bpf, docs: Formalize type notation and function semantics in ISA standard

Give a single place where the shorthand for types are defined and the
semantics of helper functions are described.

Signed-off-by: Will Hawkins <hawkinsw@obs.cr>
Acked-by: David Vernet <void@manifault.com>
Link: https://lore.kernel.org/r/20230807140651.122484-1-hawkinsw@obs.cr
Signed-off-by: Martin KaFai Lau <martin.lau@kernel.org>

Documentation/bpf/standardization/instruction-set.rst

@@ -10,9 +10,71 @@ This document specifies version 1.0 of the eBPF instruction set.
 Documentation conventions
 =========================
 
-For brevity, this document uses the type notion "u64", "u32", etc.
-to mean an unsigned integer whose width is the specified number of bits,
-and "s32", etc. to mean a signed integer of the specified number of bits.
+For brevity and consistency, this document refers to families
+of types using a shorthand syntax and refers to several expository,
+mnemonic functions when describing the semantics of instructions.
+The range of valid values for those types and the semantics of those
+functions are defined in the following subsections.
+
+Types
+-----
+This document refers to integer types with the notation `SN` to specify
+a type's signedness (`S`) and bit width (`N`), respectively.
+
+.. table:: Meaning of signedness notation.
+
+  ==== =========
+  `S`  Meaning
+  ==== =========
+  `u`  unsigned
+  `s`  signed
+  ==== =========
+
+.. table:: Meaning of bit-width notation.
+
+  ===== =========
+  `N`   Bit width
+  ===== =========
+  `8`   8 bits
+  `16`  16 bits
+  `32`  32 bits
+  `64`  64 bits
+  `128` 128 bits
+  ===== =========
+
+For example, `u32` is a type whose valid values are all the 32-bit unsigned
+numbers and `s16` is a types whose valid values are all the 16-bit signed
+numbers.
+
+Functions
+---------
+* `htobe16`: Takes an unsigned 16-bit number in host-endian format and
+  returns the equivalent number as an unsigned 16-bit number in big-endian
+  format.
+* `htobe32`: Takes an unsigned 32-bit number in host-endian format and
+  returns the equivalent number as an unsigned 32-bit number in big-endian
+  format.
+* `htobe64`: Takes an unsigned 64-bit number in host-endian format and
+  returns the equivalent number as an unsigned 64-bit number in big-endian
+  format.
+* `htole16`: Takes an unsigned 16-bit number in host-endian format and
+  returns the equivalent number as an unsigned 16-bit number in little-endian
+  format.
+* `htole32`: Takes an unsigned 32-bit number in host-endian format and
+  returns the equivalent number as an unsigned 32-bit number in little-endian
+  format.
+* `htole64`: Takes an unsigned 64-bit number in host-endian format and
+  returns the equivalent number as an unsigned 64-bit number in little-endian
+  format.
+* `bswap16`: Takes an unsigned 16-bit number in either big- or little-endian
+  format and returns the equivalent number with the same bit width but
+  opposite endianness.
+* `bswap32`: Takes an unsigned 32-bit number in either big- or little-endian
+  format and returns the equivalent number with the same bit width but
+  opposite endianness.
+* `bswap64`: Takes an unsigned 64-bit number in either big- or little-endian
+  format and returns the equivalent number with the same bit width but
+  opposite endianness.
 
 Registers and calling convention
 ================================
@@ -252,19 +314,23 @@ are supported: 16, 32 and 64.
 
 Examples:
 
-``BPF_ALU | BPF_TO_LE | BPF_END`` with imm = 16 means::
+``BPF_ALU | BPF_TO_LE | BPF_END`` with imm = 16/32/64 means::
 
   dst = htole16(dst)
+  dst = htole32(dst)
+  dst = htole64(dst)
 
-``BPF_ALU | BPF_TO_BE | BPF_END`` with imm = 64 means::
+``BPF_ALU | BPF_TO_BE | BPF_END`` with imm = 16/32/64 means::
 
+  dst = htobe16(dst)
+  dst = htobe32(dst)
   dst = htobe64(dst)
 
 ``BPF_ALU64 | BPF_TO_LE | BPF_END`` with imm = 16/32/64 means::
 
-  dst = bswap16 dst
-  dst = bswap32 dst
-  dst = bswap64 dst
+  dst = bswap16(dst)
+  dst = bswap32(dst)
+  dst = bswap64(dst)
 
 Jump instructions
 -----------------