bpf, docs: clarify sign extension of 64-bit use of 32-bit imm

imm is defined as a 32-bit signed integer.

{MOV, K, ALU64} says it does "dst = src" (where src is 'imm') and it
does do dst = (s64)imm, which in that sense does sign extend imm. The MOVSX
instruction is explained as sign extending, so added the example of
{MOV, K, ALU64} to make this more clear.

{JLE, K, JMP} says it does "PC += offset if dst <= src" (where src is 'imm',
and the comparison is unsigned). This was apparently ambiguous to some
readers as to whether the comparison was "dst <= (u64)(u32)imm" or
"dst <= (u64)(s64)imm" so added an example to make this more clear.

v1 -> v2: Address comments from Yonghong

Signed-off-by: Dave Thaler <dthaler1968@googlemail.com>
Acked-by: Yonghong Song <yonghong.song@linux.dev>
Acked-by: David Vernet <void@manifault.com>
Link: https://lore.kernel.org/r/20240520215255.10595-1-dthaler1968@gmail.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

Documentation/bpf/standardization/instruction-set.rst

@@ -393,6 +393,19 @@ The ``MOVSX`` instruction does a move operation with sign extension.
 operands into 64-bit operands.  Unlike other arithmetic instructions,
 ``MOVSX`` is only defined for register source operands (``X``).
 
+``{MOV, K, ALU64}`` means::
+
+  dst = (s64)imm
+
+``{MOV, X, ALU}`` means::
+
+  dst = (u32)src
+
+``{MOVSX, X, ALU}`` with 'offset' 8 means::
+
+  dst = (u32)(s32)(s8)src
+
+
 The ``NEG`` instruction is only defined when the source bit is clear
 (``K``).
 
@@ -491,6 +504,10 @@ Example:
 
 where 's>=' indicates a signed '>=' comparison.
 
+``{JLE, K, JMP}`` means::
+
+  if dst <= (u64)(s64)imm goto +offset
+
 ``{JA, K, JMP32}`` means::
 
   gotol +imm