> On Wed, Feb 22, 2023 at 3:23 PM Jose E. Marchesi
> <jose.marchesi@xxxxxxxxxx> wrote:
>>
>>
>> > On Mon, Feb 20, 2023 at 2:37 PM Dave Thaler
>> > <dthaler1968=40googlemail.com@xxxxxxxxxxxxxx> wrote:
>> >>
>> >> From: Dave Thaler <dthaler@xxxxxxxxxxxxx>
>> >>
>> >> Document the discussion from the email thread on the IETF bpf list,
>> >> where it was explained that the raw format varies by endianness
>> >> of the processor.
>> >>
>> >> Signed-off-by: Dave Thaler <dthaler@xxxxxxxxxxxxx>
>> >>
>> >> Acked-by: David Vernet <void@xxxxxxxxxxxxx>
>> >> ---
>> >>
>> >> V1 -> V2: rebased on top of latest master
>> >> ---
>> >> Documentation/bpf/instruction-set.rst | 16 ++++++++++++++--
>> >> 1 file changed, 14 insertions(+), 2 deletions(-)
>> >>
>> >> diff --git a/Documentation/bpf/instruction-set.rst b/Documentation/bpf/instruction-set.rst
>> >> index af515de5fc3..1d473f060fa 100644
>> >> --- a/Documentation/bpf/instruction-set.rst
>> >> +++ b/Documentation/bpf/instruction-set.rst
>> >> @@ -38,8 +38,9 @@ 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 is as follows, where MSB and LSB mean the most significant
>> >> -bits and least significant bits, respectively:
>> >> +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:
>> >>
>> >> ============= ======= ======= ======= ============
>> >> 32 bits (MSB) 16 bits 4 bits 4 bits 8 bits (LSB)
>> >> @@ -63,6 +64,17 @@ imm offset src_reg dst_reg opcode
>> >> **opcode**
>> >> operation to perform
>> >>
>> >> +and as follows for a big-endian processor:
>> >> +
>> >> +============= ======= ==================== =============== ============
>> >> +32 bits (MSB) 16 bits 4 bits 4 bits 8 bits (LSB)
>> >> +============= ======= ==================== =============== ============
>> >> +immediate offset destination register source register opcode
>> >> +============= ======= ==================== =============== ============
>> >
>> > I've changed it to:
>> > imm offset dst_reg src_reg opcode
>> >
>> > to match the little endian table,
>> > but now one of the tables feels wrong.
>> > The encoding is always done by applying C standard to the struct:
>> > struct bpf_insn {
>> > __u8 code; /* opcode */
>> > __u8 dst_reg:4; /* dest register */
>> > __u8 src_reg:4; /* source register */
>> > __s16 off; /* signed offset */
>> > __s32 imm; /* signed immediate constant */
>> > };
>> > I'm not sure how to express this clearly in the table.
>>
>> Perhaps it would be simpler to document how the instruction 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
>> instructions look like once loaded (as a 64-bit word) by a little-endian
>> or big-endian kernel?
>>
>> Stored little-endian BPF instructions:
>>
>> code src_reg dst_reg off imm
>>
>> foo-le.o: file format elf64-bpfle
>>
>> 0000000000000000 <.text>:
>> 0: 07 01 00 00 ef be ad de r1 += 0xdeadbeef
>>
>> Stored big-endian BPF instructions:
>>
>> code dst_reg src_reg off imm
>>
>> foo-be.o: file format elf64-bpfbe
>>
>> 0000000000000000 <.text>:
>> 0: 07 10 00 00 de ad be ef r1 += 0xdeadbeef
>>
>> i.e. in the stored bytes the code always comes first, then the
>> registers, then the offset, then the immediate, regardless of
>> endianness.
>>
>> This may be easier to understand by implementors looking to generate
>> and/or consume bytes conforming BPF instructions.
>
> +1
> I like this format more as well.
> Maybe we can drop the table and use a diagram of a kind ?
>
> opcode src dst offset imm assembly
> 07 0 1 00 00 ef be ad de r1 += 0xdeadbeef // little
> 07 1 0 00 00 de ad be ef r1 += 0xdeadbeef // big
Good idea. What about something like this:
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
I changed the immediate because 0xdeadbeef is negative and it may be
confusing in the assembly part: strictly it would be r1 += -559038737.
