netdev - Re: [PATCH v8 bpf-next 02/10] veth: Add driver XDP

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Message-ID: <c84a15fe-0863-067c-417d-f20ce5e9e9bc@iogearbox.net>
Date:   Tue, 7 Aug 2018 16:26:14 +0200
From:   Daniel Borkmann <daniel@...earbox.net>
To:     Toshiaki Makita <makita.toshiaki@....ntt.co.jp>,
        Alexei Starovoitov <ast@...nel.org>
Cc:     netdev@...r.kernel.org, Jesper Dangaard Brouer <brouer@...hat.com>,
        Jakub Kicinski <jakub.kicinski@...ronome.com>,
        John Fastabend <john.fastabend@...il.com>
Subject: Re: [PATCH v8 bpf-next 02/10] veth: Add driver XDP

On 08/03/2018 09:58 AM, Toshiaki Makita wrote:
> This is the basic implementation of veth driver XDP.
> 
> Incoming packets are sent from the peer veth device in the form of skb,
> so this is generally doing the same thing as generic XDP.
> 
> This itself is not so useful, but a starting point to implement other
> useful veth XDP features like TX and REDIRECT.
> 
> This introduces NAPI when XDP is enabled, because XDP is now heavily
> relies on NAPI context. Use ptr_ring to emulate NIC ring. Tx function
> enqueues packets to the ring and peer NAPI handler drains the ring.
> 
> Currently only one ring is allocated for each veth device, so it does
> not scale on multiqueue env. This can be resolved by allocating rings
> on the per-queue basis later.
> 
> Note that NAPI is not used but netif_rx is used when XDP is not loaded,
> so this does not change the default behaviour.
> 
> v6:
> - Check skb->len only when allocation is needed.
> - Add __GFP_NOWARN to alloc_page() as it can be triggered by external
>   events.
> 
> v3:
> - Fix race on closing the device.
> - Add extack messages in ndo_bpf.
> 
> v2:
> - Squashed with the patch adding NAPI.
> - Implement adjust_tail.
> - Don't acquire consumer lock because it is guarded by NAPI.
> - Make poll_controller noop since it is unnecessary.
> - Register rxq_info on enabling XDP rather than on opening the device.
> 
> Signed-off-by: Toshiaki Makita <makita.toshiaki@....ntt.co.jp>
[...]
> +
> +static struct sk_buff *veth_xdp_rcv_skb(struct veth_priv *priv,
> +					struct sk_buff *skb)
> +{
> +	u32 pktlen, headroom, act, metalen;
> +	void *orig_data, *orig_data_end;
> +	struct bpf_prog *xdp_prog;
> +	int mac_len, delta, off;
> +	struct xdp_buff xdp;
> +
> +	rcu_read_lock();
> +	xdp_prog = rcu_dereference(priv->xdp_prog);
> +	if (unlikely(!xdp_prog)) {
> +		rcu_read_unlock();
> +		goto out;
> +	}
> +
> +	mac_len = skb->data - skb_mac_header(skb);
> +	pktlen = skb->len + mac_len;
> +	headroom = skb_headroom(skb) - mac_len;
> +
> +	if (skb_shared(skb) || skb_head_is_locked(skb) ||
> +	    skb_is_nonlinear(skb) || headroom < XDP_PACKET_HEADROOM) {

Hmm, I think this is not fully correct. What happens if you have cloned skbs as
e.g. the case with TCP? This would also need a full expensive unclone to make the
data private as expected by XDP (this is basically a similar issue in generic
XDP). It may potentially be worth to also share the code here with generic XDP
implementation given it's quite similar?

> +		struct sk_buff *nskb;
> +		int size, head_off;
> +		void *head, *start;
> +		struct page *page;
> +
> +		size = SKB_DATA_ALIGN(VETH_XDP_HEADROOM + pktlen) +
> +		       SKB_DATA_ALIGN(sizeof(struct skb_shared_info));
> +		if (size > PAGE_SIZE)
> +			goto drop;
> +
> +		page = alloc_page(GFP_ATOMIC | __GFP_NOWARN);
> +		if (!page)
> +			goto drop;
> +
> +		head = page_address(page);
> +		start = head + VETH_XDP_HEADROOM;
> +		if (skb_copy_bits(skb, -mac_len, start, pktlen)) {
> +			page_frag_free(head);
> +			goto drop;
> +		}
> +
> +		nskb = veth_build_skb(head,
> +				      VETH_XDP_HEADROOM + mac_len, skb->len,
> +				      PAGE_SIZE);
> +		if (!nskb) {
> +			page_frag_free(head);
> +			goto drop;
> +		}
> +
> +		skb_copy_header(nskb, skb);
> +		head_off = skb_headroom(nskb) - skb_headroom(skb);
> +		skb_headers_offset_update(nskb, head_off);
> +		if (skb->sk)
> +			skb_set_owner_w(nskb, skb->sk);
> +		consume_skb(skb);
> +		skb = nskb;
> +	}
> +
> +	xdp.data_hard_start = skb->head;
> +	xdp.data = skb_mac_header(skb);
> +	xdp.data_end = xdp.data + pktlen;
> +	xdp.data_meta = xdp.data;
> +	xdp.rxq = &priv->xdp_rxq;
> +	orig_data = xdp.data;
> +	orig_data_end = xdp.data_end;
> +
> +	act = bpf_prog_run_xdp(xdp_prog, &xdp);
> +
> +	switch (act) {
> +	case XDP_PASS:
> +		break;
> +	default:
> +		bpf_warn_invalid_xdp_action(act);
> +	case XDP_ABORTED:
> +		trace_xdp_exception(priv->dev, xdp_prog, act);
> +	case XDP_DROP:
> +		goto drop;
> +	}
> +	rcu_read_unlock();
> +
> +	delta = orig_data - xdp.data;
> +	off = mac_len + delta;
> +	if (off > 0)
> +		__skb_push(skb, off);
> +	else if (off < 0)
> +		__skb_pull(skb, -off);
> +	skb->mac_header -= delta;
> +	off = xdp.data_end - orig_data_end;
> +	if (off != 0)
> +		__skb_put(skb, off);
> +	skb->protocol = eth_type_trans(skb, priv->dev);
> +
> +	metalen = xdp.data - xdp.data_meta;
> +	if (metalen)
> +		skb_metadata_set(skb, metalen);
> +out:
> +	return skb;
> +drop:
> +	rcu_read_unlock();
> +	kfree_skb(skb);
> +	return NULL;
> +}