netdev - Re: [PATCH v6 bpf-next 4/9] veth: Handle xdp

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Date:   Tue, 31 Jul 2018 19:40:08 +0900
From:   Toshiaki Makita <makita.toshiaki@....ntt.co.jp>
To:     Jesper Dangaard Brouer <brouer@...hat.com>
Cc:     Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>, netdev@...r.kernel.org,
        Jakub Kicinski <jakub.kicinski@...ronome.com>,
        John Fastabend <john.fastabend@...il.com>,
        "Karlsson, Magnus" <magnus.karlsson@...el.com>,
        Björn Töpel <bjorn.topel@...el.com>
Subject: Re: [PATCH v6 bpf-next 4/9] veth: Handle xdp_frames in xdp napi ring

On 2018/07/31 19:26, Jesper Dangaard Brouer wrote:
> 
> Context needed from: [PATCH v6 bpf-next 2/9] veth: Add driver XDP
> 
> On Mon, 30 Jul 2018 19:43:44 +0900
> Toshiaki Makita <makita.toshiaki@....ntt.co.jp> wrote:
> 
>> +static struct sk_buff *veth_build_skb(void *head, int headroom, int len,
>> +				      int buflen)
>> +{
>> +	struct sk_buff *skb;
>> +
>> +	if (!buflen) {
>> +		buflen = SKB_DATA_ALIGN(headroom + len) +
>> +			 SKB_DATA_ALIGN(sizeof(struct skb_shared_info));
>> +	}
>> +	skb = build_skb(head, buflen);
>> +	if (!skb)
>> +		return NULL;
>> +
>> +	skb_reserve(skb, headroom);
>> +	skb_put(skb, len);
>> +
>> +	return skb;
>> +}
> 
> 
> On Mon, 30 Jul 2018 19:43:46 +0900
> Toshiaki Makita <makita.toshiaki@....ntt.co.jp> wrote:
> 
>> +static struct sk_buff *veth_xdp_rcv_one(struct veth_priv *priv,
>> +					struct xdp_frame *frame)
>> +{
>> +	int len = frame->len, delta = 0;
>> +	struct bpf_prog *xdp_prog;
>> +	unsigned int headroom;
>> +	struct sk_buff *skb;
>> +
>> +	rcu_read_lock();
>> +	xdp_prog = rcu_dereference(priv->xdp_prog);
>> +	if (likely(xdp_prog)) {
>> +		struct xdp_buff xdp;
>> +		u32 act;
>> +
>> +		xdp.data_hard_start = frame->data - frame->headroom;
>> +		xdp.data = frame->data;
>> +		xdp.data_end = frame->data + frame->len;
>> +		xdp.data_meta = frame->data - frame->metasize;
>> +		xdp.rxq = &priv->xdp_rxq;
>> +
>> +		act = bpf_prog_run_xdp(xdp_prog, &xdp);
>> +
>> +		switch (act) {
>> +		case XDP_PASS:
>> +			delta = frame->data - xdp.data;
>> +			len = xdp.data_end - xdp.data;
>> +			break;
>> +		default:
>> +			bpf_warn_invalid_xdp_action(act);
>> +		case XDP_ABORTED:
>> +			trace_xdp_exception(priv->dev, xdp_prog, act);
>> +		case XDP_DROP:
>> +			goto err_xdp;
>> +		}
>> +	}
>> +	rcu_read_unlock();
>> +
>> +	headroom = frame->data - delta - (void *)frame;
>> +	skb = veth_build_skb(frame, headroom, len, 0);
> 
> Here you are adding an assumption that struct xdp_frame is always
> located in-the-top of the packet-data area.  I tried hard not to add
> such a dependency!  You can calculate the beginning of the frame from
> the xdp_frame->data pointer.
> 
> Why not add such a dependency?  Because for AF_XDP zero-copy, we cannot
> make such an assumption.  
> 
> Currently, when an RX-queue is in AF-XDP-ZC mode (MEM_TYPE_ZERO_COPY)
> the packet will get dropped when calling convert_to_xdp_frame(), but as
> the TODO comment indicated in convert_to_xdp_frame() this is not the
> end-goal. 
> 
> The comment in convert_to_xdp_frame(), indicate we need a full
> alloc+copy, but that is actually not necessary, if we can just use
> another memory area for struct xdp_frame, and a pointer to data.  Thus,
> allowing devmap-redir to work-ZC and allow cpumap-redir to do the copy
> on the remote CPU.

Thanks for pointing this out.
Seems you are saying xdp_frame area is not reusable. That means we
reduce usable headroom on every REDIRECT. I wanted to avoid this but
actually it is impossible, right?

>> +	if (!skb) {
>> +		xdp_return_frame(frame);
>> +		goto err;
>> +	}
>> +
>> +	memset(frame, 0, sizeof(*frame));
>> +	skb->protocol = eth_type_trans(skb, priv->dev);
>> +err:
>> +	return skb;
>> +err_xdp:
>> +	rcu_read_unlock();
>> +	xdp_return_frame(frame);
>> +
>> +	return NULL;
>> +}
> 
> 

-- 
Toshiaki Makita