Subject: Re: [PATCH v2 net-next 2/2] xdp: add multi-buff support for xdp running in generic mode

From: Lorenzo Bianconi <[email-masked]>
Date: Thu, 30 Nov 2023 11:51:05 +0100

> 
> 
> On 11/30/23 10:11, Lorenzo Bianconi wrote:
> > Similar to native xdp, do not always linearize the skb in
> > netif_receive_generic_xdp routine but create a non-linear xdp_buff to be
> > processed by the eBPF program. This allow to add  multi-buffer support
> > for xdp running in generic mode.
> > 
> > Signed-off-by: Lorenzo Bianconi <[email-masked]>
> > ---
> >   net/core/dev.c | 144 ++++++++++++++++++++++++++++++++++++++++---------
> >   1 file changed, 119 insertions(+), 25 deletions(-)
> > 
> > diff --git a/net/core/dev.c b/net/core/dev.c
> > index 4df68d7f04a2..0d08e755bb7f 100644
> > --- a/net/core/dev.c
> > +++ b/net/core/dev.c
> > @@ -4853,6 +4853,12 @@ u32 bpf_prog_run_generic_xdp(struct sk_buff *skb, struct xdp_buff *xdp,
> >   	xdp_init_buff(xdp, frame_sz, &rxqueue->xdp_rxq);
> >   	xdp_prepare_buff(xdp, hard_start, skb_headroom(skb) - mac_len,
> >   			 skb_headlen(skb) + mac_len, true);
> > +	if (skb_is_nonlinear(skb)) {
> > +		skb_shinfo(skb)->xdp_frags_size = skb->data_len;
> > +		xdp_buff_set_frags_flag(xdp);
> > +	} else {
> > +		xdp_buff_clear_frags_flag(xdp);
> > +	}
> >   	orig_data_end = xdp->data_end;
> >   	orig_data = xdp->data;
> > @@ -4882,6 +4888,14 @@ u32 bpf_prog_run_generic_xdp(struct sk_buff *skb, struct xdp_buff *xdp,
> >   		skb->len += off; /* positive on grow, negative on shrink */
> >   	}
> > +	/* XDP frag metadata (e.g. nr_frags) are updated in eBPF helpers
> > +	 * (e.g. bpf_xdp_adjust_tail), we need to update data_len here.
> > +	 */
> > +	if (xdp_buff_has_frags(xdp))
> > +		skb->data_len = skb_shinfo(skb)->xdp_frags_size;
> > +	else
> > +		skb->data_len = 0;
> > +
> >   	/* check if XDP changed eth hdr such SKB needs update */
> >   	eth = (struct ethhdr *)xdp->data;
> >   	if ((orig_eth_type != eth->h_proto) ||
> > @@ -4915,54 +4929,134 @@ u32 bpf_prog_run_generic_xdp(struct sk_buff *skb, struct xdp_buff *xdp,
> >   	return act;
> >   }
> > -static u32 netif_receive_generic_xdp(struct sk_buff **pskb,
> > -				     struct xdp_buff *xdp,
> > -				     struct bpf_prog *xdp_prog)
> > +static int netif_skb_check_for_generic_xdp(struct sk_buff **pskb,
> > +					   struct bpf_prog *prog)
> 
> I like this is split out into a check function.
> 
> >   {
> >   	struct sk_buff *skb = *pskb;
> > -	u32 act = XDP_DROP;
> > -
> > -	/* Reinjected packets coming from act_mirred or similar should
> > -	 * not get XDP generic processing.
> > -	 */
> > -	if (skb_is_redirected(skb))
> > -		return XDP_PASS;
> 
> (For other reviewers)
> This reinjected check is moved further down.
> 
> > +	int err;
> > -	/* XDP packets must be linear and must have sufficient headroom
> > -	 * of XDP_PACKET_HEADROOM bytes. This is the guarantee that also
> > -	 * native XDP provides, thus we need to do it here as well.
> > +	/* XDP does not support fraglist so we need to linearize
> > +	 * the skb.
> >   	 */
> > -	if (skb_cloned(skb) || skb_is_nonlinear(skb) ||
> > -	    skb_headroom(skb) < XDP_PACKET_HEADROOM) {
> > +	if (skb_has_frag_list(skb) || !prog->aux->xdp_has_frags) {
> >   		int hroom = XDP_PACKET_HEADROOM - skb_headroom(skb);
> >   		int troom = skb->tail + skb->data_len - skb->end;
> >   		/* In case we have to go down the path and also linearize,
> >   		 * then lets do the pskb_expand_head() work just once here.
> >   		 */
> > -		if (pskb_expand_head(skb,
> > -				     hroom > 0 ? ALIGN(hroom, NET_SKB_PAD) : 0,
> > -				     troom > 0 ? troom + 128 : 0, GFP_ATOMIC))
> > -			goto do_drop;
> > -		if (skb_linearize(skb))
> > -			goto do_drop;
> > +		err = pskb_expand_head(skb,
> > +				       hroom > 0 ? ALIGN(hroom, NET_SKB_PAD) : 0,
> > +				       troom > 0 ? troom + 128 : 0, GFP_ATOMIC);
> > +		if (err)
> > +			return err;
> > +
> > +		err = skb_linearize(skb);
> > +		if (err)
> > +			return err;
> > +
> > +		return 0;
> > +	}
> > +
> > +	/* XDP packets must have sufficient headroom of XDP_PACKET_HEADROOM
> > +	 * bytes. This is the guarantee that also native XDP provides,
> > +	 * thus we need to do it here as well.
> > +	 */
> > +	if (skb_cloned(skb) || skb_shinfo(skb)->nr_frags ||
> 
> I though we could allow a SKB with skb_shinfo(skb)->nr_frags (that isn't
> cloned or shared) to be processed by generic XDP without any reallocation?

I do not think so, we discussed about it with Jakub here [0]

[0] https://lore.kernel.org/netdev/[email-masked]/

> 
> So check would be: (skb_cloned(skb) || skb_shared(skb) ||)
> 
> > +	    skb_headroom(skb) < XDP_PACKET_HEADROOM) {
> 
> [Headroom trick]
> For layered devices the netstack could be the process that created the
> SKB.  If you noticed my veth patchset[4/4], when I detect an XDP-prog
> attach, I'm increasing the net_device headroom (.ndo_set_rx_headroom)
> such that netstack will allocated enough headroom to satisfy
> XDP_PACKET_HEADROOM.
> 
> [4/4] https://lore.kernel.org/netdev/169272716651.1975370.10514711233878278884.stgit@firesoul/

Ah nice, for some reason I missed this patch

> 
> 
> 
> > +		u32 mac_len = skb->data - skb_mac_header(skb);
> > +		u32 size, len, max_head_size, off;
> > +		struct sk_buff *nskb;
> > +		int i, head_off;
> > +
> > +		__skb_push(skb, mac_len);
> > +		max_head_size = SKB_WITH_OVERHEAD(PAGE_SIZE -
> > +						  XDP_PACKET_HEADROOM);
> > +		if (skb->len > max_head_size + MAX_SKB_FRAGS * PAGE_SIZE)
> > +			return -ENOMEM;
> > +
> > +		size = min_t(u32, skb->len, max_head_size);
> > +		nskb = netdev_alloc_skb(skb->dev, size + XDP_PACKET_HEADROOM);
> 
> 
> Would is be possible to use napi_alloc_skb() here?
> 
> The napi_alloc_skb() is faster than netdev_alloc_skb(), but it as name
> suggest assumes this is called under NAPI protection/context.  It
> used-to-be the case for generic XDP, but code got moved around to
> support layered devices, so I not 100% sure if this is always true (NAPI
> context).

Actually I was thinking about it and I was not 100% sure too (so I decided to
use the non-NAPI version). Any input about it?

Regards,
Lorenzo

> 
> 
> > +		if (!nskb)
> > +			return -ENOMEM;
> > +
> > +		skb_reserve(nskb, XDP_PACKET_HEADROOM);
> > +		skb_copy_header(nskb, skb);
> > +
> > +		err = skb_copy_bits(skb, 0, nskb->data, size);
> > +		if (err) {
> > +			consume_skb(nskb);
> > +			return err;
> > +		}
> > +		skb_put(nskb, size);
> > +
> > +		head_off = skb_headroom(nskb) - skb_headroom(skb);
> > +		skb_headers_offset_update(nskb, head_off);
> > +
> > +		off = size;
> > +		len = skb->len - off;
> > +		for (i = 0; i < MAX_SKB_FRAGS && off < skb->len; i++) {
> > +			struct page *page;
> > +			void *frag;
> > +
> > +			size = min_t(u32, len, PAGE_SIZE);
> > +			frag = netdev_alloc_frag(size);
> 
> Again the slower variant.
> 
> > +			if (!frag) {
> > +				consume_skb(nskb);
> > +				return -ENOMEM;
> > +			}
> > +
> > +			page = virt_to_head_page(frag);
> > +			skb_add_rx_frag(nskb, i, page,
> > +					frag - page_address(page), size, size);
> > +			err = skb_copy_bits(skb, off, frag, size);
> > +			if (err) {
> > +				consume_skb(nskb);
> > +				return err;
> > +			}
> > +
> > +			len -= size;
> > +			off += size;
> > +		}
> > +
> > +		consume_skb(skb);
> > +		*pskb = nskb;
> > +		__skb_pull(nskb, mac_len);
> >   	}
> > -	act = bpf_prog_run_generic_xdp(skb, xdp, xdp_prog);
> > +	return 0;
> > +}
> > +
> > +static u32 netif_receive_generic_xdp(struct sk_buff **pskb,
> > +				     struct xdp_buff *xdp,
> > +				     struct bpf_prog *xdp_prog)
> > +{
> > +	u32 act = XDP_DROP;
> > +
> > +	/* Reinjected packets coming from act_mirred or similar should
> > +	 * not get XDP generic processing.
> > +	 */
> > +	if (skb_is_redirected(*pskb))
> > +		return XDP_PASS;
> > +
> > +	if (netif_skb_check_for_generic_xdp(pskb, xdp_prog))
> > +		goto do_drop;
> > +
> > +	act = bpf_prog_run_generic_xdp(*pskb, xdp, xdp_prog);
> >   	switch (act) {
> >   	case XDP_REDIRECT:
> >   	case XDP_TX:
> >   	case XDP_PASS:
> >   		break;
> >   	default:
> > -		bpf_warn_invalid_xdp_action(skb->dev, xdp_prog, act);
> > +		bpf_warn_invalid_xdp_action((*pskb)->dev, xdp_prog, act);
> >   		fallthrough;
> >   	case XDP_ABORTED:
> > -		trace_xdp_exception(skb->dev, xdp_prog, act);
> > +		trace_xdp_exception((*pskb)->dev, xdp_prog, act);
> >   		fallthrough;
> >   	case XDP_DROP:
> >   	do_drop:
> > -		kfree_skb(skb);
> > +		kfree_skb(*pskb);
> >   		break;
> >   	}
> 
> Overall I like the patch :-)
> 
> Are we missing more things to allow GRO packets getting processed by generic
> XDP?
> 
> --Jesper