[PATCH v2] bpf, docs: Fix small typo and define semantics of sign extension

* [PATCH v2] bpf, docs: Fix small typo and define semantics of sign extension
@ 2023-08-08 21:25 Will Hawkins
  2023-08-08 21:25 ` [Bpf] " Will Hawkins
  2023-08-08 23:50 ` patchwork-bot+netdevbpf
  0 siblings, 2 replies; 3+ messages in thread
From: Will Hawkins @ 2023-08-08 21:25 UTC (permalink / raw)
  To: bpf, bpf; +Cc: Will Hawkins, David Vernet

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>

---
 .../bpf/standardization/instruction-set.rst   | 39 ++++++++++++++-----
 1 file changed, 30 insertions(+), 9 deletions(-)

 Changelog:
   v0 -> v1:
     - Separated from an earlier patch -- fly free, patch!
   v1 -> v2:
     - Fix commit message -- s/eBPF/BPF/
     - Add David's ack.

diff --git a/Documentation/bpf/standardization/instruction-set.rst b/Documentation/bpf/standardization/instruction-set.rst
index 25be958130dc..4f73e9dc8d9e 100644
--- a/Documentation/bpf/standardization/instruction-set.rst
+++ b/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::
-- 
2.41.0


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