--- /dev/null
+ function tracer guts
+ ====================
+ By Mike Frysinger
+
+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.
+
+Ideally, everyone who wishes to retain performance while supporting tracing in
+their kernel should make it all the way to dynamic ftrace support.
+
+
+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 minimum with checking things before returning. That also
+means the code flow should usually be 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_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 is 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 is strictly required, but you should use them anyway 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;
+ /* passing frame pointer up is optional -- see below */
+ prepare_ftrace_return(frompc, selfpc, frame_pointer);
+
+ /* restore all state needed by the ABI */
+}
+#endif
+
+For information on how to implement prepare_ftrace_return(), simply look at the
+x86 version (the frame pointer passing is optional; see the next section for
+more information). 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_FUNCTION_GRAPH_FP_TEST
+---------------------------
+
+An arch may pass in a unique value (frame pointer) to both the entering and
+exiting of a function. On exit, the value is compared and if it does not
+match, then it will panic the kernel. This is largely a sanity check for bad
+code generation with gcc. If gcc for your port sanely updates the frame
+pointer under different optimization levels, then ignore this option.
+
+However, adding support for it isn't terribly difficult. In your assembly code
+that calls prepare_ftrace_return(), pass the frame pointer as the 3rd argument.
+Then in the C version of that function, do what the x86 port does and pass it
+along to ftrace_push_return_trace() instead of a stub value of 0.
+
+Similarly, when you call ftrace_return_to_handler(), pass it the frame pointer.
+
+
+HAVE_FTRACE_NMI_ENTER
+---------------------
+
+If you can't trace NMI functions, then skip this option.
+
+<details to be filled>
+
+
+HAVE_SYSCALL_TRACEPOINTS
+------------------------
+
+You need very few things to get the syscalls tracing in an arch.
+
+- Support HAVE_ARCH_TRACEHOOK (see arch/Kconfig).
+- Have a NR_syscalls variable in <asm/unistd.h> that provides the number
+ of syscalls supported by the arch.
+- Support the TIF_SYSCALL_TRACEPOINT thread flags.
+- Put the trace_sys_enter() and trace_sys_exit() tracepoints calls from ptrace
+ in the ptrace syscalls tracing path.
+- If the system call table on this arch is more complicated than a simple array
+ of addresses of the system calls, implement an arch_syscall_addr to return
+ the address of a given system call.
+- If the symbol names of the system calls do not match the function names on
+ this arch, define ARCH_HAS_SYSCALL_MATCH_SYM_NAME in asm/ftrace.h and
+ implement arch_syscall_match_sym_name with the appropriate logic to return
+ true if the function name corresponds with the symbol name.
+- Tag this arch as HAVE_SYSCALL_TRACEPOINTS.
+
+
+HAVE_FTRACE_MCOUNT_RECORD
+-------------------------
+
+See scripts/recordmcount.pl for more info. Just fill in the arch-specific
+details for how to locate the addresses of mcount call sites via objdump.
+This option doesn't make much sense without also implementing dynamic ftrace.
+
+
+HAVE_DYNAMIC_FTRACE
+-------------------
+
+You will first need HAVE_FTRACE_MCOUNT_RECORD and HAVE_FUNCTION_TRACER, so
+scroll your reader back up if you got over eager.
+
+Once those are out of the way, you will need to implement:
+ - asm/ftrace.h:
+ - MCOUNT_ADDR
+ - ftrace_call_adjust()
+ - struct dyn_arch_ftrace{}
+ - asm code:
+ - mcount() (new stub)
+ - ftrace_caller()
+ - ftrace_call()
+ - ftrace_stub()
+ - C code:
+ - ftrace_dyn_arch_init()
+ - ftrace_make_nop()
+ - ftrace_make_call()
+ - ftrace_update_ftrace_func()
+
+First you will need to fill out some arch details in your asm/ftrace.h.
+
+Define MCOUNT_ADDR as the address of your mcount symbol similar to:
+ #define MCOUNT_ADDR ((unsigned long)mcount)
+Since no one else will have a decl for that function, you will need to:
+ extern void mcount(void);
+
+You will also need the helper function ftrace_call_adjust(). Most people
+will be able to stub it out like so:
+ static inline unsigned long ftrace_call_adjust(unsigned long addr)
+ {
+ return addr;
+ }
+<details to be filled>
+
+Lastly you will need the custom dyn_arch_ftrace structure. If you need
+some extra state when runtime patching arbitrary call sites, this is the
+place. For now though, create an empty struct:
+ struct dyn_arch_ftrace {
+ /* No extra data needed */
+ };
+
+With the header out of the way, we can fill out the assembly code. While we
+did already create a mcount() function earlier, dynamic ftrace only wants a
+stub function. This is because the mcount() will only be used during boot
+and then all references to it will be patched out never to return. Instead,
+the guts of the old mcount() will be used to create a new ftrace_caller()
+function. Because the two are hard to merge, it will most likely be a lot
+easier to have two separate definitions split up by #ifdefs. Same goes for
+the ftrace_stub() as that will now be inlined in ftrace_caller().
+
+Before we get confused anymore, let's check out some pseudo code so you can
+implement your own stuff in assembly:
+
+void mcount(void)
+{
+ return;
+}
+
+void ftrace_caller(void)
+{
+ /* save all state needed by the ABI (see paragraph above) */
+
+ unsigned long frompc = ...;
+ unsigned long selfpc = <return address> - MCOUNT_INSN_SIZE;
+
+ftrace_call:
+ ftrace_stub(frompc, selfpc);
+
+ /* restore all state needed by the ABI */
+
+ftrace_stub:
+ return;
+}
+
+This might look a little odd at first, but keep in mind that we will be runtime
+patching multiple things. First, only functions that we actually want to trace
+will be patched to call ftrace_caller(). Second, since we only have one tracer
+active at a time, we will patch the ftrace_caller() function itself to call the
+specific tracer in question. That is the point of the ftrace_call label.
+
+With that in mind, let's move on to the C code that will actually be doing the
+runtime patching. You'll need a little knowledge of your arch's opcodes in
+order to make it through the next section.
+
+Every arch has an init callback function. If you need to do something early on
+to initialize some state, this is the time to do that. Otherwise, this simple
+function below should be sufficient for most people:
+
+int __init ftrace_dyn_arch_init(void)
+{
+ return 0;
+}
+
+There are two functions that are used to do runtime patching of arbitrary
+functions. The first is used to turn the mcount call site into a nop (which
+is what helps us retain runtime performance when not tracing). The second is
+used to turn the mcount call site into a call to an arbitrary location (but
+typically that is ftracer_caller()). See the general function definition in
+linux/ftrace.h for the functions:
+ ftrace_make_nop()
+ ftrace_make_call()
+The rec->ip value is the address of the mcount call site that was collected
+by the scripts/recordmcount.pl during build time.
+
+The last function is used to do runtime patching of the active tracer. This
+will be modifying the assembly code at the location of the ftrace_call symbol
+inside of the ftrace_caller() function. So you should have sufficient padding
+at that location to support the new function calls you'll be inserting. Some
+people will be using a "call" type instruction while others will be using a
+"branch" type instruction. Specifically, the function is:
+ ftrace_update_ftrace_func()
+
+
+HAVE_DYNAMIC_FTRACE + HAVE_FUNCTION_GRAPH_TRACER
+------------------------------------------------
+
+The function grapher needs a few tweaks in order to work with dynamic ftrace.
+Basically, you will need to:
+ - update:
+ - ftrace_caller()
+ - ftrace_graph_call()
+ - ftrace_graph_caller()
+ - implement:
+ - ftrace_enable_ftrace_graph_caller()
+ - ftrace_disable_ftrace_graph_caller()
+
+<details to be filled>
+Quick notes:
+ - add a nop stub after the ftrace_call location named ftrace_graph_call;
+ stub needs to be large enough to support a call to ftrace_graph_caller()
+ - update ftrace_graph_caller() to work with being called by the new
+ ftrace_caller() since some semantics may have changed
+ - ftrace_enable_ftrace_graph_caller() will runtime patch the
+ ftrace_graph_call location with a call to ftrace_graph_caller()
+ - ftrace_disable_ftrace_graph_caller() will runtime patch the
+ ftrace_graph_call location with nops
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