netdev - Re: [PATCH bpf-next V2 0/7] xdp: Allow BPF to set RX hints for XDP

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Message-ID: <aJIEvK0CU_BqqgPQ@lore-rh-laptop>
Date: Tue, 5 Aug 2025 15:18:52 +0200
From: Lorenzo Bianconi <lorenzo@...nel.org>
To: Jakub Kicinski <kuba@...nel.org>
Cc: Jesper Dangaard Brouer <hawk@...nel.org>,
	Stanislav Fomichev <stfomichev@...il.com>, bpf@...r.kernel.org,
	netdev@...r.kernel.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, Jesse Brandeburg <jbrandeburg@...udflare.com>
Subject: Re: [PATCH bpf-next V2 0/7] xdp: Allow BPF to set RX hints for
 XDP_REDIRECTed packets

On Aug 01, Jakub Kicinski wrote:
> On Thu, 31 Jul 2025 23:18:12 +0200 Lorenzo Bianconi wrote:
> > IIUC the 'set' proposal (please correct me if I am wrong), the eBPF program
> > running on the NIC that is receiving the packet from the wire is supposed
> > to set (or update) the hw metadata info (e.g. RX HASH or RX checksum) in
> > the RX DMA descriptor associated to the packet to be successively consumed.
> > Am I right?
> 
> I was thinking of doing the SET on the veth side. Basically the
> metadata has to be understood by the stack only at the xdp->skb
> transition point. So we can delay the SET until that moment, carrying
> the information in program-specific format.

ack, I am fine to delay the translation of the HW metadata from a HW
specific format (the one contained in the DMA descriptor) to the network one
when they are consumed to create the SKB (the veth driver in this case) but I
guess we need to copy the info contained in the DMA descriptor into a buffer
that is still valid when veth driver consumes them since the DMA descriptor
can be no longer available at that time. Do you agree or am I missing
something?

Regards,
Lorenzo

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