netdev - Re: [PATCH bpf-next] net: xdp: introduce XDP_PACKET_HEADROOM

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Date:   Fri, 18 Mar 2022 20:54:51 +0100
From:   Lorenzo Bianconi <lorenzo.bianconi@...hat.com>
To:     Jakub Kicinski <kuba@...nel.org>
Cc:     Lorenzo Bianconi <lorenzo@...nel.org>, bpf@...r.kernel.org,
        netdev@...r.kernel.org, davem@...emloft.net, ast@...nel.org,
        daniel@...earbox.net, brouer@...hat.com, pabeni@...hat.com,
        toke@...hat.com, andrii@...nel.org, nbd@....name
Subject: Re: [PATCH bpf-next] net: xdp: introduce XDP_PACKET_HEADROOM_MIN for
 veth and generic-xdp

On Mar 18, Jakub Kicinski wrote:
> On Fri, 18 Mar 2022 20:19:29 +0100 Lorenzo Bianconi wrote:
> > diff --git a/net/core/dev.c b/net/core/dev.c
> > index ba69ddf85af6..92d560e648ab 100644
> > --- a/net/core/dev.c
> > +++ b/net/core/dev.c
> > @@ -4737,7 +4737,7 @@ static u32 netif_receive_generic_xdp(struct sk_buff *skb,
> >  	 * native XDP provides, thus we need to do it here as well.
> >  	 */
> >  	if (skb_cloned(skb) || skb_is_nonlinear(skb) ||
> > -	    skb_headroom(skb) < XDP_PACKET_HEADROOM) {
> > +	    skb_headroom(skb) < XDP_PACKET_HEADROOM_MIN) {
> >  		int hroom = XDP_PACKET_HEADROOM - skb_headroom(skb);
> >  		int troom = skb->tail + skb->data_len - skb->end;
> >  
> 
> IIUC the initial purpose of SKB mode was to be able to test or
> experiment with XDP "until drivers add support". If that's still
> the case the semantics of XDP SKB should be as close to ideal
> XDP implementation as possible.

XDP in skb-mode is useful if we want to perform a XDP_REDIRECT from
an ethernet driver into a wlan device since mac80211 requires a skb.

> 
> We had a knob for specifying needed headroom, is that thing not 
> working / not a potentially cleaner direction?
> 

which one do you mean? I guess it would be useful :)

Regards,
Lorenzo

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