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+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\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).
+
+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).