Subject: Re: [PATCH RFC bpf-next 07/20] xdp: Track if metadata is supported in xdp_frame <> xdp_buff conversions

From: Lorenzo Bianconi <[email-masked]>
Date: Mon, 10 Mar 2025 12:10:40 +0100

[...]
> > > 
> 
> I'm fairly sure that all drivers support XDP_REDIRECT.
> Except didn't Lorenzo add a feature bit for this?
> (so, some drivers might explicitly not-support this)

I think most of the drivers support XDP_REDIRECT. IIRC just some vf
implementations (e.g. ixgbevf or thunder/nicvf do not support XDP_REDIRECT).
Maybe nfp is a special case.

Regards,
Lorenzo

> 
> > > So maybe we need to fix those drivers first, if there are any.
> > 
> > Most drivers don't support metadata unfortunately:
> > 
> > > rg -U "xdp_prepare_buff\([^)]*false\);" drivers/net/
> > drivers/net/tun.c
> > 1712:		xdp_prepare_buff(&xdp, buf, pad, len, false);
> > 
> > drivers/net/ethernet/microsoft/mana/mana_bpf.c
> > 94:	xdp_prepare_buff(xdp, buf_va, XDP_PACKET_HEADROOM, pkt_len, false);
> > 
> > drivers/net/ethernet/marvell/mvneta.c
> > 2344:	xdp_prepare_buff(xdp, data, pp->rx_offset_correction + MVNETA_MH_SIZE,
> > 2345:			 data_len, false);
> > 
> > drivers/net/ethernet/marvell/octeontx2/nic/otx2_txrx.c
> > 1436:	xdp_prepare_buff(&xdp, hard_start, OTX2_HEAD_ROOM,
> > 1437:			 cqe->sg.seg_size, false);
> > 
> > drivers/net/ethernet/socionext/netsec.c
> > 1021:		xdp_prepare_buff(&xdp, desc->addr, NETSEC_RXBUF_HEADROOM,
> > 1022:				 pkt_len, false);
> > 
> > drivers/net/ethernet/google/gve/gve_rx.c
> > 740:	xdp_prepare_buff(&new, frame, headroom, len, false);
> > 859:		xdp_prepare_buff(&xdp, page_info->page_address +
> > 860:				 page_info->page_offset, GVE_RX_PAD,
> > 861:				 len, false);
> > 
> > drivers/net/ethernet/marvell/mvpp2/mvpp2_main.c
> > 3984:			xdp_prepare_buff(&xdp, data,
> > 3985:					 MVPP2_MH_SIZE + MVPP2_SKB_HEADROOM,
> > 3986:					 rx_bytes, false);
> > 
> > drivers/net/ethernet/aquantia/atlantic/aq_ring.c
> > 794:		xdp_prepare_buff(&xdp, hard_start, rx_ring->page_offset,
> > 795:				 buff->len, false);
> > 
> > drivers/net/ethernet/cavium/thunder/nicvf_main.c
> > 554:	xdp_prepare_buff(&xdp, hard_start, data - hard_start, len, false);
> > 
> > drivers/net/ethernet/ti/cpsw_new.c
> > 348:		xdp_prepare_buff(&xdp, pa, headroom, size, false);
> > 
> > drivers/net/ethernet/freescale/enetc/enetc.c
> > 1710:	xdp_prepare_buff(xdp_buff, hard_start - rx_ring->buffer_offset,
> > 1711:			 rx_ring->buffer_offset, size, false);
> > 
> > drivers/net/ethernet/ti/am65-cpsw-nuss.c
> > 1335:		xdp_prepare_buff(&xdp, page_addr, AM65_CPSW_HEADROOM,
> > 1336:				 pkt_len, false);
> > 
> > drivers/net/ethernet/ti/cpsw.c
> > 403:		xdp_prepare_buff(&xdp, pa, headroom, size, false);
> > 
> > drivers/net/ethernet/sfc/rx.c
> > 289:	xdp_prepare_buff(&xdp, *ehp - EFX_XDP_HEADROOM, EFX_XDP_HEADROOM,
> > 290:			 rx_buf->len, false);
> > 
> > drivers/net/ethernet/mediatek/mtk_eth_soc.c
> > 2097:			xdp_prepare_buff(&xdp, data, MTK_PP_HEADROOM, pktlen,
> > 2098:					 false);
> > 
> > drivers/net/ethernet/sfc/siena/rx.c
> > 291:	xdp_prepare_buff(&xdp, *ehp - EFX_XDP_HEADROOM, EFX_XDP_HEADROOM,
> > 292:			 rx_buf->len, false)
> > 
> > I don't know if it's just because no one has added calls to
> > skb_metadata_set() in yet, or if there's a more fundamental reason.
> > 
> 
> I simply think driver developers have been lazy.
> 
> If someone want some easy kernel commits, these drivers should be fairly
> easy to fix...
> 
> > I think they all reserve some amount of headroom, but not always the
> > full XDP_PACKET_HEADROOM. Eg sfc:
> > 
> 
> The Intel drivers use 192 (AFAIK if that is still true). The API ended
> up supporting non-standard XDP_PACKET_HEADROOM, due to the Intel
> drivers, when XDP support was added to those (which is a long time ago now).
> 
> > drivers/net/ethernet/sfc/net_driver.h:
> > /* Non-standard XDP_PACKET_HEADROOM and tailroom to satisfy XDP_REDIRECT and
> >   * still fit two standard MTU size packets into a single 4K page.
> >   */
> > #define EFX_XDP_HEADROOM	128
> > 
> 
> This is smaller than most drivers, but still have enough headroom for
> xdp_frame + traits.
> 
> > If it's just because skb_metadata_set() is missing, I can take the
> > patches from this series that adds a "generic" XDP -> skb hook ("trait:
> > Propagate presence of traits to sk_buff"), have it call
> > skb_metadata_set(), and try to add it to all the drivers in a separate
> > series.
> > 
> 
> I think someone should cleanup those drivers and add support.
> 
> --Jesper
> 
> > > >   	/* Lifetime of xdp_rxq_info is limited to NAPI/enqueue time,
> > > >   	 * while mem_type is valid on remote CPU.
> > > >   	 */
> > > > @@ -369,6 +374,8 @@ void xdp_convert_frame_to_buff(const struct xdp_frame *frame,
> > > >   	xdp->data = frame->data;
> > > >   	xdp->data_end = frame->data + frame->len;
> > > >   	xdp->data_meta = frame->data - frame->metasize;
> > > > +	if (frame->meta_unsupported)
> > > > +		xdp_set_data_meta_invalid(xdp);
> > > >   	xdp->frame_sz = frame->frame_sz;
> > > >   	xdp->flags = frame->flags;
> > > >   }
> > > > @@ -396,6 +403,7 @@ int xdp_update_frame_from_buff(const struct xdp_buff *xdp,
> > > >   	xdp_frame->len  = xdp->data_end - xdp->data;
> > > >   	xdp_frame->headroom = headroom - sizeof(*xdp_frame);
> > > >   	xdp_frame->metasize = metasize;
> > > > +	xdp_frame->meta_unsupported = xdp_data_meta_unsupported(xdp);
> > > >   	xdp_frame->frame_sz = xdp->frame_sz;
> > > >   	xdp_frame->flags = xdp->flags;
> > > 
> > > Thanks,
> > > Olek
>