Message-ID: <27edae8a-f1a8-4000-ac1e-fd4d5b01e807@iogearbox.net>
Date: Wed, 9 Jul 2025 16:00:10 +0200
From: Daniel Borkmann <daniel@...earbox.net>
To: Stanislav Fomichev <stfomichev@...il.com>
Cc: Jesper Dangaard Brouer <hawk@...nel.org>,
 "Song, Yoong Siang" <yoong.siang.song@...el.com>,
 "David S . Miller" <davem@...emloft.net>, Eric Dumazet
 <edumazet@...gle.com>, Jakub Kicinski <kuba@...nel.org>,
 Paolo Abeni <pabeni@...hat.com>, Simon Horman <horms@...nel.org>,
 Jonathan Corbet <corbet@....net>, Alexei Starovoitov <ast@...nel.org>,
 John Fastabend <john.fastabend@...il.com>,
 Stanislav Fomichev <sdf@...ichev.me>, Andrii Nakryiko <andrii@...nel.org>,
 Martin KaFai Lau <martin.lau@...ux.dev>, Eduard Zingerman
 <eddyz87@...il.com>, Song Liu <song@...nel.org>,
 Yonghong Song <yonghong.song@...ux.dev>, KP Singh <kpsingh@...nel.org>,
 Hao Luo <haoluo@...gle.com>, Jiri Olsa <jolsa@...nel.org>,
 Mykola Lysenko <mykolal@...com>, Shuah Khan <shuah@...nel.org>,
 Magnus Karlsson <magnus.karlsson@...il.com>, Björn Töpel
 <bjorn@...nel.org>, Maciej Fijalkowski <maciej.fijalkowski@...el.com>,
 Jonathan Lemon <jonathan.lemon@...il.com>,
 "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
 "linux-doc@...r.kernel.org" <linux-doc@...r.kernel.org>,
 "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
 "bpf@...r.kernel.org" <bpf@...r.kernel.org>,
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Subject: Re: [PATCH bpf-next,v3 2/2] selftests/bpf: Enhance XDP Rx metadata
 handling

On 7/7/25 5:03 PM, Stanislav Fomichev wrote:
> On 07/04, Daniel Borkmann wrote:
>> On 7/4/25 11:58 AM, Jesper Dangaard Brouer wrote:
>>> On 04/07/2025 03.17, Song, Yoong Siang wrote:
>>>> On Friday, July 4, 2025 1:05 AM, Jesper Dangaard Brouer <hawk@...nel.org> wrote:
>>>>> On 02/07/2025 18.57, Song Yoong Siang wrote:
>>>>>> Introduce the XDP_METADATA_SIZE macro as a conservative measure to
>>>>>> accommodate any metadata areas reserved by Ethernet devices.
>>>>>
>>>>> This seems like a sloppy workaround :-(
>>>>>
>>>>> To me, the problem arise because AF_XDP is lacking the ability to
>>>>> communicate the size of the data_meta area.  If we had this capability,
>>>>> then we could allow the IGC driver to take some of the space, have the
>>>>> BPF-prog expand it futher (bpf_xdp_adjust_meta) and then userspace
>>>>> AF_XDP would simply be able to see the size of the data_meta area, and
>>>>> apply the struct xdp_meta at right offset.
>>>>>
>>>> Thanks for your input.
>>>>
>>>> I agree with you that the implementation will be simple if user application
>>>> able to get the size of data_meta area. The intention of this patch set is to let
>>>> developer aware of such limitations before we have a perfect solution.
>>>>
>>>> Btw, do you got any suggestion on how to expose the metadata length?
>>>> I not sure whether xdp_desc.options is a simple and good idea or not?
>>>
>>> That is a question to the AF_XDP maintainers... added them to this email.
>>>
>>> /* Rx/Tx descriptor */
>>> struct xdp_desc {
>>>       __u64 addr;
>>>       __u32 len;
>>>       __u32 options;
>>> };
>>>
>>> As far as I know, the xdp_desc.options field isn't used, right?
>>
>> The options holds flags, see also XDP_PKT_CONTD and XDP_TX_METADATA.
>>
>>> (Please AF_XDP experts, please verify below statements:)
>>> Something else we likely want to document: The available headroom in the
>>> AF_XDP frame.  When accessing the metadata in userspace AF_XDP we do a
>>> negative offset from the UMEM packet pointer.  IIRC on RX the available
>>> headroom will be either 255 or 192 bytes (depending on NIC drivers).
>>>
>>> Slightly confusing when AF_XDP transmitting from userspace the UMEM
>>> headroom is default zero (XSK_UMEM__DEFAULT_FRAME_HEADROOM is zero).
>>> This is configurable via xsk_umem_config.frame_headroom, like I did in
>>> this example[1].
>>>
>>> Maybe I did something wrong in[1], because I see that the new method is
>>> setting xsk_umem_config.tx_metadata_len + flag XDP_UMEM_TX_METADATA_LEN.
>>> This is nicely documented in [2]. How does this interact with setting
>>> xsk_umem_config.frame_headroom ?
>>
>> If you request XDP_UMEM_TX_METADATA_LEN then on TX side you can fill
>> struct xsk_tx_metadata before the start of packet data, that is,
>> meta = data - sizeof(struct xsk_tx_metadata). The validity of the
>> latter is indicated via desc->options |= XDP_TX_METADATA and then
>> you fill meta->flags with things like XDP_TXMD_FLAGS_CHECKSUM to
>> tell that the related fields are valid (ex. request.csum_start,
>> request.csum_offset) and that you expect the driver to do the
>> offload with this info. This is also what I mentioned in the other
>> thread some time ago that imho it would make sense to have this also
>> on RX side somewhat similar to virtio_net_hdr..
> 
> Let's at least document the current behavior where some (small minority of)
> drivers can reuse the rx metadata area for some of its state? If we want
> to improve on that by adding another knob, we can follow up?
> (but I remember last time it was discussed, about a year ago, people
> were not enthusiastic about another parameter exported as uapi)

But its still fundamentally broken no? Unless there is no harm for BPF devs
to override that rx metadata area when the pkt later on goes up the stack, but
it sounds this is not the case here. Iiuc, Yoong is trying a different approach
now to prepend before data_hard_start [0]? Then if BPF prog needs it, igc
already implements xmo_rx_timestamp callback which can copy it from there.

   [0] https://lore.kernel.org/bpf/20250707191742.662aeffb@kernel.org/

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