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bpf, docs: Add explanation of endianness 

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@microsoft.com>
Acked-by: David Vernet <void@manifault.com>
Link: https://lore.kernel.org/r/20230220223742.1347-1-dthaler1968@googlemail.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
This commit is contained in:
Dave Thaler 2023-02-20 22:37:42 +00:00 committed by Alexei Starovoitov
parent 9fa0289285
commit 746ce76712
1 changed files with 14 additions and 2 deletions
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16
Documentation/bpf/instruction-set.rst
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@ -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)
============= ======= ======= ======= ============
imm offset dst_reg src_reg opcode
============= ======= ======= ======= ============
Multi-byte fields ('imm' and 'offset') are similarly stored in
the byte order of the processor.
Note that most instructions do not use all of the fields.
Unused fields shall be cleared to zero.