netdev - Re: [PATCH] net-xdp: netdev attribute to control xdpgeneric skb linearization

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Date:   Fri, 24 Jan 2020 09:15:42 -0800
From:   Luigi Rizzo <lrizzo@...gle.com>
To:     Toke Høiland-Jørgensen <toke@...hat.com>
Cc:     Daniel Borkmann <daniel@...earbox.net>, netdev@...r.kernel.org,
        Jesper Dangaard Brouer <hawk@...nel.org>,
        "David S. Miller" <davem@...emloft.net>, sameehj@...zon.com
Subject: Re: [PATCH] net-xdp: netdev attribute to control xdpgeneric skb linearization

On Fri, Jan 24, 2020 at 7:31 AM Toke Høiland-Jørgensen <toke@...hat.com> wrote:
>
> Luigi Rizzo <lrizzo@...gle.com> writes:
>
...
> > My motivation for this change is that enforcing those guarantees has
> > significant cost (even for native xdp in the cases I mentioned - mtu >
> > 1 page, hw LRO, header split), and this is an interim solution to make
> > generic skb usable without too much penalty.
>
> Sure, that part I understand; I just don't like that this "interim"
> solution makes generic and native XDP diverge further in their
> semantics...

As a matter of fact I think it would make full sense to use the same approach
to control whether native xdp should pay the price converting to linear buffers
when the hw cannot guarantee that.

To me this seems to be a case of "perfect is enemy of good":..

cheers
luigi


>
> > In the long term I think it would be good if the xdp program could
> > express its requirements at load time ("i just need header, I need at
> > least 18 bytes of headroom..") and have the netdev or nic driver
> > reconfigure as appropriate.
>
> This may be interesting to include in the XDP feature detection
> capabilities we've been discussing for some time. Our current thinking
> is that the verifier should detect what a program does, rather than the
> program having to explicitly declare what features it needs. See
> https://github.com/xdp-project/xdp-project/blob/master/xdp-project.org#notes-implementation-plan
> for some notes on this :)
>
> -Toke
>