On Sat, 7 May 2022 17:12:36 +0100
Quentin Armitage <quen...@armitage.org.uk> wrote:
> 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, reread the time and TSC once
> delta * NSEC_PER_SEC > (1 << 63). In order to ensure that there
> is no overflow if there is a several second gap between calls to
> pcapng_tsc_to_ns() the actual check to reread the clock is:
> delta > ((1ULL << 63) / NSEC_PER_SEC)
>
> Fixes: 8d23ce8f5ee ("pcapng: add new library for writing pcapng files")
> Cc: sta...@dpdk.org
>
> Signed-off-by: Quentin Armitage <quen...@armitage.org.uk>
What about something like this instead.
diff --git a/lib/pcapng/rte_pcapng.c b/lib/pcapng/rte_pcapng.c
index 90b2f5bc6905..c5534301bf2c 100644
--- 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 +35,39 @@ struct rte_pcapng {
};
/* For converting TSC cycles to PCAPNG ns format */
-struct pcapng_time {
+#define TICK_SCALE 16u
+static struct {
uint64_t ns;
uint64_t cycles;
+ struct rte_reciprocal_u64 inverse;
} pcapng_time;
RTE_INIT(pcapng_init)
{
struct timespec ts;
+ uint64_t scale_tick_per_ns;
pcapng_time.cycles = rte_get_tsc_cycles();
clock_gettime(CLOCK_REALTIME, &ts);
pcapng_time.ns = rte_timespec_to_ns(&ts);
+
+ scale_tick_per_ns = (rte_get_tsc_hz() * TICK_SCALE) / NSEC_PER_SEC;
+ pcapng_time.inverse = rte_reciprocal_value_u64(scale_tick_per_ns);
}
/* PCAPNG timestamps are in nanoseconds */
static uint64_t pcapng_tsc_to_ns(uint64_t cycles)
{
- uint64_t delta;
+ uint64_t delta, elapsed;
delta = cycles - pcapng_time.cycles;
- return pcapng_time.ns + (delta * NSEC_PER_SEC) / rte_get_tsc_hz();
+
+ /* Compute elapsed time in nanoseconds scaled by TICK_SCALE
+ * since the start of the capture.
+ * With scale of 4 this will roll over in 36 years.
+ */
+ elapsed = rte_reciprocal_divide_u64(delta, &pcapng_time.inverse);
+ return pcapng_time.ns + elapsed / TICK_SCALE;
}
/* length of option including padding */
