netdev - Re: [PATCH bpf-next V2 3/7] net: xdp: Add kfuncs to store hw metadata in xdp

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Message-ID: <85eee028-2784-4bcd-b9a9-9e1bdf0799f3@kernel.org>
Date: Thu, 3 Jul 2025 13:41:02 +0200
From: Jesper Dangaard Brouer <hawk@...nel.org>
To: bpf@...r.kernel.org, netdev@...r.kernel.org,
 Jakub Kicinski <kuba@...nel.org>, lorenzo@...nel.org
Cc: Alexei Starovoitov <ast@...nel.org>,
 Daniel Borkmann <borkmann@...earbox.net>,
 Eric Dumazet <eric.dumazet@...il.com>, "David S. Miller"
 <davem@...emloft.net>, Paolo Abeni <pabeni@...hat.com>, sdf@...ichev.me,
 kernel-team@...udflare.com, arthur@...hurfabre.com, jakub@...udflare.com
Subject: Re: [PATCH bpf-next V2 3/7] net: xdp: Add kfuncs to store hw metadata
 in xdp_buff

On 02/07/2025 16.58, Jesper Dangaard Brouer wrote:
> From: Lorenzo Bianconi<lorenzo@...nel.org>
> 
> Introduce the following kfuncs to store hw metadata provided by the NIC
> into the xdp_buff struct:
> 
> - rx-hash: bpf_xdp_store_rx_hash
> - rx-vlan: bpf_xdp_store_rx_vlan
> - rx-hw-ts: bpf_xdp_store_rx_ts
> 
> Signed-off-by: Lorenzo Bianconi<lorenzo@...nel.org>
> Signed-off-by: Jesper Dangaard Brouer<hawk@...nel.org>
> ---
>   include/net/xdp.h |    5 +++++
>   net/core/xdp.c    |   45 +++++++++++++++++++++++++++++++++++++++++++++
>   2 files changed, 50 insertions(+)
> 
> diff --git a/net/core/xdp.c b/net/core/xdp.c
> index bd3110fc7ef8..1ffba57714ea 100644
> --- a/net/core/xdp.c
> +++ b/net/core/xdp.c
> @@ -963,12 +963,57 @@ __bpf_kfunc int bpf_xdp_metadata_rx_vlan_tag(const struct xdp_md *ctx,
[...]
> +__bpf_kfunc int bpf_xdp_store_rx_ts(struct xdp_md *ctx, u64 ts)
> +{
> +	struct xdp_buff *xdp = (struct xdp_buff *)ctx;
> +	struct skb_shared_info *sinfo = xdp_get_shared_info_from_buff(xdp);
> +	struct skb_shared_hwtstamps *shwt = &sinfo->hwtstamps;
> +
> +	shwt->hwtstamp = ts;

Here we are storing into the SKB shared_info struct.  This is located at
the SKB data tail.  Thus, this will very likely cause a cache-miss.

What about storing it into xdp->rx_meta and then starting a prefetch for
shared_info?  (and updating patch-4 that moved it into SKB)

(Reviewers should be aware that writing into the xdp_frame headroom
(xdp->rx_meta) likely isn't a cache-miss, because all drivers does a
prefetchw for this memory prior to running BPF-prog).

> +	xdp->flags |= XDP_FLAGS_META_RX_TS;
> +
> +	return 0;
> +}