From: Dave Thaler <dthaler@xxxxxxxxxxxxx>
Explain helper functions.
Kernel functions and bpf to bpf calls are covered in
a later commit in this set ("Add extended call instructions").
Signed-off-by: Dave Thaler <dthaler@xxxxxxxxxxxxx>
---
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 29b599c70..f9e56d9d5 100644
--- a/Documentation/bpf/instruction-set.rst
+++ b/Documentation/bpf/instruction-set.rst
@@ -242,7 +242,7 @@ 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
@@ -253,6 +253,22 @@ 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:
+
+ u64 function(u64 r1, u64 r2, u64 r3, u64 r4, u64 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
