netdev - Re: [PATCH v3 net-next 1/4] bpf: xdp: Allow head adjustment in XDP prog

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Date:   Wed, 7 Dec 2016 08:37:58 -0800
From:   Alexei Starovoitov <alexei.starovoitov@...il.com>
To:     Jakub Kicinski <kubakici@...pl>
Cc:     Daniel Borkmann <daniel@...earbox.net>,
        Martin KaFai Lau <kafai@...com>, netdev@...r.kernel.org,
        Alexei Starovoitov <ast@...com>,
        Brenden Blanco <bblanco@...mgrid.com>,
        David Miller <davem@...emloft.net>,
        Jesper Dangaard Brouer <brouer@...hat.com>,
        John Fastabend <john.fastabend@...il.com>,
        Saeed Mahameed <saeedm@...lanox.com>,
        Tariq Toukan <tariqt@...lanox.com>,
        Kernel Team <kernel-team@...com>
Subject: Re: [PATCH v3 net-next 1/4] bpf: xdp: Allow head adjustment in XDP
 prog

On Wed, Dec 07, 2016 at 11:41:12AM +0000, Jakub Kicinski wrote:
> > I see nothing wrong if this is exposed/made visible in the usual way through
> > ethtool -k as well. I guess at least that would be the expected way to query
> > for such driver capabilities.
> 
> +1 on exposing this to user space.  Whether via ethtool -k or a
> separate XDP-specific netlink message is mostly a question of whether
> we expect the need to expose more complex capabilities than bits.

I'm very much against using NETIF_F_ flags and exposing this to user space.
I see this xdp feature flag as temporary workaround until all drivers
support adjust_head() helper. It is very much a fundamental feature for xdp.
Without being able to add/remove headers the usability of xdp becomes very limited.

If you guys dont like extra ndo_xdp command, I'd suggest to do
"if (prog->xdp_adjust_head)" check in the driver and if driver doesn't
support it yet, just fail XDP_SETUP_PROG command.
imo that will be more flexible interface, since in the future drivers
can fail on different combination of features and simple boolean flag
unlikely to serve us for long time.