X-Git-Url: http://git.droids-corp.org/?a=blobdiff_plain;f=doc%2Fguides%2Fprog_guide%2Fbpf_lib.rst;h=1feb7734a358a4eea7be9f4a35ad1ed55d2f6993;hb=bc778a17fa46f1ec69180c7f660585155eefc318;hp=9c728da7bdc66557f9ced4c1e5422e8b942cdf0b;hpb=6861c01001ac3b1869f5cf1f492809875f753e67;p=dpdk.git

diff --git a/doc/guides/prog_guide/bpf_lib.rst b/doc/guides/prog_guide/bpf_lib.rst
index 9c728da7bd..1feb7734a3 100644
--- a/doc/guides/prog_guide/bpf_lib.rst
+++ b/doc/guides/prog_guide/bpf_lib.rst
@@ -27,6 +27,35 @@ The library API provides the following basic operations:
 
 *   Load BPF program from the ELF file and install callback to execute it on given ethdev port/queue.
 
+Packet data load instructions
+-----------------------------
+
+DPDK supports two non-generic instructions: ``(BPF_ABS | size | BPF_LD)``
+and ``(BPF_IND | size | BPF_LD)`` which are used to access packet data.
+These instructions can only be used when execution context is a pointer to
+``struct rte_mbuf`` and have seven implicit operands.
+Register ``R6`` is an implicit input that must contain pointer to ``rte_mbuf``.
+Register ``R0`` is an implicit output which contains the data fetched from the
+packet. Registers ``R1-R5`` are scratch registers
+and must not be used to store the data across these instructions.
+These instructions have implicit program exit condition as well. When
+eBPF program is trying to access the data beyond the packet boundary,
+the interpreter will abort the execution of the program. JIT compilers
+therefore must preserve this property. ``src_reg`` and ``imm32`` fields are
+explicit inputs to these instructions.
+For example, ``(BPF_IND | BPF_W | BPF_LD)`` means:
+
+.. code-block:: c
+
+    uint32_t tmp;
+    R0 = rte_pktmbuf_read((const struct rte_mbuf *)R6,  src_reg + imm32,
+	sizeof(tmp), &tmp);
+    if (R0 == NULL) return FAILED;
+    R0 = ntohl(*(uint32_t *)R0);
+
+and ``R1-R5`` were scratched.
+
+
 Not currently supported eBPF features
 -------------------------------------
 
@@ -34,5 +63,4 @@ Not currently supported eBPF features
  - cBPF
  - tail-pointer call
  - eBPF MAP
- - skb
  - external function calls for 32-bit platforms