Re: [PATCH net-next v8 5/5] net: stmmac: rework stmmac_rx to support XDP rx multi-buff

[Nicolai Buchwitz <nb@tipi-net.de>]

On 18.10.2026 19:07, Lorenzo Bianconi wrote:
> The preceding commit dropped frames split across multiple descriptors, 
> or
> using a second buffer, while XDP was enabled. This prevented the
> per-descriptor XDP path from re-parsing frame tails and from applying 
> FCS
> handling to truncated buffers.
> 
> Rework stmmac_rx() to accumulate all the descriptors that make up a 
> frame
> in an xdp_buff, so the XDP program runs on the full (possibly 
> fragmented)
> packet instead of just the first buffer. Frames split across 
> descriptors
> or buffers are now passed to programs loaded with BPF_F_XDP_HAS_FRAGS;
> programs without that capability continue to drop fragmented frames. 
> When
> the frame is not consumed by the program, assemble the skb from the 
> head
> buffer and the collected fragments via napi_build_skb()/
> xdp_update_skb_frags_info().
> 
> Note that XDP still rejects an MTU larger than ETH_DATA_LEN
> (stmmac_xdp_set_prog(), stmmac_change_mtu()) and disables SPH, so an
> ordinary frame fits in a single buffer. This path is therefore 
> exercised
> by frames that do span several descriptors (for example jumbo frames 
> the
> MAC admits through its jumbo-enable bit) and is groundwork for 
> extending
> XDP to jumbo/SPH; it does not enable XDP jumbo support by itself.
> 
> To do so, store the in-progress xdp_buff in rx_q->state instead of the
> partially built skb, so the accumulated head and fragments survive a 
> NAPI
> poll boundary (mid-frame dma_own or dirty_rx break). The state is saved
> only while a frame is in progress and cleared once it completes, 
> leaving
> it untouched when the poll does not process anything (e.g. netpoll 
> invoked
> with a zero budget).
> 
> In addition:
> - advertise NETDEV_XDP_ACT_RX_SG, since the RX path can now deliver
>   non-linear buffers to XDP;
> - build the skb head with napi_build_skb() passing xdp->frame_sz, so
>   skb_shinfo() lands on the same shared_info the fragments were 
> accumulated
>   into;
> - carry the XDP metadata written with bpf_xdp_adjust_meta() into the 
> skb
>   via skb_metadata_set(), so it is no longer dropped and TC/BPF 
> programs
>   can read it through data_meta;
> - attach fragments with xdp_buff_add_frag(), which initializes all the
>   shared_info fields and takes care of the pfmemalloc bit;
> - charge fragments the whole page_pool page size (xdp->frame_sz) as
>   truesize instead of the old dma_buf_sz; with SPH/buf2 most frames 
> carry
>   a fragment, so skb->truesize grows by roughly 1.5x on 4K pages and 2x 
> on
>   64K pages, which reduces the effective socket receive buffer. The new
>   value is more accurate now that a fragment is a full page_pool page, 
> but
>   it is a behavior change;
> - release the buffers collected in the xdp_buff when a frame is dropped
>   on RX errors; the previous code relied on dev_kfree_skb() recycling 
> the
>   skb frags, which no longer applies now that the head and fragments 
> live
>   in an xdp_buff rather than a partially built skb;
> - drop the whole frame when it would exceed MAX_SKB_FRAGS, where the
>   previous skb_add_rx_frag() calls were unbounded;
> - strip the Ethernet FCS from the accumulated xdp_buff with a 
> driver-local
>   tail shrink (stmmac_xdp_shrink_tail()) for both linear and fragmented
>   frames, instead of trimming the skb or the first descriptor length.
> 
> Signed-off-by: Lorenzo Bianconi <lorenzo.bianconi@oss.qualcomm.com>

> [...]

Reviewed-by: Nicolai Buchwitz <nb@tipi-net.de>

Thanks,
Nicolai