[PATCH v2 2/2] bpf, doc: Improve MOV/MOVSX documentation and add examples

[Vineet Gupta <vineet.gupta@linux.dev>]

 - Add missing form {MOV, K, ALU}.

 - Add some assembly (pseudo-C) snippets.

 - Rearrange: MOV content comes before MOVSX.

 - MOVSX content itself rearranged: canonical sign extension variant
   for {8,16,32}-> 64 moved ahead of the special variant which only
   sign extends to 32 and zeroes out the upper bits.

 - Remove the hyphen '-' in "sign-extension" to make grep hit all
   instances with one pattern.

Signed-off-by: Vineet Gupta <vineet.gupta@linux.dev>
---
Changes since v1
  - Address review comments from AI CI-bot
  - Added missing {MOV, K, ALU}
---
 .../bpf/standardization/instruction-set.rst   | 47 +++++++++++++++----
 1 file changed, 37 insertions(+), 10 deletions(-)

diff --git a/Documentation/bpf/standardization/instruction-set.rst b/Documentation/bpf/standardization/instruction-set.rst
index 96181565906f..c55628448898 100644
--- a/Documentation/bpf/standardization/instruction-set.rst
+++ b/Documentation/bpf/standardization/instruction-set.rst
@@ -414,25 +414,52 @@ etc. This specification requires that signed modulo MUST use truncated division
 
    a % n = a - n * trunc(a / n)
 
-The ``MOVSX`` instruction does a move operation with sign extension.
-``{MOVSX, X, ALU}`` :term:`sign extends<Sign Extend>` 8-bit and 16-bit operands into
-32-bit operands, and zeroes the remaining upper 32 bits.
-``{MOVSX, X, ALU64}`` :term:`sign extends<Sign Extend>` 8-bit, 16-bit, and 32-bit
-operands into 64-bit operands.  Unlike other arithmetic instructions,
-``MOVSX`` is only defined for register source operands (``X``).
+For move operations, the ``MOV`` instruction has a few different forms.
+
+``{MOV, X, ALU64}`` means::
+
+  dst = src
+
+e.g. ``r1 = r2``
 
 ``{MOV, K, ALU64}`` means::
 
   dst = (s64)imm
 
-``{MOV, X, ALU}`` means::
+e.g. ``r1 = -4``
+     ``r5 = 9282009``
+
+``{MOV, K, ALU}`` means::
+
+  dst = (u32)imm
+
+e.g. ``w1 = -4``
+     ``w5 = 7302004``
+
+``{MOV, X, ALU}`` has zero extension semantics (upper 32 bits are zeroed)::
 
   dst = (u32)src
 
+e.g. w5 = w9
+
+The ``MOVSX`` instruction does a move operation with sign extension and has
+a couple of forms.
+
+``{MOVSX, X, ALU64}`` :term:`sign extends<Sign Extend>` 8-bit, 16-bit, and 32-bit
+operands into 64-bit operands.
+
+e.g. ``r1 = (s8)r2``
+
+The ``{MOVSX, X, ALU}`` form has slightly different semantics: it
+:term:`sign extends<Sign Extend>` 8-bit and 16-bit operands into
+32-bit operands, and zeroes the remaining upper 32 bits (similar to ``MOV``).
+
 ``{MOVSX, X, ALU}`` with 'offset' 8 means::
 
   dst = (u32)(s32)(s8)src
 
+Unlike other arithmetic instructions,
+``MOVSX`` is only defined for register source operands (``X``).
 
 The ``NEG`` instruction is only defined when the source bit is clear
 (``K``).
@@ -605,7 +632,7 @@ For load and store instructions (``LD``, ``LDX``, ``ST``, and ``STX``), the
     ABS            1      legacy BPF packet access (absolute)   `Legacy BPF Packet access instructions`_
     IND            2      legacy BPF packet access (indirect)   `Legacy BPF Packet access instructions`_
     MEM            3      regular load and store operations     `Regular load and store operations`_
-    MEMSX          4      sign-extension load operations        `Sign-extension load operations`_
+    MEMSX          4      sign extension load operations        `Sign extension load operations`_
     ATOMIC         6      atomic operations                     `Atomic operations`_
     =============  =====  ====================================  =============
 
@@ -649,10 +676,10 @@ instructions that transfer data between a register and memory.
 Where '<size>' is one of: ``B``, ``H``, ``W``, or ``DW``, and
 'unsigned size' is one of: u8, u16, u32, or u64.
 
-Sign-extension load operations
+Sign extension load operations
 ------------------------------
 
-The ``MEMSX`` mode modifier is used to encode :term:`sign-extension<Sign Extend>` load
+The ``MEMSX`` mode modifier is used to encode :term:`sign extension<Sign Extend>` load
 instructions that transfer data between a register and memory.
 
 ``{MEMSX, <size>, LDX}`` means::
-- 
2.53.0