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Re: [Bpf] Sign extension ISA question

Yonghong Song <yonghong.song@linux.dev> Wed, 17 January 2024 03:48 UTC

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Subject: Re: [Bpf] Sign extension ISA question
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On 1/16/24 5:56 PM, dthaler1968@googlemail.com wrote:
> Yonghong Song <yonghong.song@linux.dev> wrote:
>>> Is there any semantic difference between the following two instructions?
>>>
>>> {.opcode = BPF_ALU64 | BPF_MOV | BPF_K, .offset = 0, .imm = -1}
>> This is supported. Sign extension of -1 will be put into ALU64 reg.
>>
>>> {.opcode = BPF_ALU64 | BPF_MOVSX | BPF_K, .offset = 32, .imm = -1}
>> This is not supported. BPF_MOVSX only supports register extension.
>> We should make it clear in the doc.
> Is that limitation a Linux-specific implementation statement? (i.e., put into
> linux-notes.txt)
>
> Or that the meaning is undefined for all runtimes and could be used
> for some other purpose in the future?  (i.e., put into instruction-set.rst)
>
> For now I'll interpret it as the latter.

You are right. The
   {.opcode = BPF_ALU64 | BPF_MOVSX | BPF_K, .offset = 32, .imm = -1}
is not supported by bpf ISA. Currently, it will be an illegal encoding
from kernel perspective.

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