1 VPPINFRA (Infrastructure)
2 =========================
4 The files associated with the VPP Infrastructure layer are located in
5 the ./src/vppinfra folder.
7 VPPinfra is a collection of basic c-library services, quite
8 sufficient to build standalone programs to run directly on bare metal.
9 It also provides high-performance dynamic arrays, hashes, bitmaps,
10 high-precision real-time clock support, fine-grained event-logging, and
11 data structure serialization.
13 One fair comment / fair warning about vppinfra: you can\'t always tell a
14 macro from an inline function from an ordinary function simply by name.
15 Macros are used to avoid function calls in the typical case, and to
16 cause (intentional) side-effects.
18 Vppinfra has been around for almost 20 years and tends not to change
19 frequently. The VPP Infrastructure layer contains the following
25 Vppinfra vectors are ubiquitous dynamically resized arrays with by user
26 defined \"headers\". Many vpppinfra data structures (e.g. hash, heap,
27 pool) are vectors with various different headers.
29 The memory layout looks like this:
32 User header (optional, uword aligned)
33 Alignment padding (if needed)
34 Vector length in elements
35 User's pointer -> Vector element 0
41 As shown above, the vector APIs deal with pointers to the 0th element of
42 a vector. Null pointers are valid vectors of length zero.
44 To avoid thrashing the memory allocator, one often resets the length of
45 a vector to zero while retaining the memory allocation. Set the vector
46 length field to zero via the vec\_reset\_length(v) macro. \[Use the
47 macro! It's smart about NULL pointers.\]
49 Typically, the user header is not present. User headers allow for other
50 data structures to be built atop vppinfra vectors. Users may specify the
51 alignment for first data element of a vector via the \[vec\]()\*\_aligned
54 Vector elements can be any C type e.g. (int, double, struct bar). This
55 is also true for data types built atop vectors (e.g. heap, pool, etc.).
56 Many macros have \_a variants supporting alignment of vector elements
57 and \_h variants supporting non-zero-length vector headers. The \_ha
58 variants support both. Additionally cacheline alignment within a
59 vector element structure can be specified using the
60 \[CLIB_CACHE_LINE_ALIGN_MARK\]() macro.
62 Inconsistent usage of header and/or alignment related macro variants
63 will cause delayed, confusing failures.
65 Standard programming error: memorize a pointer to the ith element of a
66 vector, and then expand the vector. Vectors expand by 3/2, so such code
67 may appear to work for a period of time. Correct code almost always
68 memorizes vector **indices** which are invariant across reallocations.
70 In typical application images, one supplies a set of global functions
71 designed to be called from gdb. Here are a few examples:
73 - vl(v) - prints vec\_len(v)
74 - pe(p) - prints pool\_elts(p)
75 - pifi(p, index) - prints pool\_is\_free\_index(p, index)
76 - debug\_hex\_bytes (p, nbytes) - hex memory dump nbytes starting at p
78 Use the "show gdb" debug CLI command to print the current set.
83 Vppinfra bitmaps are dynamic, built using the vppinfra vector APIs.
84 Quite handy for a variety jobs.
89 Vppinfra pools combine vectors and bitmaps to rapidly allocate and free
90 fixed-size data structures with independent lifetimes. Pools are perfect
91 for allocating per-session structures.
96 Vppinfra provides several hash flavors. Data plane problems involving
97 packet classification / session lookup often use
98 ./src/vppinfra/bihash\_template.\[ch\] bounded-index extensible
99 hashes. These templates are instantiated multiple times, to efficiently
100 service different fixed-key sizes.
102 Bihashes are thread-safe. Read-locking is not required. A simple
103 spin-lock ensures that only one thread writes an entry at a time.
105 The original vppinfra hash implementation in
106 ./src/vppinfra/hash.\[ch\] are simple to use, and are often used in
107 control-plane code which needs exact-string-matching.
109 In either case, one almost always looks up a key in a hash table to
110 obtain an index in a related vector or pool. The APIs are simple enough,
111 but one must take care when using the unmanaged arbitrary-sized key
112 variant. Hash\_set\_mem (hash\_table, key\_pointer, value) memorizes
113 key\_pointer. It is usually a bad mistake to pass the address of a
114 vector element as the second argument to hash\_set\_mem. It is perfectly
115 fine to memorize constant string addresses in the text segment.
120 Vppinfra includes high-precision, low-cost timing services. The
121 datatype clib_time_t and associated functions reside in
122 ./src/vppinfra/time.\[ch\]. Call clib_time_init (clib_time_t \*cp) to
123 initialize the clib_time_t object.
125 Clib_time_init(...) can use a variety of different ways to establish
126 the hardware clock frequency. At the end of the day, vppinfra
127 timekeeping takes the attitude that the operating system's clock is
128 the closest thing to a gold standard it has handy.
130 When properly configured, NTP maintains kernel clock synchronization
131 with a highly accurate off-premises reference clock. Notwithstanding
132 network propagation delays, a synchronized NTP client will keep the
133 kernel clock accurate to within 50ms or so.
135 Why should one care? Simply put, oscillators used to generate CPU
136 ticks aren't super accurate. They work pretty well, but a 0.1% error
137 wouldn't be out of the question. That's a minute and a half's worth of
138 error in 1 day. The error changes constantly, due to temperature
139 variation, and a host of other physical factors.
141 It's far too expensive to use system calls for timing, so we're left
142 with the problem of continously adjusting our view of the CPU tick
143 register's clocks_per_second parameter.
145 The clock rate adjustment algorithm measures the number of cpu ticks
146 and the "gold standard" reference time across an interval of
147 approximately 16 seconds. We calculate clocks_per_second for the
148 interval: use rdtsc (on x86_64) and a system call to get the latest
149 cpu tick count and the kernel's latest nanosecond timestamp. We
150 subtract the previous interval end values, and use exponential
151 smoothing to merge the new clock rate sample into the clocks_per_second
154 As of this writing, we maintain the clock rate by way of the following
155 first-order differential equation:
159 clocks_per_second(t) = clocks_per_second(t-1) * K + sample_cps(t)*(1-K)
160 where K = e**(-1.0/3.75);
163 This yields a per observation "half-life" of 1 minute. Empirically,
164 the clock rate converges within 5 minutes, and appears to maintain
165 near-perfect agreement with the kernel clock in the face of ongoing
166 NTP time adjustments.
168 See ./src/vppinfra/time.c:clib_time_verify_frequency(...) to look at
169 the rate adjustment algorithm. The code rejects frequency samples
170 corresponding to the sort of adjustment which might occur if someone
171 changes the gold standard kernel clock by several seconds.
173 ### Monotonic timebase support
175 Particularly during system initialization, the "gold standard" system
176 reference clock can change by a large amount, in an instant. It's not
177 a best practice to yank the reference clock - in either direction - by
178 hours or days. In fact, some poorly-constructed use-cases do so.
180 To deal with this reality, clib_time_now(...) returns the number of
181 seconds since vpp started, *guaranteed to be monotonically
182 increasing, no matter what happens to the system reference clock*.
184 This is first-order important, to avoid breaking every active timer in
185 the system. The vpp host stack alone may account for tens of millions
186 of active timers. It's utterly impractical to track down and fix
187 timers, so we must deal with the issue at the timebase level.
189 Here's how it works. Prior to adjusting the clock rate, we collect the
190 kernel reference clock and the cpu clock:
193 /* Ask the kernel and the CPU what time it is... */
194 now_reference = unix_time_now ();
195 now_clock = clib_cpu_time_now ();
198 Compute changes for both clocks since the last rate adjustment,
199 roughly 15 seconds ago:
202 /* Compute change in the reference clock */
203 delta_reference = now_reference - c->last_verify_reference_time;
205 /* And change in the CPU clock */
206 delta_clock_in_seconds = (f64) (now_clock - c->last_verify_cpu_time) *
207 c->seconds_per_clock;
210 Delta_reference is key. Almost 100% of the time, delta_reference and
211 delta_clock_in_seconds are identical modulo one system-call
212 time. However, NTP or a privileged user can yank the system reference
213 time - in either direction - by an hour, a day, or a decade.
215 As described above, clib_time_now(...) must return monotonically
216 increasing answers to the question "how long has it been since vpp
217 started, in seconds." To do that, the clock rate adjustment algorithm
218 begins by recomputing the initial reference time:
221 c->init_reference_time += (delta_reference - delta_clock_in_seconds);
224 It's easy to convince yourself that if the reference clock changes by
225 15.000000 seconds and the cpu clock tick time changes by 15.000000
226 seconds, the initial reference time won't change.
228 If, on the other hand, delta_reference is -86400.0 and delta clock is
229 15.0 - reference time jumped backwards by exactly one day in a
230 15-second rate update interval - we add -86415.0 to the initial
233 Given the corrected initial reference time, we recompute the total
234 number of cpu ticks which have occurred since the corrected initial
235 reference time, at the current clock tick rate:
238 c->total_cpu_time = (now_reference - c->init_reference_time)
239 * c->clocks_per_second;
245 Vppinfra format is roughly equivalent to printf.
247 Format has a few properties worth mentioning. Format's first argument is
248 a (u8 \*) vector to which it appends the result of the current format
249 operation. Chaining calls is very easy:
254 result = format (0, "junk = %d, ", junk);
255 result = format (result, "more junk = %d\n", more_junk);
258 As previously noted, NULL pointers are perfectly proper 0-length
259 vectors. Format returns a (u8 \*) vector, **not** a C-string. If you
260 wish to print a (u8 \*) vector, use the "%v" format string. If you need
261 a (u8 \*) vector which is also a proper C-string, either of these
267 result = format (result, "<whatever>%c", 0);
270 Remember to vec\_free() the result if appropriate. Be careful not to
271 pass format an uninitialized (u8 \*).
273 Format implements a particularly handy user-format scheme via the "%U"
274 format specification. For example:
277 u8 * format_junk (u8 * s, va_list *va)
279 junk = va_arg (va, u32);
280 s = format (s, "%s", junk);
284 result = format (0, "junk = %U, format_junk, "This is some junk");
287 format\_junk() can invoke other user-format functions if desired. The
288 programmer shoulders responsibility for argument type-checking. It is
289 typical for user format functions to blow up spectacularly if the
290 va\_arg(va, type) macros don't match the caller's idea of reality.
295 Vppinfra unformat is vaguely related to scanf, but considerably more
298 A typical use case involves initializing an unformat\_input\_t from
299 either a C-string or a (u8 \*) vector, then parsing via unformat() as
303 unformat_input_t input;
305 unformat_init_string (&input, "<some-C-string>");
307 unformat_init_vector (&input, <u8-vector>);
310 Then loop parsing individual elements:
313 while (unformat_check_input (&input) != UNFORMAT_END_OF_INPUT)
315 if (unformat (&input, "value1 %d", &value1))
316 ;/* unformat sets value1 */
317 else if (unformat (&input, "value2 %d", &value2)
318 ;/* unformat sets value2 */
320 return clib_error_return (0, "unknown input '%U'",
321 format_unformat_error, input);
325 As with format, unformat implements a user-unformat function capability
326 via a "%U" user unformat function scheme. Generally, one can trivially
327 transform "format (s, "foo %d", foo) -> "unformat (input, "foo %d", &foo)".
329 Unformat implements a couple of handy non-scanf-like format specifiers:
332 unformat (input, "enable %=", &enable, 1 /* defaults to 1 */);
333 unformat (input, "bitzero %|", &mask, (1<<0));
334 unformat (input, "bitone %|", &mask, (1<<1));
338 The phrase "enable %=" means "set the supplied variable to the default
339 value" if unformat parses the "enable" keyword all by itself. If
340 unformat parses "enable 123" set the supplied variable to 123.
342 We could clean up a number of hand-rolled "verbose" + "verbose %d"
343 argument parsing codes using "%=".
345 The phrase "bitzero %|" means "set the specified bit in the supplied
346 bitmask" if unformat parses "bitzero". Although it looks like it could
347 be fairly handy, it's very lightly used in the code base.
349 `%_` toggles whether or not to skip input white space.
351 For transition from skip to no-skip in middle of format string, skip input white space. For example, the following:
354 fmt = "%_%d.%d%_->%_%d.%d%_"
355 unformat (input, fmt, &one, &two, &three, &four);
357 matches input "1.2 -> 3.4".
358 Without this, the space after -> does not get skipped.
363 ### How to parse a single input line
365 Debug CLI command functions MUST NOT accidentally consume input
366 belonging to other debug CLI commands. Otherwise, it's impossible to
367 script a set of debug CLI commands which "work fine" when issued one
370 This bit of code is NOT correct:
373 /* Eats script input NOT beloging to it, and chokes! */
374 while (unformat_check_input (input) != UNFORMAT_END_OF_INPUT)
376 if (unformat (input, ...))
378 else if (unformat (input, ...))
381 return clib_error_return (0, "parse error: '%U'",
382 format_unformat_error, input);
387 When executed as part of a script, such a function will return "parse
388 error: '<next-command-text>'" every time, unless it happens to be the
389 last command in the script.
391 Instead, use "unformat_line_input" to consume the rest of a line's
392 worth of input - everything past the path specified in the
393 VLIB_CLI_COMMAND declaration.
395 For example, unformat_line_input with "my_command" set up as shown
396 below and user input "my path is clear" will produce an
397 unformat_input_t that contains "is clear".
400 VLIB_CLI_COMMAND (...) = {
405 Here's a bit of code which shows the required mechanics, in full:
408 static clib_error_t *
409 my_command_fn (vlib_main_t * vm,
410 unformat_input_t * input,
411 vlib_cli_command_t * cmd)
413 unformat_input_t _line_input, *line_input = &_line_input;
415 clib_error_t *error = 0;
417 if (!unformat_user (input, unformat_line_input, line_input))
421 * Here, UNFORMAT_END_OF_INPUT is at the end of the line we consumed,
422 * not at the end of the script...
424 while (unformat_check_input (line_input) != UNFORMAT_END_OF_INPUT)
426 if (unformat (line_input, "this %u", &this))
428 else if (unformat (line_input, "that %u", &that))
432 error = clib_error_return (0, "parse error: '%U'",
433 format_unformat_error, line_input);
438 <do something based on "this" and "that", etc>
441 unformat_free (line_input);
445 VLIB_CLI_COMMAND (my_command, static) = {
447 .function = my_command_fn",
454 Vppinfra errors and warnings
455 ----------------------------
457 Many functions within the vpp dataplane have return-values of type
458 clib\_error\_t \*. Clib\_error\_t's are arbitrary strings with a bit of
459 metadata \[fatal, warning\] and are easy to announce. Returning a NULL
460 clib\_error\_t \* indicates "A-OK, no error."
462 Clib\_warning(format-args) is a handy way to add debugging
463 output; clib warnings prepend function:line info to unambiguously locate
464 the message source. Clib\_unix\_warning() adds perror()-style Linux
465 system-call information. In production images, clib\_warnings result in
471 Vppinfra serialization support allows the programmer to easily serialize
472 and unserialize complex data structures.
474 The underlying primitive serialize/unserialize functions use network
475 byte-order, so there are no structural issues serializing on a
476 little-endian host and unserializing on a big-endian host.