[bpf-next,v2] bpf, docs: clarify sign extension of 64-bit use of 32-bit imm

Commit Message

Dave Thaler May 20, 2024, 9:52 p.m. UTC

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 <[e-mail]>
---
 .../bpf/standardization/instruction-set.rst     | 17 +++++++++++++++++
 1 file changed, 17 insertions(+)

Comments

Yonghong Song May 21, 2024, 2:20 a.m. UTC | #1

On 5/20/24 3:52 PM, Dave Thaler wrote:
> 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 <[e-mail]>

Acked-by: Yonghong Song <[e-mail]>

David Vernet May 22, 2024, 6:21 p.m. UTC | #2

On Mon, May 20, 2024 at 02:52:55PM -0700, Dave Thaler wrote:
> 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 <[e-mail]>

Acked-by: David Vernet <[e-mail]>

[e-mail] May 25, 2024, 5:50 p.m. UTC | #3

Hello:

This patch was applied to bpf/bpf-next.git (master)
by Alexei Starovoitov <[e-mail]>:

On Mon, 20 May 2024 14:52:55 -0700 you wrote:
> 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.
> 
> [...]

Here is the summary with links:
  - [bpf-next,v2] bpf, docs: clarify sign extension of 64-bit use of 32-bit imm
    https://git.kernel.org/bpf/bpf-next/c/4e1215d9a190

You are awesome, thank you!

Patch

diff --git a/Documentation/bpf/standardization/instruction-set.rst b/Documentation/bpf/standardization/instruction-set.rst
index 997560aba..7bb1281c5 100644
--- a/Documentation/bpf/standardization/instruction-set.rst
+++ b/Documentation/bpf/standardization/instruction-set.rst
@@ -385,6 +385,19 @@  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``).
 
@@ -486,6 +499,10 @@  Example:
 
 where 's>=' indicates a signed '>=' comparison.
 
+``{JLE, K, JMP}`` means::
+
+  if dst <= (u64)(s64)imm goto +offset
+
 ``{JA, K, JMP32}`` means::
 
   gotol +imm