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/3f365af4be1fe6a0ef77e751ff9... [2] https://www.kernel.org/doc/html/v6.12/networking/xsk-tx-metadata.html
--Jesper