netdev - Re: [PATCH 3/4] bpf, docs: Generate nicer tables for instruction encodings

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Date:   Mon, 3 Jan 2022 10:57:12 +0100
From:   Christoph Hellwig <hch@....de>
To:     Alexei Starovoitov <alexei.starovoitov@...il.com>
Cc:     Christoph Hellwig <hch@....de>, Jonathan Corbet <corbet@....net>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Andrii Nakryiko <andrii@...nel.org>,
        Martin KaFai Lau <kafai@...com>,
        Song Liu <songliubraving@...com>, Yonghong Song <yhs@...com>,
        John Fastabend <john.fastabend@...il.com>,
        KP Singh <kpsingh@...nel.org>, linux-doc@...r.kernel.org,
        netdev@...r.kernel.org, bpf@...r.kernel.org
Subject: Re: [PATCH 3/4] bpf, docs: Generate nicer tables for instruction
 encodings

On Thu, Dec 30, 2021 at 04:43:24PM -0800, Alexei Starovoitov wrote:
> > +  ========  =====  =========================
> > +  code      value  description
> > +  ========  =====  =========================
> > +  BPF_JA    0x00   BPF_JMP only
> >    BPF_JEQ   0x10
> >    BPF_JGT   0x20
> >    BPF_JGE   0x30
> >    BPF_JSET  0x40
> 
> Not your fault, but the new table looks odd with
> only some opcodes documented.
> Same issue with BPF_ALU table.
> In the past the documented opcodes were for eBPF only and
> not documented in both, so it wasn't that bad.
> At least there was a reason for discrepancy.
> Now it just odd.
> May be add a comment to all rows?

Yes, having the description everywhere would be good.  But I'll have to
do research to actually figure out what should go in there for some.

> > +  =============  =====  =====================
> > +  mode modifier  value  description
> > +  =============  =====  =====================
> > +  BPF_IMM        0x00   used for 64-bit mov
> > +  BPF_ABS        0x20
> > +  BPF_IND        0x40
> > +  BPF_MEM        0x60
> 
> May be say here that ABS and IND are legacy for compat with classic only?
> and MEM is the most common modifier for load/store?

Sure.