netdev - Re: [PATCH bpf-next V1 7/7] net: xdp: update documentation for xdp-rx-metadata.rst

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Message-ID: <aEj6nqH85uBe2IlW@mini-arch>
Date: Tue, 10 Jun 2025 20:40:14 -0700
From: Stanislav Fomichev <stfomichev@...il.com>
To: Lorenzo Bianconi <lorenzo@...nel.org>
Cc: Toke Høiland-Jørgensen <toke@...hat.com>,
	Daniel Borkmann <daniel@...earbox.net>,
	Jesper Dangaard Brouer <hawk@...nel.org>, bpf@...r.kernel.org,
	netdev@...r.kernel.org, Jakub Kicinski <kuba@...nel.org>,
	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, Magnus Karlsson <magnus.karlsson@...el.com>,
	Maciej Fijalkowski <maciej.fijalkowski@...el.com>
Subject: Re: [PATCH bpf-next V1 7/7] net: xdp: update documentation for
 xdp-rx-metadata.rst

On 06/11, Lorenzo Bianconi wrote:
> > Daniel Borkmann <daniel@...earbox.net> writes:
> > 
> [...]
> > >> 
> > >> Why not have a new flag for bpf_redirect that transparently stores all
> > >> available metadata? If you care only about the redirect -> skb case.
> > >> Might give us more wiggle room in the future to make it work with
> > >> traits.
> > >
> > > Also q from my side: If I understand the proposal correctly, in order to fully
> > > populate an skb at some point, you have to call all the bpf_xdp_metadata_* kfuncs
> > > to collect the data from the driver descriptors (indirect call), and then yet
> > > again all equivalent bpf_xdp_store_rx_* kfuncs to re-store the data in struct
> > > xdp_rx_meta again. This seems rather costly and once you add more kfuncs with
> > > meta data aren't you better off switching to tc(x) directly so the driver can
> > > do all this natively? :/
> > 
> > I agree that the "one kfunc per metadata item" scales poorly. IIRC, the
> > hope was (back when we added the initial HW metadata support) that we
> > would be able to inline them to avoid the function call overhead.
> > 
> > That being said, even with half a dozen function calls, that's still a
> > lot less overhead from going all the way to TC(x). The goal of the use
> > case here is to do as little work as possible on the CPU that initially
> > receives the packet, instead moving the network stack processing (and
> > skb allocation) to a different CPU with cpumap.
> > 
> > So even if the *total* amount of work being done is a bit higher because
> > of the kfunc overhead, that can still be beneficial because it's split
> > between two (or more) CPUs.
> > 
> > I'm sure Jesper has some concrete benchmarks for this lying around
> > somewhere, hopefully he can share those :)
> 
> Another possible approach would be to have some utility functions (not kfuncs)
> used to 'store' the hw metadata in the xdp_frame that are executed in each
> driver codebase before performing XDP_REDIRECT. The downside of this approach
> is we need to parse the hw metadata twice if the eBPF program that is bounded
> to the NIC is consuming these info. What do you think?

That's the option I was asking about. I'm assuming we should be able
to reuse existing xmo metadata callbacks for this. We should be able
to hide it from the drivers also hopefully.