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

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Date:   Wed, 07 Dec 2016 12:04:13 -0500 (EST)
From:   David Miller <davem@...emloft.net>
To:     alexei.starovoitov@...il.com
Cc:     kubakici@...pl, daniel@...earbox.net, kafai@...com,
        netdev@...r.kernel.org, ast@...com, bblanco@...mgrid.com,
        brouer@...hat.com, john.fastabend@...il.com, saeedm@...lanox.com,
        tariqt@...lanox.com, kernel-team@...com
Subject: Re: [PATCH v3 net-next 1/4] bpf: xdp: Allow head adjustment in XDP
 prog

From: Alexei Starovoitov <alexei.starovoitov@...il.com>
Date: Wed, 7 Dec 2016 08:37:58 -0800

> 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.

Indeed, if the eventual plan is to have all drivers be required to
support a fundamental set of XDP features then exporting this in any
way to userspace is not a good idea.