[PATCH 09/15] ebpf-docs: Explain helper functions

[dthaler1968@googlemail.com]

From: Dave Thaler <dthaler@microsoft.com>

Signed-off-by: Dave Thaler <dthaler@microsoft.com>
---
 Documentation/bpf/instruction-set.rst | 18 +++++++++++++++++-
 1 file changed, 17 insertions(+), 1 deletion(-)

diff --git a/Documentation/bpf/instruction-set.rst b/Documentation/bpf/instruction-set.rst
index 2987234eb..926957830 100644
--- a/Documentation/bpf/instruction-set.rst
+++ b/Documentation/bpf/instruction-set.rst
@@ -245,7 +245,7 @@ BPF_JSET  0x40   PC += off if dst & src
 BPF_JNE   0x50   PC += off if dst != src
 BPF_JSGT  0x60   PC += off if dst > src     signed
 BPF_JSGE  0x70   PC += off if dst >= src    signed
-BPF_CALL  0x80   function call
+BPF_CALL  0x80   function call              see `Helper functions`_
 BPF_EXIT  0x90   function / program return  BPF_JMP only
 BPF_JLT   0xa0   PC += off if dst < src     unsigned
 BPF_JLE   0xb0   PC += off if dst <= src    unsigned
@@ -256,6 +256,22 @@ BPF_JSLE  0xd0   PC += off if dst <= src    signed
 The eBPF program needs to store the return value into register R0 before doing a
 BPF_EXIT.
 
+Helper functions
+~~~~~~~~~~~~~~~~
+Helper functions are a concept whereby BPF programs can call into a
+set of function calls exposed by the eBPF runtime.  Each helper
+function is identified by an integer used in a ``BPF_CALL`` instruction.
+The available helper functions may differ for each eBPF program type.
+
+Conceptually, each helper function is implemented with a commonly shared function
+signature defined as:
+
+  uint64_t function(uint64_t r1, uint64_t r2, uint64_t r3, uint64_t r4, uint64_t r5)
+
+In actuality, each helper function is defined as taking between 0 and 5 arguments,
+with the remaining registers being ignored.  The definition of a helper function
+is responsible for specifying the type (e.g., integer, pointer, etc.) of the value returned,
+the number of arguments, and the type of each argument.
 
 Load and store instructions
 ===========================
-- 
2.33.4