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

[sdf@google.com]

On 10/19, dthaler1968@googlemail.com wrote:
> From: Dave Thaler <dthaler@microsoft.com>

> 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@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 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