[4/4] bpf, docs: Explain helper functions

Commit Message

Dave Thaler Oct. 19, 2022, 6:38 p.m. UTC

From: Dave Thaler <[e-mail]>

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 <[e-mail]>
---
 Documentation/bpf/instruction-set.rst | 18 +++++++++++++++++-
 1 file changed, 17 insertions(+), 1 deletion(-)

Comments

Stanislav Fomichev Oct. 19, 2022, 8:54 p.m. UTC | #1

On 10/19, [e-mail] wrote:
> From: Dave Thaler <[e-mail]>

> 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 <[e-mail]>
> ---
>   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

(the series looks good to me, but I'm assuming Alexei will take a look at
these anywey because he did for the previous submissions)

Not really related to the patch, but in general, quoting from the above:
"BPF programs can call ... by the eBPF runtime". I know we do it all the  
time
and use BPF/eBPF interchangeably, but should we try to be consistent at  
least
within the same page?

$ grep -r 'eBPF program' Documentation/bpf/ | wc -l
34
$ grep -r ' BPF program' Documentation/bpf/ | wc -l
97

> +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

Patch

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
 ===========================