linux-kernel - Re: [PATCH RFC 0/4] Add XDP RX queue index metadata via kfuncs

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Message-ID: <87y0q23j2w.fsf@cloudflare.com>
Date: Thu, 25 Sep 2025 12:47:03 +0200
From: Jakub Sitnicki <jakub@...udflare.com>
To: Mehdi Ben Hadj Khelifa <mehdi.benhadjkhelifa@...il.com>
Cc: davem@...emloft.net,  edumazet@...gle.com,  kuba@...nel.org,
  pabeni@...hat.com,  donald.hunter@...il.com,  andrew+netdev@...n.ch,
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Subject: Re: [PATCH RFC 0/4] Add XDP RX queue index metadata via kfuncs

On Thu, Sep 25, 2025 at 12:28 PM +01, Mehdi Ben Hadj Khelifa wrote:
> On 9/25/25 11:18 AM, Jakub Sitnicki wrote:
>> On Thu, Sep 25, 2025 at 11:54 AM +01, Mehdi Ben Hadj Khelifa wrote:
>>> On 9/25/25 10:43 AM, Jakub Sitnicki wrote:
>>>> On Tue, Sep 23, 2025 at 10:00 PM +01, Mehdi Ben Hadj Khelifa wrote:
>>>>>    This patch series is intended to make a base for setting
>>>>>    queue_index in the xdp_rxq_info struct in bpf/cpumap.c to
>>>>>    the right index. Although that part I still didn't figure
>>>>>    out yet,I m searching for my guidance to do that as well
>>>>>    as for the correctness of the patches in this series.
>>>> What is the use case/movtivation behind this work?
>>>
>>> The goal of the work is to have xdp programs have the correct packet RX queue
>>> index after being redirected through cpumap because currently the queue_index
>>> gets unset or more accurately set to 0 as a default in xdp_rxq_info. This is my
>>> current understanding.I still have to know how I can propogate that HW hint from
>>> the NICs to the function where I need it.
>> This explains what this series does, the desired end state of
>> information passing, but not why is does it - how that information is
>> going to be consumed? To what end?
>
> In my vision,The queue index propagated correctly through cpumap can help xdp
> programs use it for things such as per queue load balancing,Adaptive RSS tuning
> and even maybe for DDoS mitigation where they can drop traffic per queue.I mean
> if these aren't correct intents or if they don't justify the added code, I can
> abort working on it. Even if they weren't I need more guidance on how I can have
> that metadata from HW hints...

Both filtering or load balancing you'd want to do early on - in the XDP
program invoked on receive from NIC, which as Stanislav pointed out
already has access to the RX queue index in its context. Not on the
remote CPU after spending cycles on a redirect.

And even if you wanted to pass that information to the remote XDP
program, to do something with it, you can already store it in custom XDP
metadata [1].

So while perhaps there is something that you can't do today but would be
useful, I don't know what it is. Hence my question about the use case.

[1] https://docs.ebpf.io/linux/helper-function/bpf_xdp_adjust_meta/