netdev - Re: [xdp-hints] Re: [PATCH bpf-next v3 11/12] mlx5: Support RX XDP metadata

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Date:   Fri, 9 Dec 2022 08:45:24 -0800
From:   Jakub Kicinski <kuba@...nel.org>
To:     Toke Høiland-Jørgensen <toke@...hat.com>
Cc:     Stanislav Fomichev <sdf@...gle.com>,
        Alexei Starovoitov <alexei.starovoitov@...il.com>,
        bpf <bpf@...r.kernel.org>, Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Andrii Nakryiko <andrii@...nel.org>,
        Martin KaFai Lau <martin.lau@...ux.dev>,
        Song Liu <song@...nel.org>, Yonghong Song <yhs@...com>,
        John Fastabend <john.fastabend@...il.com>,
        KP Singh <kpsingh@...nel.org>, Hao Luo <haoluo@...gle.com>,
        Jiri Olsa <jolsa@...nel.org>,
        Saeed Mahameed <saeedm@...dia.com>,
        David Ahern <dsahern@...il.com>,
        Willem de Bruijn <willemb@...gle.com>,
        Jesper Dangaard Brouer <brouer@...hat.com>,
        Anatoly Burakov <anatoly.burakov@...el.com>,
        Alexander Lobakin <alexandr.lobakin@...el.com>,
        Magnus Karlsson <magnus.karlsson@...il.com>,
        Maryam Tahhan <mtahhan@...hat.com>, xdp-hints@...-project.net,
        Network Development <netdev@...r.kernel.org>
Subject: Re: [xdp-hints] Re: [PATCH bpf-next v3 11/12] mlx5: Support RX XDP
 metadata

On Fri, 09 Dec 2022 15:42:37 +0100 Toke Høiland-Jørgensen wrote:
> If we expect the program to do out of band probing, we could just get
> rid of the _supported() functions entirely?
> 
> I mean, to me, the whole point of having the separate _supported()
> function for each item was to have a lower-overhead way of checking if
> the metadata item was supported. But if the overhead is not actually
> lower (because both incur a function call), why have them at all? Then
> we could just change the implementation from this:
> 
> bool mlx5e_xdp_rx_hash_supported(const struct xdp_md *ctx)
> {
> 	const struct mlx5_xdp_buff *_ctx = (void *)ctx;
> 
> 	return _ctx->xdp.rxq->dev->features & NETIF_F_RXHASH;
> }
> 
> u32 mlx5e_xdp_rx_hash(const struct xdp_md *ctx)
> {
> 	const struct mlx5_xdp_buff *_ctx = (void *)ctx;
> 
> 	return be32_to_cpu(_ctx->cqe->rss_hash_result);
> }
> 
> to this:
> 
> u32 mlx5e_xdp_rx_hash(const struct xdp_md *ctx)
> {
> 	const struct mlx5_xdp_buff *_ctx = (void *)ctx;
> 
> 	if (!(_ctx->xdp.rxq->dev->features & NETIF_F_RXHASH))
>                 return 0;
> 
> 	return be32_to_cpu(_ctx->cqe->rss_hash_result);
> }

Are there no corner cases? E.g. in case of an L2 frame you'd then
expect a hash of 0? Rather than no hash? 

If I understand we went for the _supported() thing to make inlining 
the check easier than inlining the actual read of the field.
But we're told inlining is a bit of a wait.. so isn't the motivation
for the _supported() pretty much gone? And we should we go back to
returning an error from the actual read?

Is partial inlining hard? (inline just the check and generate a full
call for the read, ending up with the same code as with _supported())