bpf, docs: Clarify PC use in instruction-set.rst

[linux-block.git] / Documentation / bpf / standardization / instruction-set.rst

diff --git a/Documentation/bpf/standardization/instruction-set.rst b/Documentation/bpf/standardization/instruction-set.rst
index b44bdacd0195782d8e9170243404e629907a7ada..997560abadabea75e81014fbd9ca554b8686ba53 100644 (file)
--- a/Documentation/bpf/standardization/instruction-set.rst
+++ b/Documentation/bpf/standardization/instruction-set.rst
@@ -469,6 +469,12 @@ JSLT      0xc    any      PC += offset if dst < src          signed
 JSLE      0xd    any      PC += offset if dst <= src         signed
 ========  =====  =======  =================================  ===================================================
 
+where 'PC' denotes the program counter, and the offset to increment by
+is in units of 64-bit instructions relative to the instruction following
+the jump instruction.  Thus 'PC += 1' skips execution of the next
+instruction if it's a basic instruction or results in undefined behavior
+if the next instruction is a 128-bit wide instruction.
+
 The BPF program needs to store the return value into register R0 before doing an
 ``EXIT``.