--- /dev/null
+ function tracer guts
+ ====================
+
+Introduction
+------------
+
+Here we will cover the architecture pieces that the common function tracing
+code relies on for proper functioning. Things are broken down into increasing
+complexity so that you can start simple and at least get basic functionality.
+
+Note that this focuses on architecture implementation details only. If you
+want more explanation of a feature in terms of common code, review the common
+ftrace.txt file.
+
+
+Prerequisites
+-------------
+
+Ftrace relies on these features being implemented:
+ STACKTRACE_SUPPORT - implement save_stack_trace()
+ TRACE_IRQFLAGS_SUPPORT - implement include/asm/irqflags.h
+
+
+HAVE_FUNCTION_TRACER
+--------------------
+
+You will need to implement the mcount and the ftrace_stub functions.
+
+The exact mcount symbol name will depend on your toolchain. Some call it
+"mcount", "_mcount", or even "__mcount". You can probably figure it out by
+running something like:
+ $ echo 'main(){}' | gcc -x c -S -o - - -pg | grep mcount
+ call mcount
+We'll make the assumption below that the symbol is "mcount" just to keep things
+nice and simple in the examples.
+
+Keep in mind that the ABI that is in effect inside of the mcount function is
+*highly* architecture/toolchain specific. We cannot help you in this regard,
+sorry. Dig up some old documentation and/or find someone more familiar than
+you to bang ideas off of. Typically, register usage (argument/scratch/etc...)
+is a major issue at this point, especially in relation to the location of the
+mcount call (before/after function prologue). You might also want to look at
+how glibc has implemented the mcount function for your architecture. It might
+be (semi-)relevant.
+
+The mcount function should check the function pointer ftrace_trace_function
+to see if it is set to ftrace_stub. If it is, there is nothing for you to do,
+so return immediately. If it isn't, then call that function in the same way
+the mcount function normally calls __mcount_internal -- the first argument is
+the "frompc" while the second argument is the "selfpc" (adjusted to remove the
+size of the mcount call that is embedded in the function).
+
+For example, if the function foo() calls bar(), when the bar() function calls
+mcount(), the arguments mcount() will pass to the tracer are:
+ "frompc" - the address bar() will use to return to foo()
+ "selfpc" - the address bar() (with _mcount() size adjustment)
+
+Also keep in mind that this mcount function will be called *a lot*, so
+optimizing for the default case of no tracer will help the smooth running of
+your system when tracing is disabled. So the start of the mcount function is
+typically the bare min with checking things before returning. That also means
+the code flow should usually kept linear (i.e. no branching in the nop case).
+This is of course an optimization and not a hard requirement.
+
+Here is some pseudo code that should help (these functions should actually be
+implemented in assembly):
+
+void ftrace_stub(void)
+{
+ return;
+}
+
+void mcount(void)
+{
+ /* save any bare state needed in order to do initial checking */
+
+ extern void (*ftrace_trace_function)(unsigned long, unsigned long);
+ if (ftrace_trace_function != ftrace_stub)
+ goto do_trace;
+
+ /* restore any bare state */
+
+ return;
+
+do_trace:
+
+ /* save all state needed by the ABI (see paragraph above) */
+
+ unsigned long frompc = ...;
+ unsigned long selfpc = <return address> - MCOUNT_INSN_SIZE;
+ ftrace_trace_function(frompc, selfpc);
+
+ /* restore all state needed by the ABI */
+}
+
+Don't forget to export mcount for modules !
+extern void mcount(void);
+EXPORT_SYMBOL(mcount);
+
+
+HAVE_FUNCTION_TRACE_MCOUNT_TEST
+-------------------------------
+
+This is an optional optimization for the normal case when tracing is turned off
+in the system. If you do not enable this Kconfig option, the common ftrace
+code will take care of doing the checking for you.
+
+To support this feature, you only need to check the function_trace_stop
+variable in the mcount function. If it is non-zero, there is no tracing to be
+done at all, so you can return.
+
+This additional pseudo code would simply be:
+void mcount(void)
+{
+ /* save any bare state needed in order to do initial checking */
+
++ if (function_trace_stop)
++ return;
+
+ extern void (*ftrace_trace_function)(unsigned long, unsigned long);
+ if (ftrace_trace_function != ftrace_stub)
+...
+
+
+HAVE_FUNCTION_GRAPH_TRACER
+--------------------------
+
+Deep breath ... time to do some real work. Here you will need to update the
+mcount function to check ftrace graph function pointers, as well as implement
+some functions to save (hijack) and restore the return address.
+
+The mcount function should check the function pointers ftrace_graph_return
+(compare to ftrace_stub) and ftrace_graph_entry (compare to
+ftrace_graph_entry_stub). If either of those are not set to the relevant stub
+function, call the arch-specific function ftrace_graph_caller which in turn
+calls the arch-specific function prepare_ftrace_return. Neither of these
+function names are strictly required, but you should use them anyways to stay
+consistent across the architecture ports -- easier to compare & contrast
+things.
+
+The arguments to prepare_ftrace_return are slightly different than what are
+passed to ftrace_trace_function. The second argument "selfpc" is the same,
+but the first argument should be a pointer to the "frompc". Typically this is
+located on the stack. This allows the function to hijack the return address
+temporarily to have it point to the arch-specific function return_to_handler.
+That function will simply call the common ftrace_return_to_handler function and
+that will return the original return address with which, you can return to the
+original call site.
+
+Here is the updated mcount pseudo code:
+void mcount(void)
+{
+...
+ if (ftrace_trace_function != ftrace_stub)
+ goto do_trace;
+
++#ifdef CONFIG_FUNCTION_GRAPH_TRACER
++ extern void (*ftrace_graph_return)(...);
++ extern void (*ftrace_graph_entry)(...);
++ if (ftrace_graph_return != ftrace_stub ||
++ ftrace_graph_entry != ftrace_graph_entry_stub)
++ ftrace_graph_caller();
++#endif
+
+ /* restore any bare state */
+...
+
+Here is the pseudo code for the new ftrace_graph_caller assembly function:
+#ifdef CONFIG_FUNCTION_GRAPH_TRACER
+void ftrace_graph_caller(void)
+{
+ /* save all state needed by the ABI */
+
+ unsigned long *frompc = &...;
+ unsigned long selfpc = <return address> - MCOUNT_INSN_SIZE;
+ prepare_ftrace_return(frompc, selfpc);
+
+ /* restore all state needed by the ABI */
+}
+#endif
+
+For information on how to implement prepare_ftrace_return(), simply look at
+the x86 version. The only architecture-specific piece in it is the setup of
+the fault recovery table (the asm(...) code). The rest should be the same
+across architectures.
+
+Here is the pseudo code for the new return_to_handler assembly function. Note
+that the ABI that applies here is different from what applies to the mcount
+code. Since you are returning from a function (after the epilogue), you might
+be able to skimp on things saved/restored (usually just registers used to pass
+return values).
+
+#ifdef CONFIG_FUNCTION_GRAPH_TRACER
+void return_to_handler(void)
+{
+ /* save all state needed by the ABI (see paragraph above) */
+
+ void (*original_return_point)(void) = ftrace_return_to_handler();
+
+ /* restore all state needed by the ABI */
+
+ /* this is usually either a return or a jump */
+ original_return_point();
+}
+#endif
+
+
+HAVE_FTRACE_NMI_ENTER
+---------------------
+
+If you can't trace NMI functions, then skip this option.
+
+<details to be filled>
+
+
+HAVE_FTRACE_SYSCALLS
+---------------------
+
+<details to be filled>
+
+
+HAVE_FTRACE_MCOUNT_RECORD
+-------------------------
+
+See scripts/recordmcount.pl for more info.
+
+<details to be filled>
+
+
+HAVE_DYNAMIC_FTRACE
+---------------------
+
+<details to be filled>
git://ftp.safe.ca / safe / jmp / linux-2.6 / commitdiff