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bpf, docs: Clarify that MOVSX is only for BPF_X not BPF_K

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Per discussion on the mailing list at
https://mailarchive.ietf.org/arch/msg/bpf/uQiqhURdtxV_ZQOTgjCdm-seh74/
the MOVSX operation is only defined to support register extension.

The document didn't previously state this and incorrectly implied
that one could use an immediate value.

Signed-off-by: Dave Thaler <dthaler1968@gmail.com>
Acked-by: David Vernet <void@manifault.com>
Acked-by: Yonghong Song <yonghong.song@linux.dev>
Link: https://lore.kernel.org/r/20240118232954.27206-1-dthaler1968@gmail.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
This commit is contained in:
Dave Thaler 2024-01-18 15:29:54 -08:00 committed by Alexei Starovoitov
parent b3f086a7a1
commit 20e109ea98
1 changed files with 2 additions and 1 deletions
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3
Documentation/bpf/standardization/instruction-set.rst
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@ -317,7 +317,8 @@ The ``BPF_MOVSX`` instruction does a move operation with sign extension.
``BPF_ALU | BPF_MOVSX`` :term:`sign extends<Sign Extend>` 8-bit and 16-bit operands into 32
bit operands, and zeroes the remaining upper 32 bits.
``BPF_ALU64 | BPF_MOVSX`` :term:`sign extends<Sign Extend>` 8-bit, 16-bit, and 32-bit
operands into 64 bit operands.
operands into 64 bit operands. Unlike other arithmetic instructions,
``BPF_MOVSX`` is only defined for register source operands (``BPF_X``).
Shift operations use a mask of 0x3F (63) for 64-bit operations and 0x1F (31)
for 32-bit operations.