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

[Jesper Dangaard Brouer <hawk@kernel.org>]

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?


(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 ?


[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