netdev - Re: [RFC bpf-next 0/7] bpf: netdev TX metadata

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Date: Tue, 13 Jun 2023 20:54:26 -0700
From: David Ahern <dsahern@...nel.org>
To: Jakub Kicinski <kuba@...nel.org>, Stanislav Fomichev <sdf@...gle.com>
Cc: bpf@...r.kernel.org, ast@...nel.org, daniel@...earbox.net,
 andrii@...nel.org, martin.lau@...ux.dev, song@...nel.org, yhs@...com,
 john.fastabend@...il.com, kpsingh@...nel.org, haoluo@...gle.com,
 jolsa@...nel.org, willemb@...gle.com, magnus.karlsson@...el.com,
 bjorn@...nel.org, maciej.fijalkowski@...el.com, netdev@...r.kernel.org
Subject: Re: [RFC bpf-next 0/7] bpf: netdev TX metadata

On 6/13/23 9:31 PM, Jakub Kicinski wrote:
> On Mon, 12 Jun 2023 10:23:00 -0700 Stanislav Fomichev wrote:
>> The goal of this series is to add two new standard-ish places
>> in the transmit path:
>>
>> 1. Right before the packet is transmitted (with access to TX
>>    descriptors)

If a device requires multiple Tx descriptors per skb or multibuf frame,
how would that be handled within the XDP API?

> 
> I'm not sure that the Tx descriptors can be populated piecemeal.

If it is host memory before the pidx move, why would that matter? Do you
have a specific example in mind?

> If we were ever to support more standard offload features, which
> require packet geometry (hdr offsets etc.) to be described "call
> per feature" will end up duplicating arguments, and there will be
> a lot of args..
> 
> And if there is an SKB path in the future combining the normal SKB
> offloads with the half-rendered descriptors may be a pain.

Once the descriptor(s) is (are) populated, the skb is irrelevant is it
not? Only complication that comes to mind is wanting to add or remove
headers (e.g., tunnels) which will be much more complicated at this
point, but might still be possible on a per NIC (and maybe version) basis.