netdev - Re: [PATCH RFC bpf-next 01/20] trait: limited KV store for packet metadata

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Message-ID: <CAADnVQ+OShaA37-=B4-GWTQQ8p4yPw3TgYLPTkbHMJLYhr48kg@mail.gmail.com>
Date: Thu, 6 Mar 2025 22:36:26 -0800
From: Alexei Starovoitov <alexei.starovoitov@...il.com>
To: arthur@...hurfabre.com
Cc: Network Development <netdev@...r.kernel.org>, bpf <bpf@...r.kernel.org>, 
	Jakub Sitnicki <jakub@...udflare.com>, Jesper Dangaard Brouer <hawk@...nel.org>, Yan Zhai <yan@...udflare.com>, 
	jbrandeburg@...udflare.com, Toke Høiland-Jørgensen <thoiland@...hat.com>, 
	lbiancon@...hat.com, Arthur Fabre <afabre@...udflare.com>
Subject: Re: [PATCH RFC bpf-next 01/20] trait: limited KV store for packet metadata

On Wed, Mar 5, 2025 at 6:33 AM <arthur@...hurfabre.com> wrote:
>
> +struct __trait_hdr {
> +       /* Values are stored ordered by key, immediately after the header.
> +        *
> +        * The size of each value set is stored in the header as two bits:
> +        *  - 00: Not set.
> +        *  - 01: 2 bytes.
> +        *  - 10: 4 bytes.
> +        *  - 11: 8 bytes.

...

> +        *  - hweight(low) + hweight(high)<<1 is offset.

the comment doesn't match the code

> +        */
> +       u64 high;
> +       u64 low;

...

> +static __always_inline int __trait_total_length(struct __trait_hdr h)
> +{
> +       return (hweight64(h.low) << 1) + (hweight64(h.high) << 2)
> +               // For size 8, we only get 4+2=6. Add another 2 in.
> +               + (hweight64(h.high & h.low) << 1);
> +}

This is really cool idea, but 2 byte size doesn't feel that useful.
How about:
- 00: Not set.
- 01: 4 bytes.
- 10: 8 bytes.
- 11: 16 bytes.

4 byte may be useful for ipv4, 16 for ipv6, and 8 is just a good number.
And compute the same way with 3 popcount with extra +1 to shifts.