From 4e1215d9a1903fc9e976aa8903674d050c7af5ff Mon Sep 17 00:00:00 2001 From: Dave Thaler Date: Mon, 20 May 2024 14:52:55 -0700 Subject: [PATCH] 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 Acked-by: Yonghong Song Acked-by: David Vernet Link: https://lore.kernel.org/r/20240520215255.10595-1-dthaler1968@gmail.com Signed-off-by: Alexei Starovoitov --- .../bpf/standardization/instruction-set.rst | 17 +++++++++++++++++ 1 file changed, 17 insertions(+) diff --git a/Documentation/bpf/standardization/instruction-set.rst b/Documentation/bpf/standardization/instruction-set.rst index 02e2490433c6..6b11d1109f12 100644 --- a/Documentation/bpf/standardization/instruction-set.rst +++ b/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