netdev - Re: [RFC bpf-next 1/4] net: xdp: introduce flags field in xdp_buff and xdp

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Date:   Fri, 28 May 2021 19:33:15 -0600
From:   David Ahern <dsahern@...il.com>
To:     Tom Herbert <tom@...bertland.com>,
        Lorenzo Bianconi <lorenzo@...nel.org>
Cc:     bpf@...r.kernel.org,
        Linux Kernel Network Developers <netdev@...r.kernel.org>,
        "David S. Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Alexei Starovoitov <ast@...nel.org>,
        Eelco Chaudron <echaudro@...hat.com>,
        magnus.karlsson@...el.com,
        Toke Høiland-Jørgensen <toke@...hat.com>,
        Jesper Dangaard Brouer <brouer@...hat.com>, bjorn@...nel.org,
        Maciej Fijałkowski (Intel) 
        <maciej.fijalkowski@...el.com>,
        john fastabend <john.fastabend@...il.com>
Subject: Re: [RFC bpf-next 1/4] net: xdp: introduce flags field in xdp_buff
 and xdp_frame

On 5/28/21 3:18 PM, Tom Herbert wrote:
> On Fri, May 28, 2021 at 10:44 AM Lorenzo Bianconi <lorenzo@...nel.org> wrote:
>>
>> Introduce flag field in xdp_buff and xdp_frame data structure in order
>> to report xdp_buffer metadata. For the moment just hw checksum hints
>> are defined but flags field will be reused for xdp multi-buffer
>> For the moment just CHECKSUM_UNNECESSARY is supported.
>> CHECKSUM_COMPLETE will need to set csum value in metada space.
>>
> Lorenzo,
> 
> This isn't sufficient for the checksum-unnecessary interface, we'd
> also need ability to set csum_level for cases the device validated
> more than one checksum.

That's on me. The original patch was for XDP_REDIRECT to VMs and the
VIRTIO_NET_HDR_ API does not support csum_level.
VIRTIO_NET_HDR_F_DATA_VALID means CHECKSUM_UNNECESSARY, an API
implemented 10 years ago.

> 
> IMO, we shouldn't support CHECKSUM_UNNECESSARY for new uses like this.
> For years now, the Linux community has been pleading with vendors to
> provide CHECKSUM_COMPLETE which is far more useful and robust than
> CHECSUM_UNNECESSARY, and yet some still haven't got with the program
> even though we see more and more instances where CHECKSUM_UNNECESSARY
> doesn't even work at all (e.g. cases with SRv6, new encaps device
> doesn't understand). I believe it's time to take a stand! :-)
>

There is no new hardware or new feature at play here. This about XDP
frames getting the checksum validation setting that an skb enjoys today.
You are taking a stand against S/W equivalency with the existing NICs?
That basically penalizes XDP, continuing to limit its usefulness with
very well established use cases that could benefit from it.