[PATCH V3] bpf, docs: Document BPF insn encoding in term of stored bytes

["Jose E. Marchesi" <jose.marchesi@oracle.com>]

[Changes from V2:
- Use src and dst consistently in the document.
- Use a more graphical depiction of the 128-bit instruction.
- Remove `Where:' fragment.
- Clarify that unused bits are reserved and shall be zeroed.]

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>
---
 Documentation/bpf/instruction-set.rst | 63 ++++++++++++++-------------
 1 file changed, 33 insertions(+), 30 deletions(-)

diff --git a/Documentation/bpf/instruction-set.rst b/Documentation/bpf/instruction-set.rst
index 01802ed9b29b..fae2e48d6a0b 100644
--- a/Documentation/bpf/instruction-set.rst
+++ b/Documentation/bpf/instruction-set.rst
@@ -38,15 +38,11 @@ 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.
 
 **imm**
   signed integer immediate value
@@ -54,48 +50,55 @@ imm            offset   src_reg  dst_reg  opcode
 **offset**
   signed integer offset used with pointer arithmetic
 
-**src_reg**
+**src**
   the source register number (0-10), except where otherwise specified
   (`64-bit immediate instructions`_ reuse this field for other purposes)
 
-**dst_reg**
+**dst**
   destination register number (0-10)
 
 **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
 
 Note that most instructions do not use all of the fields.
 Unused fields shall be cleared to zero.
 
-As discussed below in `64-bit immediate instructions`_, a 64-bit immediate
-instruction uses a 64-bit immediate value that is constructed as follows.
-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.
+As discussed below in `64-bit immediate instructions`_, a 64-bit
+immediate instruction uses a 64-bit immediate value that is
+constructed as follows.  The 64 bits following the basic instruction
+contain a pseudo instruction using the same format but with opcode,
+dst, src, 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
+  .-----------------------------.
+  |                             |
+  code:8 regs:16 offset:16 imm:32 unused:32 imm:32
+                                  |              |
+                                  '--------------'
+                                 pseudo instruction
 
 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 are reserved and shall be cleared to zero.
 
 Instruction classes
 -------------------
@@ -137,7 +140,7 @@ code            source  instruction class
   source  value  description
   ======  =====  ==============================================
   BPF_K   0x00   use 32-bit 'imm' value as source operand
-  BPF_X   0x08   use 'src_reg' register value as source operand
+  BPF_X   0x08   use 'src' register value as source operand
   ======  =====  ==============================================
 
 **instruction class**
-- 
2.30.2