Re: [Bpf] [PATCH] bpf, docs: Document BPF insn encoding in term of stored bytes

Jose E. Marchesi
2023-02-27T18:06:09Z

Hi David.

Thanks for the comments, and sorry for the rst formatting mistakes. I
forgot to add the :: before the indented blocks. I'm resending with
these.

> On Fri, Feb 24, 2023 at 09:04:07PM +0100, Jose E. Marchesi wrote:
>>
>> This patch modifies instruction-set.rst so it documents the encoding
>> of BPF instructions in terms of how the bytes are stored (be it in an
>> ELF file or as bytes in a memory buffer to be loaded into the kernel
>> or some other BPF consumer) as opposed to how the instruction looks
>> like once loaded.
>>
>> This is hopefully easier to understand by implementors looking to
>> generate and/or consume bytes conforming BPF instructions.
>>
>> The patch also clarifies that the unused bytes in a pseudo-instruction
>> shall be cleared with zeros.
>>
>> Signed-off-by: Jose E. Marchesi <jose.marchesi@oracle.com>
>
> Hi Jose,
>
> Thanks for writing this up.
>
>> ---
>> Documentation/bpf/instruction-set.rst | 43 +++++++++++++--------------
>> 1 file changed, 21 insertions(+), 22 deletions(-)
>>
>> diff --git a/Documentation/bpf/instruction-set.rst b/Documentation/bpf/instruction-set.rst
>> index 01802ed9b29b..9b28c0e15bb6 100644
>> --- a/Documentation/bpf/instruction-set.rst
>> +++ b/Documentation/bpf/instruction-set.rst
>> @@ -38,15 +38,13 @@ eBPF has two instruction encodings:
>> * the wide instruction encoding, which appends a second 64-bit immediate (i.e.,
>> constant) value after the basic instruction for a total of 128 bits.
>>
>> -The basic instruction encoding looks as follows for a little-endian processor,
>> -where MSB and LSB mean the most significant bits and least significant bits,
>> -respectively:
>> +The fields conforming an encoded basic instruction are stored in the
>> +following order:
>>
>> -============= ======= ======= ======= ============
>> -32 bits (MSB) 16 bits 4 bits 4 bits 8 bits (LSB)
>> -============= ======= ======= ======= ============
>> -imm offset src_reg dst_reg opcode
>> -============= ======= ======= ======= ============
>> + opcode:8 src:4 dst:4 offset:16 imm:32 // In little-endian BPF.
>> + opcode:8 dst:4 src:4 offset:16 imm:32 // In big-endian BPF.
>
> Unfortunately this won't render correctly. It'll look something like
> this:
>
> The fields conforming an encoded basic instruction are stored in the following order:
>
> opcode:8 src:4 dst:4 offset:16 imm:32 // In little-endian
> BPF. opcode:8 dst:4 src:4 offset:16 imm:32 // In big-endian BPF.
>
> You'll have to add some extra newlines. You can test out your changes
> with:
>
> make SPHINXDIRS=bpf htmldocs
>
> And then the output is put in Documentation/output/bpf
>
> In general, this is sort of the problem we have with rst. We want to
> strike a balance between readable in a text editor, and readable when
> rendered in a web browser. I think we can strike such a balance here,
> but it'll probably require a bit of rst-fu. As described below, I think
> we can fix this with a literal code block by just adding a : to order:
>
>> +The fields conforming an encoded basic instruction are stored in the
>> +following order::
>
>
>> +
>> +Where,
>>
>> **imm**
>> signed integer immediate value
>> @@ -64,16 +62,17 @@ imm offset src_reg dst_reg opcode
>> **opcode**
>> operation to perform
>>
>> -and as follows for a big-endian processor:
>> +Note that the contents of multi-byte fields ('imm' and 'offset') are
>> +stored using big-endian byte ordering in big-endian BPF and
>> +little-endian byte ordering in little-endian BPF.
>>
>> -============= ======= ======= ======= ============
>> -32 bits (MSB) 16 bits 4 bits 4 bits 8 bits (LSB)
>> -============= ======= ======= ======= ============
>> -imm offset dst_reg src_reg opcode
>> -============= ======= ======= ======= ============
>> +For example:
>>
>> -Multi-byte fields ('imm' and 'offset') are similarly stored in
>> -the byte order of the processor.
>> + opcode offset imm assembly
>> + src dst
>> + 07 0 1 00 00 44 33 22 11 r1 += 0x11223344 // little
>> + dst src
>> + 07 1 0 00 00 11 22 33 44 r1 += 0x11223344 // big
>
> This also won't render. rst will think it's a "definition list" (see
> [0]), so it's interpreting the line with '// big' as a term that will be
> defined on the next line.
>
> [0]: https://www.sphinx-doc.org/en/master/usage/restructuredtext/basics.html#lists-and-quote-like-blocks
>
> If you do "For example::" it should render correctly. This applies
> elsewhere to the patch. Let's just make all of these literal code
> blocks.
>
> Thanks,
> David
>
>>
>> Note that most instructions do not use all of the fields.
>> Unused fields shall be cleared to zero.
>> @@ -84,18 +83,18 @@ The 64 bits following the basic instruction contain a pseudo instruction
>> using the same format but with opcode, dst_reg, src_reg, and offset all set to zero,
>> and imm containing the high 32 bits of the immediate value.
>>
>> -================= ==================
>> -64 bits (MSB) 64 bits (LSB)
>> -================= ==================
>> -basic instruction pseudo instruction
>> -================= ==================
>> +This is depicted in the following figure:
>> +
>> + basic_instruction pseudo_instruction
>> + code:8 regs:16 offset:16 imm:32 | unused:32 imm:32
>>
>> Thus the 64-bit immediate value is constructed as follows:
>>
>> imm64 = (next_imm << 32) | imm
>>
>> where 'next_imm' refers to the imm value of the pseudo instruction
>> -following the basic instruction.
>> +following the basic instruction. The unused bytes in the pseudo
>> +instruction shall be cleared to zero.
>>
>> Instruction classes
>> -------------------
>> --
>> 2.30.2
>>
>> --
>> Bpf mailing list
>> Bpf@ietf.org
>> https://www.ietf.org/mailman/listinfo/bpf