netdev - Re: [PATCH v6 bpf-next 04/17] bpf: introduce new alu insn BPF

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Date:   Mon, 6 May 2019 08:57:06 -0700
From:   Alexei Starovoitov <alexei.starovoitov@...il.com>
To:     Jiong Wang <jiong.wang@...ronome.com>
Cc:     daniel@...earbox.net, bpf@...r.kernel.org, netdev@...r.kernel.org,
        oss-drivers@...ronome.com
Subject: Re: [PATCH v6 bpf-next 04/17] bpf: introduce new alu insn BPF_ZEXT
 for explicit zero extension

On Fri, May 03, 2019 at 11:42:31AM +0100, Jiong Wang wrote:
> This patch introduce new alu32 insn BPF_ZEXT, and allocate the unused
> opcode 0xe0 to it.
> 
> Compared with the other alu32 insns, zero extension on low 32-bit is the
> only semantics for this instruction. It also allows various JIT back-ends
> to do optimal zero extension code-gen.
> 
> BPF_ZEXT is supposed to be encoded with BPF_ALU only, and is supposed to be
> generated by the latter 32-bit optimization code inside verifier for those
> arches that do not support hardware implicit zero extension only.
> 
> It is not supposed to be used in user's program directly at the moment.
> Therefore, no need to recognize it inside generic verification code. It
> just need to be supported for execution on interpreter or related JIT
> back-ends.

uapi and the doc define it, but "it is not supposed to be used" ?!

> Signed-off-by: Jiong Wang <jiong.wang@...ronome.com>
> ---
>  Documentation/networking/filter.txt | 10 ++++++++++
>  include/uapi/linux/bpf.h            |  3 +++
>  kernel/bpf/core.c                   |  4 ++++
>  tools/include/uapi/linux/bpf.h      |  3 +++
>  4 files changed, 20 insertions(+)
> 
> diff --git a/Documentation/networking/filter.txt b/Documentation/networking/filter.txt
> index 319e5e0..1cb3e42 100644
> --- a/Documentation/networking/filter.txt
> +++ b/Documentation/networking/filter.txt
> @@ -903,6 +903,16 @@ If BPF_CLASS(code) == BPF_ALU or BPF_ALU64 [ in eBPF ], BPF_OP(code) is one of:
>    BPF_MOV   0xb0  /* eBPF only: mov reg to reg */
>    BPF_ARSH  0xc0  /* eBPF only: sign extending shift right */
>    BPF_END   0xd0  /* eBPF only: endianness conversion */
> +  BPF_ZEXT  0xe0  /* eBPF BPF_ALU only: zero-extends low 32-bit */
> +
> +Compared with BPF_ALU | BPF_MOV which zero-extends low 32-bit implicitly,
> +BPF_ALU | BPF_ZEXT zero-extends low 32-bit explicitly. Such zero extension is

wait. that's an excellent observation. alu|mov is exactly it.
we do not need another insn.
we probably can teach the verifier to recognize <<32, >>32 and replace with mov32