Message-ID: <941fece6-9660-4aa8-91ed-346b0c2d97c1@gmail.com>
Date: Thu, 25 Sep 2025 13:03:11 +0100
From: Mehdi Ben Hadj Khelifa <mehdi.benhadjkhelifa@...il.com>
To: Jakub Sitnicki <jakub@...udflare.com>
Cc: davem@...emloft.net, edumazet@...gle.com, kuba@...nel.org,
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Subject: Re: [PATCH RFC 0/4] Add XDP RX queue index metadata via kfuncs

On 9/25/25 11:47 AM, Jakub Sitnicki wrote:
> 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/
> 
Very clear,
I will abort working on this since it can be passed as a custom XDP 
metadata [1] until further valid use cases or when it proves to be more 
useful.
Thank you for your review and time.
Best Regards,
Mehdi

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

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