netdev - Re: [PATCH v2 net-next 6/9] bpf: helpers: add bpf_xdp_adjust_mb

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Date:   Thu, 3 Sep 2020 18:13:58 -0700
From:   Alexei Starovoitov <alexei.starovoitov@...il.com>
To:     Lorenzo Bianconi <lorenzo@...nel.org>
Cc:     netdev@...r.kernel.org, bpf@...r.kernel.org, davem@...emloft.net,
        lorenzo.bianconi@...hat.com, brouer@...hat.com,
        echaudro@...hat.com, sameehj@...zon.com, kuba@...nel.org,
        john.fastabend@...il.com, daniel@...earbox.net, ast@...nel.org,
        shayagr@...zon.com
Subject: Re: [PATCH v2 net-next 6/9] bpf: helpers: add
 bpf_xdp_adjust_mb_header helper

On Thu, Sep 03, 2020 at 10:58:50PM +0200, Lorenzo Bianconi wrote:
> +BPF_CALL_2(bpf_xdp_adjust_mb_header, struct  xdp_buff *, xdp,
> +	   int, offset)
> +{
> +	void *data_hard_end, *data_end;
> +	struct skb_shared_info *sinfo;
> +	int frag_offset, frag_len;
> +	u8 *addr;
> +
> +	if (!xdp->mb)
> +		return -EOPNOTSUPP;
> +
> +	sinfo = xdp_get_shared_info_from_buff(xdp);
> +
> +	frag_len = skb_frag_size(&sinfo->frags[0]);
> +	if (offset > frag_len)
> +		return -EINVAL;
> +
> +	frag_offset = skb_frag_off(&sinfo->frags[0]);
> +	data_end = xdp->data_end + offset;
> +
> +	if (offset < 0 && (-offset > frag_offset ||
> +			   data_end < xdp->data + ETH_HLEN))
> +		return -EINVAL;
> +
> +	data_hard_end = xdp_data_hard_end(xdp); /* use xdp->frame_sz */
> +	if (data_end > data_hard_end)
> +		return -EINVAL;
> +
> +	addr = page_address(skb_frag_page(&sinfo->frags[0])) + frag_offset;
> +	if (offset > 0) {
> +		memcpy(xdp->data_end, addr, offset);
> +	} else {
> +		memcpy(addr + offset, xdp->data_end + offset, -offset);
> +		memset(xdp->data_end + offset, 0, -offset);
> +	}
> +
> +	skb_frag_size_sub(&sinfo->frags[0], offset);
> +	skb_frag_off_add(&sinfo->frags[0], offset);
> +	xdp->data_end = data_end;
> +
> +	return 0;
> +}

wait a sec. Are you saying that multi buffer XDP actually should be skb based?
If that's what mvneta driver is doing that's fine, but that is not a
reasonable requirement to put on all other drivers.