introduced helper to retrieve packet timestamp in Unix Epoch (#318)

This commit aims to introduce an helper to ease packet's timestamp computation.
Moreover, the new value will refer to the Unix Epoch instead of kernel timer.

Signed-off-by: Simone Magnani <simonemagnani.96@gmail.com>

Documentation/developers/dataplane.rst

@@ -56,11 +56,31 @@ The VLAN handling in TC and XDP eBPF programs is a little bit different, so poly
 - uint8_t **pcn_vlan_pop_tag** (struct CTXTYPE* pkt);
 - uint8_t **pcn_vlan_push_tag** (struct CTXTYPE* pkt, u16 eth_proto, u32 vlan_id);
 
+Packet Timestamping
+*******************
+
+So far we know that computing packet's timestamp in eBPF is quite tricky, due to the lack of usable kernel helpers.
+The only supported function is **bpf_ktime_get_ns()**, which returns the value of a kernel MONOTONIC clock, meaning that
+if the device has gone to sleep/suspend that clock is not updated. Moreover, in the **ctx** structure the field **ctx->tstamp**
+has not a standard behaviour: we tested that this value can be:
+
+- empty
+- Unix epoch timestamp
+- Unknown (maybe kernel timer) timestamp
+
+We have introduced an helper function to compute such timestamp referring to Unix Epoch: **pcn_get_packet_epoch(ctx)**.
+What basically happens is that to all eBPF program is injected a flag **_EPOCH_BASE** containing a precomputed value. This value
+represents the **Unix_Epoch - Kernel_timer** at time X. Calling the helper would increase this value by **bpf_ktime_get_ns()**
+nanoseconds, thanks to you obtain the Unix Epoch time of when you call such helper.
+
+In the following release, it will be possible to compute the exact timestamp even though the device sleeps/suspends due to
+the introduction of a new **bpf_ktime_get_ns()** function, which in addition will consider the inactivity time (CLOCK_BOOTTIME instead of CLOCK_MONOTONIC).
 
 Known limitations
 *****************
 - Since you cannot send a packet on multiple ports, multicast, broadcast or any similar functionality has to be implemented in the control path.
 - The support for multiple eBPF programs is not yet documented.
+- Timestamp will not be exact if the device running Polycube sleeps/suspends during its execution.
 
 
 Debugging the data plane

src/polycubed/src/base_cube.cpp

@@ -21,12 +21,32 @@
 namespace polycube {
 namespace polycubed {
 
+/** Function to compute a base Unix Epoch timestamp to be used as reference by
+ * all the timestamp computed using pcn_get_packet_epoch(ctx).
+ *
+ * Supposing the packet arrives at time X and we precomputed Unix Epoch and
+ * kernel timer at time Y:
+ *
+ * Epoch_at_X = Epoch_at_Y + (Kernel_at_X - Kernel_at_Y)
+ *
+ * where Kernel_at_X is given by the function bpf_ktime_get_ns()
+ * */
+uint64_t genBaseTime() {
+  // getting system clock (to retrieve Unix Epoch)
+  auto epoch = std::chrono::system_clock::now();
+  // getting the same time as bpf_ktime_get_ns()
+  auto up = std::chrono::steady_clock::now();
+  // return value to be summed to packet's bpf_ktime_get_ns()
+  return epoch.time_since_epoch().count() - up.time_since_epoch().count();
+}
+
 std::vector<std::string> BaseCube::cflags_ = {
     std::string("-D_POLYCUBE_MAX_NODES=") +
         std::to_string(PatchPanel::_POLYCUBE_MAX_NODES),
     std::string("-D_POLYCUBE_MAX_BPF_PROGRAMS=") +
         std::to_string(_POLYCUBE_MAX_BPF_PROGRAMS),
     std::string("-D_POLYCUBE_MAX_PORTS=") + std::to_string(_POLYCUBE_MAX_PORTS),
+    std::string("-D_EPOCH_BASE=") + std::to_string(genBaseTime()),
 };
 
 BaseCube::BaseCube(const std::string &name, const std::string &service_name,
@@ -496,6 +516,15 @@ int pcn_vlan_pop_tag(struct CTXTYPE *pkt);
 static __always_inline
 int pcn_vlan_push_tag(struct CTXTYPE *pkt, u16 eth_proto, u32 vlan_id);
 
+/* helper to get packet timestamp related to Unix Epoch */
+static __always_inline
+uint64_t pcn_get_packet_tstamp(struct CTXTYPE *ctx) {
+  /* Too much variadic, do not know a priori what timestamp is
+  if(ctx->tstamp != 0) {
+    return bpf_ntohl(ctx->tstamp);
+  }*/
+  return _EPOCH_BASE + bpf_ktime_get_ns();
+}
 )";
 
 }  // namespace polycubed