bpf, docs: Fix small typo and define semantics of sign extension

Add additional precision on the semantics of the sign extension
operations in BPF. In addition, fix a very minor typo.

Signed-off-by: Will Hawkins <hawkinsw@obs.cr>
Acked-by: David Vernet <void@manifault.com>
Link: https://lore.kernel.org/r/20230808212503.197834-1-hawkinsw@obs.cr
Signed-off-by: Martin KaFai Lau <martin.lau@kernel.org>

Documentation/bpf/standardization/instruction-set.rst

@@ -76,6 +76,27 @@ Functions
   format and returns the equivalent number with the same bit width but
   opposite endianness.
 
+
+Definitions
+-----------
+
+.. glossary::
+
+  Sign Extend
+    To `sign extend an` ``X`` `-bit number, A, to a` ``Y`` `-bit number, B  ,` means to
+
+    #. Copy all ``X`` bits from `A` to the lower ``X`` bits of `B`.
+    #. Set the value of the remaining ``Y`` - ``X`` bits of `B` to the value of
+       the  most-significant bit of `A`.
+
+.. admonition:: Example
+
+  Sign extend an 8-bit number ``A`` to a 16-bit number ``B`` on a big-endian platform:
+  ::
+
+    A:          10000110
+    B: 11111111 10000110
+
 Registers and calling convention
 ================================
 
@@ -234,7 +255,7 @@ BPF_SMOD   0x90   1        dst = (src != 0) ? (dst s% src) : dst
 BPF_XOR    0xa0   0        dst ^= src
 BPF_MOV    0xb0   0        dst = src
 BPF_MOVSX  0xb0   8/16/32  dst = (s8,s16,s32)src
-BPF_ARSH   0xc0   0        sign extending dst >>= (src & mask)
+BPF_ARSH   0xc0   0        :term:`sign extending<Sign Extend>` dst >>= (src & mask)
 BPF_END    0xd0   0        byte swap operations (see `Byte swap instructions`_ below)
 =========  =====  =======  ==========================================================
 
@@ -266,22 +287,22 @@ where '(u32)' indicates that the upper 32 bits are zeroed.
 Note that most instructions have instruction offset of 0. Only three instructions
 (``BPF_SDIV``, ``BPF_SMOD``, ``BPF_MOVSX``) have a non-zero offset.
 
-The devision and modulo operations support both unsigned and signed flavors.
+The division and modulo operations support both unsigned and signed flavors.
 
 For unsigned operations (``BPF_DIV`` and ``BPF_MOD``), for ``BPF_ALU``,
 'imm' is interpreted as a 32-bit unsigned value. For ``BPF_ALU64``,
-'imm' is first sign extended from 32 to 64 bits, and then interpreted as
-a 64-bit unsigned value.
+'imm' is first :term:`sign extended<Sign Extend>` from 32 to 64 bits, and then
+interpreted as a 64-bit unsigned value.
 
 For signed operations (``BPF_SDIV`` and ``BPF_SMOD``), for ``BPF_ALU``,
 'imm' is interpreted as a 32-bit signed value. For ``BPF_ALU64``, 'imm'
-is first sign extended from 32 to 64 bits, and then interpreted as a
-64-bit signed value.
+is first :term:`sign extended<Sign Extend>` from 32 to 64 bits, and then
+interpreted as a 64-bit signed value.
 
 The ``BPF_MOVSX`` instruction does a move operation with sign extension.
-``BPF_ALU | BPF_MOVSX`` sign extends 8-bit and 16-bit operands into 32
+``BPF_ALU | BPF_MOVSX`` :term:`sign extends<Sign Extend>` 8-bit and 16-bit operands into 32
 bit operands, and zeroes the remaining upper 32 bits.
-``BPF_ALU64 | BPF_MOVSX`` sign extends 8-bit, 16-bit, and 32-bit
+``BPF_ALU64 | BPF_MOVSX`` :term:`sign extends<Sign Extend>` 8-bit, 16-bit, and 32-bit
 operands into 64 bit operands.
 
 Shift operations use a mask of 0x3F (63) for 64-bit operations and 0x1F (31)
@@ -466,7 +487,7 @@ Where size is one of: ``BPF_B``, ``BPF_H``, ``BPF_W``, or ``BPF_DW`` and
 Sign-extension load operations
 ------------------------------
 
-The ``BPF_MEMSX`` mode modifier is used to encode sign-extension load
+The ``BPF_MEMSX`` mode modifier is used to encode :term:`sign-extension<Sign Extend>` load
 instructions that transfer data between a register and memory.
 
 ``BPF_MEMSX | <size> | BPF_LDX`` means::