Re: [Bpf] [PATCH bpf-next v2] bpf, docs: Clarify PC use in instruction-set.rst

[David Vernet <void@manifault.com>]

[-- Attachment #1: Type: text/plain, Size: 1749 bytes --]

On Fri, Apr 26, 2024 at 01:18:28PM -0700, Dave Thaler wrote:
> This patch elaborates on the use of PC by expanding the PC acronym,
> explaining the units, and the relative position to which the offset
> applies.
> 
> v1->v2: reword per feedback from Alexei
> 
> Signed-off-by: Dave Thaler <dthaler1968@googlemail.com>
> ---
>  Documentation/bpf/standardization/instruction-set.rst | 6 ++++++
>  1 file changed, 6 insertions(+)
> 
> diff --git a/Documentation/bpf/standardization/instruction-set.rst b/Documentation/bpf/standardization/instruction-set.rst
> index b44bdacd0..766f57636 100644
> --- 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 and fails verification if the
> +next instruction is a 128-bit wide instruction.

Should we say "results in undefined behavior" rather than "fails
verification"? I'm not sure if we should be dictating verifier semantics
in the ISA document.

> +
>  The BPF program needs to store the return value into register R0 before doing an
>  ``EXIT``.
>  
> -- 
> 2.40.1
> 
> -- 
> Bpf mailing list
> Bpf@ietf.org
> https://www.ietf.org/mailman/listinfo/bpf

[-- Attachment #2: signature.asc --]
[-- Type: application/pgp-signature, Size: 228 bytes --]