bpf, docs: Better document the legacy packet access instruction

Use consistent terminology and structured RST elements to better document
these two oddball instructions.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20220131183638.3934982-4-hch@lst.de

Documentation/bpf/instruction-set.rst

@@ -213,8 +213,8 @@ The mode modifier is one of:
   mode modifier  value  description
   =============  =====  ====================================
   BPF_IMM        0x00   used for 64-bit mov
-  BPF_ABS        0x20   legacy BPF packet access
+  BPF_ABS        0x20   legacy BPF packet access (absolute)
-  BPF_IND        0x40   legacy BPF packet access
+  BPF_IND        0x40   legacy BPF packet access (indirect)
   BPF_MEM        0x60   regular load and store operations
   BPF_ATOMIC     0xc0   atomic operations
   =============  =====  ====================================
@@ -294,29 +294,39 @@ eBPF has one 16-byte instruction: ``BPF_LD | BPF_DW | BPF_IMM`` which consists
 of two consecutive ``struct bpf_insn`` 8-byte blocks and interpreted as single
 instruction that loads 64-bit immediate value into a dst_reg.
 
-Packet access instructions
+Legacy BPF Packet access instructions
---------------------------
+-------------------------------------
 
-eBPF has two non-generic instructions: (BPF_ABS | <size> | BPF_LD) and
+eBPF has special instructions for access to packet data that have been
-(BPF_IND | <size> | BPF_LD) which are used to access packet data.
+carried over from classic BPF to retain the performance of legacy socket
+filters running in the eBPF interpreter.
 
-They had to be carried over from classic BPF to have strong performance of
+The instructions come in two forms: ``BPF_ABS | <size> | BPF_LD`` and
-socket filters running in eBPF interpreter. These instructions can only
+``BPF_IND | <size> | BPF_LD``.
-be used when interpreter context is a pointer to ``struct sk_buff`` and
-have seven implicit operands. Register R6 is an implicit input that must
-contain pointer to sk_buff. Register R0 is an implicit output which contains
-the data fetched from the packet. Registers R1-R5 are scratch registers
-and must not be used to store the data across BPF_ABS | BPF_LD or
-BPF_IND | BPF_LD instructions.
 
-These instructions have implicit program exit condition as well. When
+These instructions are used to access packet data and can only be used when
-eBPF program is trying to access the data beyond the packet boundary,
+the program context is a pointer to networking packet.  ``BPF_ABS``
-the interpreter will abort the execution of the program. JIT compilers
+accesses packet data at an absolute offset specified by the immediate data
-therefore must preserve this property. src_reg and imm32 fields are
+and ``BPF_IND`` access packet data at an offset that includes the value of
-explicit inputs to these instructions.
+a register in addition to the immediate data.
 
-For example, BPF_IND | BPF_W | BPF_LD means::
+These instructions have seven implicit operands:
+
+ * Register R6 is an implicit input that must contain pointer to a
+   struct sk_buff.
+ * Register R0 is an implicit output which contains the data fetched from
+   the packet.
+ * Registers R1-R5 are scratch registers that are clobbered after a call to
+   ``BPF_ABS | BPF_LD`` or ``BPF_IND`` | BPF_LD instructions.
+
+These instructions have an implicit program exit condition as well. When an
+eBPF program is trying to access the data beyond the packet boundary, the
+program execution will be aborted.
+
+``BPF_ABS | BPF_W | BPF_LD`` means::
+
+  R0 = ntohl(*(u32 *) (((struct sk_buff *) R6)->data + imm32))
+
+``BPF_IND | BPF_W | BPF_LD`` means::
 
   R0 = ntohl(*(u32 *) (((struct sk_buff *) R6)->data + src_reg + imm32))
-
-and R1 - R5 are clobbered.