RE: Sign extension ISA question

[dthaler1968@googlemail.com]

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.

Thanks,
Dave