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 09:26:14 -0800
From:   Martin KaFai Lau <kafai@...com>
To:     Alexei Starovoitov <alexei.starovoitov@...il.com>
CC:     Jakub Kicinski <kubakici@...pl>,
        Daniel Borkmann <daniel@...earbox.net>,
        <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 08:37:58AM -0800, Alexei Starovoitov wrote:
> 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.
It makes sense that adjust_head() will eventually be supported by
all xdp-capable driver.  If that is the case, lets check
prog->xdp_adjust_head inside the driver instead of adding
another ndo_xdp command which will become unuseful very soon.