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

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Date:   Mon, 21 Mar 2022 13:17:51 +0100
From:   Lorenzo Bianconi <lorenzo@...nel.org>
To:     Jakub Kicinski <kuba@...nel.org>
Cc:     Lorenzo Bianconi <lorenzo.bianconi@...hat.com>,
        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.

ack, got your point, but I guess there is still a value running the same xdp
program instead of switching to a tc one if the driver does not support
native xdp. Anyway I am fine dropping this patch.

> 
> > > 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 :)
:)

Do you mean set dev->needed_headroom based on XDP_HEADROOM if the device is
running in xdp mode, right? I guess this is doable for veth, but what is
the right value for generic-xdp? Am I missing something?

Regards,
Lorenzo

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