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Re: [Bpf] ISA: BPF_MSH and deprecated packet access instructions

dthaler1968@googlemail.com Tue, 30 January 2024 15:51 UTC

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From: dthaler1968@googlemail.com
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To: 'Alexei Starovoitov' <alexei.starovoitov@gmail.com>
Cc: "'Jose E. Marchesi'" <jose.marchesi@oracle.com>, 'Yonghong Song' <yonghong.song@linux.dev>, bpf@ietf.org, 'bpf' <bpf@vger.kernel.org>
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Date: Tue, 30 Jan 2024 07:51:10 -0800
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Subject: Re: [Bpf] ISA: BPF_MSH and deprecated packet access instructions
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Alexei Starovoitov <alexei.starovoitov@gmail.com> wrote:
[...]
> > Although the Linux verifier doesn't support them, the fact that gcc
> > does support them tells me that it's probably safest to list the DW
> > and LDX variants as deprecated as well, which is what the draft
> > already did in the appendix so that's good (nothing to change there, I
> > think).
> 
> DW never existed in classic bpf, so abs/ind never had DW flavor.
> If some assembler/compiler decided to "support" them it's on them.
> The standard must not list such things as deprecated. They never existed. So
> nothing is deprecated.

Ack, I will remove the ABS/IND + DW lines from the appendix.

> Same with MSH. BPF_LDX | BPF_MSH | BPF_B is the only insn ever existed.
> It's a legacy insn. Just like abs/ind.

Should it be listed in the legacy conformance group then?

Currently it's not mentioned in instruction-set.rst at all, so the opcode
is available to use by any new instruction.  If we do list it in instruction-set.rst
then, like abs/ind, it will be avoided by anyone proposing new instructions.

Dave

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