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| author | Dave Thaler <dthaler1968@googlemail.com> | 2024-05-20 14:52:55 -0700 |
|---|---|---|
| committer | Alexei Starovoitov <ast@kernel.org> | 2024-05-25 10:39:03 -0700 |
| commit | 4e1215d9a1903fc9e976aa8903674d050c7af5ff (patch) | |
| tree | c087489b3a7f5ffb2b73a42022a4a4ad796c70ba /Documentation/bpf | |
| parent | a985fdca5e7e665d58dc40c92a67c8b67b6291db (diff) | |
| download | lwn-4e1215d9a1903fc9e976aa8903674d050c7af5ff.tar.gz lwn-4e1215d9a1903fc9e976aa8903674d050c7af5ff.zip | |
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>
Diffstat (limited to 'Documentation/bpf')
| -rw-r--r-- | Documentation/bpf/standardization/instruction-set.rst | 17 |
1 files changed, 17 insertions, 0 deletions
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 |