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

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Date:   Fri, 18 Mar 2022 14:01:53 -0700
From:   Jakub Kicinski <kuba@...nel.org>
To:     Lorenzo Bianconi <lorenzo.bianconi@...hat.com>
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 Fri, 18 Mar 2022 20:54:51 +0100 Lorenzo Bianconi wrote:
> > 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.

Ack, I understand the use case is real, but given that the TC
alternative exists we can apply more scrutiny to the trade offs.
IMO production use of XDP skb mode would be a mistake, the thing 
is a layering violation by nature. Our time is better spent making
TC / XDP code portability effortless.

> > 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 :)

We have ndo_set_rx_headroom and dev->needed_headroom.
Sorry for brevity, I'm on the move today, referring to things 
from memory :)