffff0fff)\n\t" \
- *	       "bcc	1b" \
- *	       : "=&r" (__result) \
- *	       : "r" (__ptr), "rIL" (val) \
- *	       : "r0","r3","ip","lr","cc","memory" ); \
- *	   __result; })
- */
 
 __kuser_cmpxchg:				@ 0xffff0fc0
 
···
 
 	.align	5
 
-/*
- * Reference prototype:
- *
- *	int __kernel_get_tls(void)
- *
- * Input:
- *
- *	lr = return address
- *
- * Output:
- *
- *	r0 = TLS value
- *
- * Clobbered:
- *
- *	none
- *
- * Definition and user space usage example:
- *
- *	typedef int (__kernel_get_tls_t)(void);
- *	#define __kernel_get_tls (*(__kernel_get_tls_t *)0xffff0fe0)
- *
- * Get the TLS value as previously set via the __ARM_NR_set_tls syscall.
- *
- * This could be used as follows:
- *
- * #define __kernel_get_tls() \
- *	({ register unsigned int __val asm("r0"); \
- *         asm( "mov r0, #0xffff0fff; mov lr, pc; sub pc, r0, #31" \
- *	        : "=r" (__val) : : "lr","cc" ); \
- *	   __val; })
- */
-
 __kuser_get_tls:				@ 0xffff0fe0
 	ldr	r0, [pc, #(16 - 8)]	@ read TLS, set in kuser_get_tls_init
 	usr_ret	lr
···
 	.rep	4
 	.word	0			@ 0xffff0ff0 software TLS value, then
 	.endr				@ pad up to __kuser_helper_version
-
-/*
- * Reference declaration:
- *
- *	extern unsigned int __kernel_helper_version;
- *
- * Definition and user space usage example:
- *
- *	#define __kernel_helper_version (*(unsigned int *)0xffff0ffc)
- *
- * User space may read this to determine the curent number of helpers
- * available.
- */
 
 __kuser_helper_version:				@ 0xffff0ffc
 	.word	((__kuser_helper_end - __kuser_helper_start) >> 5)