Re: [PATCH bpf-next,v3 2/2] selftests/bpf: Enhance XDP Rx metadata handling

[Daniel Borkmann <daniel@iogearbox.net>]

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@kernel.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..

> [1] https://github.com/xdp-project/bpf-examples/blob/3f365af4be1fe6a0ef77e751ff9b12c912810453/AF_XDP-interaction/af_xdp_user.c#L423-L424
> [2] https://www.kernel.org/doc/html/v6.12/networking/xsk-tx-metadata.html
> 
> --Jesper