8 Here we will cover the architecture pieces that the common function tracing
9 code relies on for proper functioning. Things are broken down into increasing
10 complexity so that you can start simple and at least get basic functionality.
12 Note that this focuses on architecture implementation details only. If you
13 want more explanation of a feature in terms of common code, review the common
16 Ideally, everyone who wishes to retain performance while supporting tracing in
17 their kernel should make it all the way to dynamic ftrace support.
23 Ftrace relies on these features being implemented:
24 STACKTRACE_SUPPORT - implement save_stack_trace()
25 TRACE_IRQFLAGS_SUPPORT - implement include/asm/irqflags.h
31 You will need to implement the mcount and the ftrace_stub functions.
33 The exact mcount symbol name will depend on your toolchain. Some call it
34 "mcount", "_mcount", or even "__mcount". You can probably figure it out by
35 running something like:
36 $ echo 'main(){}' | gcc -x c -S -o - - -pg | grep mcount
38 We'll make the assumption below that the symbol is "mcount" just to keep things
39 nice and simple in the examples.
41 Keep in mind that the ABI that is in effect inside of the mcount function is
42 *highly* architecture/toolchain specific. We cannot help you in this regard,
43 sorry. Dig up some old documentation and/or find someone more familiar than
44 you to bang ideas off of. Typically, register usage (argument/scratch/etc...)
45 is a major issue at this point, especially in relation to the location of the
46 mcount call (before/after function prologue). You might also want to look at
47 how glibc has implemented the mcount function for your architecture. It might
50 The mcount function should check the function pointer ftrace_trace_function
51 to see if it is set to ftrace_stub. If it is, there is nothing for you to do,
52 so return immediately. If it isn't, then call that function in the same way
53 the mcount function normally calls __mcount_internal -- the first argument is
54 the "frompc" while the second argument is the "selfpc" (adjusted to remove the
55 size of the mcount call that is embedded in the function).
57 For example, if the function foo() calls bar(), when the bar() function calls
58 mcount(), the arguments mcount() will pass to the tracer are:
59 "frompc" - the address bar() will use to return to foo()
60 "selfpc" - the address bar() (with mcount() size adjustment)
62 Also keep in mind that this mcount function will be called *a lot*, so
63 optimizing for the default case of no tracer will help the smooth running of
64 your system when tracing is disabled. So the start of the mcount function is
65 typically the bare minimum with checking things before returning. That also
66 means the code flow should usually be kept linear (i.e. no branching in the nop
67 case). This is of course an optimization and not a hard requirement.
69 Here is some pseudo code that should help (these functions should actually be
70 implemented in assembly):
72 void ftrace_stub(void)
79 /* save any bare state needed in order to do initial checking */
81 extern void (*ftrace_trace_function)(unsigned long, unsigned long);
82 if (ftrace_trace_function != ftrace_stub)
85 /* restore any bare state */
91 /* save all state needed by the ABI (see paragraph above) */
93 unsigned long frompc = ...;
94 unsigned long selfpc = <return address> - MCOUNT_INSN_SIZE;
95 ftrace_trace_function(frompc, selfpc);
97 /* restore all state needed by the ABI */
100 Don't forget to export mcount for modules !
101 extern void mcount(void);
102 EXPORT_SYMBOL(mcount);
105 HAVE_FUNCTION_GRAPH_TRACER
106 --------------------------
108 Deep breath ... time to do some real work. Here you will need to update the
109 mcount function to check ftrace graph function pointers, as well as implement
110 some functions to save (hijack) and restore the return address.
112 The mcount function should check the function pointers ftrace_graph_return
113 (compare to ftrace_stub) and ftrace_graph_entry (compare to
114 ftrace_graph_entry_stub). If either of those is not set to the relevant stub
115 function, call the arch-specific function ftrace_graph_caller which in turn
116 calls the arch-specific function prepare_ftrace_return. Neither of these
117 function names is strictly required, but you should use them anyway to stay
118 consistent across the architecture ports -- easier to compare & contrast
121 The arguments to prepare_ftrace_return are slightly different than what are
122 passed to ftrace_trace_function. The second argument "selfpc" is the same,
123 but the first argument should be a pointer to the "frompc". Typically this is
124 located on the stack. This allows the function to hijack the return address
125 temporarily to have it point to the arch-specific function return_to_handler.
126 That function will simply call the common ftrace_return_to_handler function and
127 that will return the original return address with which you can return to the
130 Here is the updated mcount pseudo code:
134 if (ftrace_trace_function != ftrace_stub)
137 +#ifdef CONFIG_FUNCTION_GRAPH_TRACER
138 + extern void (*ftrace_graph_return)(...);
139 + extern void (*ftrace_graph_entry)(...);
140 + if (ftrace_graph_return != ftrace_stub ||
141 + ftrace_graph_entry != ftrace_graph_entry_stub)
142 + ftrace_graph_caller();
145 /* restore any bare state */
148 Here is the pseudo code for the new ftrace_graph_caller assembly function:
149 #ifdef CONFIG_FUNCTION_GRAPH_TRACER
150 void ftrace_graph_caller(void)
152 /* save all state needed by the ABI */
154 unsigned long *frompc = &...;
155 unsigned long selfpc = <return address> - MCOUNT_INSN_SIZE;
156 /* passing frame pointer up is optional -- see below */
157 prepare_ftrace_return(frompc, selfpc, frame_pointer);
159 /* restore all state needed by the ABI */
163 For information on how to implement prepare_ftrace_return(), simply look at the
164 x86 version (the frame pointer passing is optional; see the next section for
165 more information). The only architecture-specific piece in it is the setup of
166 the fault recovery table (the asm(...) code). The rest should be the same
167 across architectures.
169 Here is the pseudo code for the new return_to_handler assembly function. Note
170 that the ABI that applies here is different from what applies to the mcount
171 code. Since you are returning from a function (after the epilogue), you might
172 be able to skimp on things saved/restored (usually just registers used to pass
175 #ifdef CONFIG_FUNCTION_GRAPH_TRACER
176 void return_to_handler(void)
178 /* save all state needed by the ABI (see paragraph above) */
180 void (*original_return_point)(void) = ftrace_return_to_handler();
182 /* restore all state needed by the ABI */
184 /* this is usually either a return or a jump */
185 original_return_point();
190 HAVE_FUNCTION_GRAPH_FP_TEST
191 ---------------------------
193 An arch may pass in a unique value (frame pointer) to both the entering and
194 exiting of a function. On exit, the value is compared and if it does not
195 match, then it will panic the kernel. This is largely a sanity check for bad
196 code generation with gcc. If gcc for your port sanely updates the frame
197 pointer under different optimization levels, then ignore this option.
199 However, adding support for it isn't terribly difficult. In your assembly code
200 that calls prepare_ftrace_return(), pass the frame pointer as the 3rd argument.
201 Then in the C version of that function, do what the x86 port does and pass it
202 along to ftrace_push_return_trace() instead of a stub value of 0.
204 Similarly, when you call ftrace_return_to_handler(), pass it the frame pointer.
206 HAVE_FUNCTION_GRAPH_RET_ADDR_PTR
207 --------------------------------
209 An arch may pass in a pointer to the return address on the stack. This
210 prevents potential stack unwinding issues where the unwinder gets out of
211 sync with ret_stack and the wrong addresses are reported by
212 ftrace_graph_ret_addr().
214 Adding support for it is easy: just define the macro in asm/ftrace.h and
215 pass the return address pointer as the 'retp' argument to
216 ftrace_push_return_trace().
218 HAVE_FTRACE_NMI_ENTER
219 ---------------------
221 If you can't trace NMI functions, then skip this option.
223 <details to be filled>
226 HAVE_SYSCALL_TRACEPOINTS
227 ------------------------
229 You need very few things to get the syscalls tracing in an arch.
231 - Support HAVE_ARCH_TRACEHOOK (see arch/Kconfig).
232 - Have a NR_syscalls variable in <asm/unistd.h> that provides the number
233 of syscalls supported by the arch.
234 - Support the TIF_SYSCALL_TRACEPOINT thread flags.
235 - Put the trace_sys_enter() and trace_sys_exit() tracepoints calls from ptrace
236 in the ptrace syscalls tracing path.
237 - If the system call table on this arch is more complicated than a simple array
238 of addresses of the system calls, implement an arch_syscall_addr to return
239 the address of a given system call.
240 - If the symbol names of the system calls do not match the function names on
241 this arch, define ARCH_HAS_SYSCALL_MATCH_SYM_NAME in asm/ftrace.h and
242 implement arch_syscall_match_sym_name with the appropriate logic to return
243 true if the function name corresponds with the symbol name.
244 - Tag this arch as HAVE_SYSCALL_TRACEPOINTS.
247 HAVE_FTRACE_MCOUNT_RECORD
248 -------------------------
250 See scripts/recordmcount.pl for more info. Just fill in the arch-specific
251 details for how to locate the addresses of mcount call sites via objdump.
252 This option doesn't make much sense without also implementing dynamic ftrace.
258 You will first need HAVE_FTRACE_MCOUNT_RECORD and HAVE_FUNCTION_TRACER, so
259 scroll your reader back up if you got over eager.
261 Once those are out of the way, you will need to implement:
264 - ftrace_call_adjust()
265 - struct dyn_arch_ftrace{}
267 - mcount() (new stub)
272 - ftrace_dyn_arch_init()
275 - ftrace_update_ftrace_func()
277 First you will need to fill out some arch details in your asm/ftrace.h.
279 Define MCOUNT_ADDR as the address of your mcount symbol similar to:
280 #define MCOUNT_ADDR ((unsigned long)mcount)
281 Since no one else will have a decl for that function, you will need to:
282 extern void mcount(void);
284 You will also need the helper function ftrace_call_adjust(). Most people
285 will be able to stub it out like so:
286 static inline unsigned long ftrace_call_adjust(unsigned long addr)
290 <details to be filled>
292 Lastly you will need the custom dyn_arch_ftrace structure. If you need
293 some extra state when runtime patching arbitrary call sites, this is the
294 place. For now though, create an empty struct:
295 struct dyn_arch_ftrace {
296 /* No extra data needed */
299 With the header out of the way, we can fill out the assembly code. While we
300 did already create a mcount() function earlier, dynamic ftrace only wants a
301 stub function. This is because the mcount() will only be used during boot
302 and then all references to it will be patched out never to return. Instead,
303 the guts of the old mcount() will be used to create a new ftrace_caller()
304 function. Because the two are hard to merge, it will most likely be a lot
305 easier to have two separate definitions split up by #ifdefs. Same goes for
306 the ftrace_stub() as that will now be inlined in ftrace_caller().
308 Before we get confused anymore, let's check out some pseudo code so you can
309 implement your own stuff in assembly:
316 void ftrace_caller(void)
318 /* save all state needed by the ABI (see paragraph above) */
320 unsigned long frompc = ...;
321 unsigned long selfpc = <return address> - MCOUNT_INSN_SIZE;
324 ftrace_stub(frompc, selfpc);
326 /* restore all state needed by the ABI */
332 This might look a little odd at first, but keep in mind that we will be runtime
333 patching multiple things. First, only functions that we actually want to trace
334 will be patched to call ftrace_caller(). Second, since we only have one tracer
335 active at a time, we will patch the ftrace_caller() function itself to call the
336 specific tracer in question. That is the point of the ftrace_call label.
338 With that in mind, let's move on to the C code that will actually be doing the
339 runtime patching. You'll need a little knowledge of your arch's opcodes in
340 order to make it through the next section.
342 Every arch has an init callback function. If you need to do something early on
343 to initialize some state, this is the time to do that. Otherwise, this simple
344 function below should be sufficient for most people:
346 int __init ftrace_dyn_arch_init(void)
351 There are two functions that are used to do runtime patching of arbitrary
352 functions. The first is used to turn the mcount call site into a nop (which
353 is what helps us retain runtime performance when not tracing). The second is
354 used to turn the mcount call site into a call to an arbitrary location (but
355 typically that is ftracer_caller()). See the general function definition in
356 linux/ftrace.h for the functions:
359 The rec->ip value is the address of the mcount call site that was collected
360 by the scripts/recordmcount.pl during build time.
362 The last function is used to do runtime patching of the active tracer. This
363 will be modifying the assembly code at the location of the ftrace_call symbol
364 inside of the ftrace_caller() function. So you should have sufficient padding
365 at that location to support the new function calls you'll be inserting. Some
366 people will be using a "call" type instruction while others will be using a
367 "branch" type instruction. Specifically, the function is:
368 ftrace_update_ftrace_func()
371 HAVE_DYNAMIC_FTRACE + HAVE_FUNCTION_GRAPH_TRACER
372 ------------------------------------------------
374 The function grapher needs a few tweaks in order to work with dynamic ftrace.
375 Basically, you will need to:
378 - ftrace_graph_call()
379 - ftrace_graph_caller()
381 - ftrace_enable_ftrace_graph_caller()
382 - ftrace_disable_ftrace_graph_caller()
384 <details to be filled>
386 - add a nop stub after the ftrace_call location named ftrace_graph_call;
387 stub needs to be large enough to support a call to ftrace_graph_caller()
388 - update ftrace_graph_caller() to work with being called by the new
389 ftrace_caller() since some semantics may have changed
390 - ftrace_enable_ftrace_graph_caller() will runtime patch the
391 ftrace_graph_call location with a call to ftrace_graph_caller()
392 - ftrace_disable_ftrace_graph_caller() will runtime patch the
393 ftrace_graph_call location with nops