pcapng: fix timestamp wrapping in output files

In pcap_tsc_to_ns(), delta * NSEC_PER_SEC will overflow approx 8
seconds after pcap_init is called when using a TSC with a frequency
of 2.5GHz.

To avoid the overflow, update the saved time and TSC value once
delta >= tsc_hz.

Fixes: 8d23ce8f5ee ("pcapng: add new library for writing pcapng files")
Cc: stable@dpdk.org
Signed-off-by: Quentin Armitage <quentin@armitage.org.uk>
Acked-by: Stephen Hemminger <stephen@networkplumber.org>

diff --git a/lib/pcapng/rte_pcapng.c b/lib/pcapng/rte_pcapng.c
index 90b2f5bc6905e1fbd204a181dec861c267287cce..06ad712bd1eb5b252d910a09e4ac21284a202d81 100644 (file)
--- a/lib/pcapng/rte_pcapng.c
+++ b/lib/pcapng/rte_pcapng.c
@@ -19,6 +19,7 @@
 #include <rte_ether.h>
 #include <rte_mbuf.h>
 #include <rte_pcapng.h>
+#include <rte_reciprocal.h>
 #include <rte_time.h>
 
 #include "pcapng_proto.h"
@@ -34,27 +35,63 @@ struct rte_pcapng {
 };
 
 /* For converting TSC cycles to PCAPNG ns format */
-struct pcapng_time {
+static struct pcapng_time {
        uint64_t ns;
        uint64_t cycles;
+       uint64_t tsc_hz;
+       struct rte_reciprocal_u64 tsc_hz_inverse;
 } pcapng_time;
 
-RTE_INIT(pcapng_init)
+static inline void
+pcapng_init(void)
 {
        struct timespec ts;
 
        pcapng_time.cycles = rte_get_tsc_cycles();
        clock_gettime(CLOCK_REALTIME, &ts);
+       pcapng_time.cycles = (pcapng_time.cycles + rte_get_tsc_cycles()) / 2;
        pcapng_time.ns = rte_timespec_to_ns(&ts);
+
+       pcapng_time.tsc_hz = rte_get_tsc_hz();
+       pcapng_time.tsc_hz_inverse = rte_reciprocal_value_u64(pcapng_time.tsc_hz);
 }
 
 /* PCAPNG timestamps are in nanoseconds */
 static uint64_t pcapng_tsc_to_ns(uint64_t cycles)
 {
-       uint64_t delta;
-
+       uint64_t delta, secs;
+
+       if (!pcapng_time.tsc_hz)
+               pcapng_init();
+
+       /* In essence the calculation is:
+        *   delta = (cycles - pcapng_time.cycles) * NSEC_PRE_SEC / rte_get_tsc_hz()
+        * but this overflows within 4 to 8 seconds depending on TSC frequency.
+        * Instead, if delta >= pcapng_time.tsc_hz:
+        *   Increase pcapng_time.ns and pcapng_time.cycles by the number of
+        *   whole seconds in delta and reduce delta accordingly.
+        * delta will therefore always lie in the interval [0, pcapng_time.tsc_hz),
+        * which will not overflow when multiplied by NSEC_PER_SEC provided the
+        * TSC frequency < approx 18.4GHz.
+        *
+        * Currently all TSCs operate below 5GHz.
+        */
        delta = cycles - pcapng_time.cycles;
-       return pcapng_time.ns + (delta * NSEC_PER_SEC) / rte_get_tsc_hz();
+       if (unlikely(delta >= pcapng_time.tsc_hz)) {
+               if (likely(delta < pcapng_time.tsc_hz * 2)) {
+                       delta -= pcapng_time.tsc_hz;
+                       pcapng_time.cycles += pcapng_time.tsc_hz;
+                       pcapng_time.ns += NSEC_PER_SEC;
+               } else {
+                       secs = rte_reciprocal_divide_u64(delta, &pcapng_time.tsc_hz_inverse);
+                       delta -= secs * pcapng_time.tsc_hz;
+                       pcapng_time.cycles += secs * pcapng_time.tsc_hz;
+                       pcapng_time.ns += secs * NSEC_PER_SEC;
+               }
+       }
+
+       return pcapng_time.ns + rte_reciprocal_divide_u64(delta * NSEC_PER_SEC,
+                                                         &pcapng_time.tsc_hz_inverse);
 }
 
 /* length of option including padding */