On Fri, Feb 07, 2014 at 10:18:51AM +0000, AKASHI Takahiro wrote:

This patch implements arm64 specific part to support function tracers,
such as function (CONFIG_FUNCTION_TRACER), function_graph
(CONFIG_FUNCTION_GRAPH_TRACER) and function profiler
(CONFIG_FUNCTION_PROFILER).

With 'function' tracer, all the functions in the kernel are traced with
timestamps in ${sysfs}/tracing/trace. If function_graph tracer is
specified, call graph is generated.

The kernel must be compiled with -pg option so that _mcount() is inserted
at the beginning of functions. This function is called on every function's
entry as long as tracing is enabled.
In addition, function_graph tracer also needs to be able to probe function's
exit. ftrace_graph_caller() & return_to_handler do this by faking link
register's value to intercept function's return path.

More details on architecture specific requirements are described in
Documentation/trace/ftrace-design.txt.

[...]

You can use AARCH64_INSN_SIZE here.

+#include <linux/linkage.h>
+#include <asm/ftrace.h>
+
+/*
+ * Gcc with -pg will put the following code in the beginning of each function:
+ *      mov x0, x30
+ *      bl _mcount
+ *     [function's body ...]
+ * "bl _mcount" may be replaced to "bl ftrace_caller" or NOP if dynamic
+ * ftrace is enabled.
+ *
+ * Please note that x0 as an argument will not be used here because we can
+ * get lr(x30) of insturmented function at any time by winding up call stack

instrumented

+ * as long as the kernel is compiled without -fomit-frame-pointer.
+ * (or CONFIG_FRAME_POINTER, this is forced on arm64)
+ *
+ * stack layout after mcount_enter:
+ *
+ *   0 ---------------------------------------- sp of _mcount()
+ *      x29: fp of instrumented function       fp is not winded
+ *     --------------------
+ *      x30: lr of _mcount() (= instrumented function's pc)
+ * +16 ---------------------------------------- sp of instrumented function
+ *
+ *     ....
+ *
+ * +xx ---------------------------------------- fp of instrumented function
+ *      x29: fp of parent
+ *     --------------------
+ *      x30: lr of insturmented function (= parent's pc)
+ *     --------------------
+ *      xxx
+ */
+
+       .macro mcount_enter
+       stp     x29, x30, [sp, #-48]!
+       .endm

Can you elaborate in your comment about where this 48 comes from please? I
can't join it up with your ascii art.

Will