vlib: refactor trajectory trace debug feature

trajectory trace has been broken for a while because we used to save the
buffer trajectory in a vector pointed to in opaque2. This does not work
well when opaque2 is copied (eg. because of a clone) as 2 buffers end up
sharing the same vector.
This dedicates a full cacheline in the buffer metadata instead when
trajectory is compiled in. No dynamic allocation, no sharing, no tears.

Type: refactor

Change-Id: I6a028ca1b48d38f393a36979e5e452c2dd48ad3f
Signed-off-by: Benoît Ganne <bganne@cisco.com>

2 files changed, 18 insertions, 12 deletions

diff --git a/src/plugins/af_xdp/device.c b/src/plugins/af_xdp/device.c
index fabf85adfa6..35ba617d6e3 100644
--- a/src/plugins/af_xdp/device.c
+++ b/src/plugins/af_xdp/device.c
@@ -208,6 +208,18 @@ af_xdp_create_queue (vlib_main_t * vm, af_xdp_create_if_args_t * args,
   umem_config.comp_size = args->txq_size;
   umem_config.frame_size =
     sizeof (vlib_buffer_t) + vlib_buffer_get_default_data_size (vm);
+  /*
+   * Note about headroom: for some reasons, there seem to be a discrepency
+   * between 0-copy and copy mode:
+   *   - 0-copy: XDP_PACKET_HEADROOM will be added to the user headroom
+   *   - copy: nothing is added to the user headroom
+   * We privileged 0-copy and set headroom so that frame_headroom +
+   * XDP_PACKET_HEADROOM == sizeof(vlib_buffer_t), ie data will correctly
+   * point to vlib_buffer_t->data for 0-copy. In copy mode, we have to add
+   * XDP_PACKET_HEADROOM to desc offset during refill.
+   */
+  STATIC_ASSERT (sizeof (vlib_buffer_t) >= XDP_PACKET_HEADROOM, "wrong size");
+  umem_config.frame_headroom = sizeof (vlib_buffer_t) - XDP_PACKET_HEADROOM;
   umem_config.flags = XDP_UMEM_UNALIGNED_CHUNK_FLAG;
   if (xsk_umem__create
       (umem, uword_to_pointer (vm->buffer_main->buffer_mem_start, void *),
diff --git a/src/plugins/af_xdp/input.c b/src/plugins/af_xdp/input.c
index 65eee75cb92..da422bdccaf 100644
--- a/src/plugins/af_xdp/input.c
+++ b/src/plugins/af_xdp/input.c
@@ -130,17 +130,11 @@ af_xdp_device_input_refill (vlib_main_t * vm,
 
   /*
    * Note about headroom: for some reasons, there seem to be a discrepency
-   * between 0-copy and copy mode:
-   *   - 0-copy: XDP_PACKET_HEADROOM will be added to the user headroom
-   *   - copy: nothing is added to the user headroom
-   * We privileged 0-copy and set headroom to 0. As XDP_PACKET_HEADROOM ==
-   * sizeof(vlib_buffer_t), data will correctly point to vlib_buffer_t->data.
-   * In copy mode, we have to add sizeof(vlib_buffer_t) to desc offset during
-   * refill.
+   * between 0-copy and copy mode. See
+   * src/plugins/af_xdp/device.c:af_xdp_create_queue()
    */
-  STATIC_ASSERT (sizeof (vlib_buffer_t) == XDP_PACKET_HEADROOM, "wrong size");
-#define bi2addr(bi) \
-  (((bi) << CLIB_LOG2_CACHE_LINE_BYTES) + (copy ? sizeof(vlib_buffer_t) : 0))
+#define bi2addr(bi)                                                           \
+  (((bi) << CLIB_LOG2_CACHE_LINE_BYTES) + (copy ? XDP_PACKET_HEADROOM : 0))
 
 wrap_around:
 
@@ -218,8 +212,8 @@ af_xdp_device_input_bufs (vlib_main_t * vm, const af_xdp_device_t * ad,
   const u32 mask = rxq->rx.mask;
   u32 n = n_rx, *bi = bis, bytes = 0;
 
-#define addr2bi(addr) \
-  (((addr) - (copy ? sizeof(vlib_buffer_t) : 0)) >> CLIB_LOG2_CACHE_LINE_BYTES)
+#define addr2bi(addr)                                                         \
+  (((addr) - (copy ? XDP_PACKET_HEADROOM : 0)) >> CLIB_LOG2_CACHE_LINE_BYTES)
 
   while (n >= 1)
     {